Understanding the function of the AMP Indicator (GEN Indicator in a P1800) in your vintage Volvo, and how to proceed when it comes ON.  Including Original Equipment Charging System Checks, Troubleshooting and Repair

First published 2005, R. Kwas, Revisions Ongoing  [Comments Added] 

The AMP Indicator glowing in all its glory...and if it does this all of a sudden, while
 we're traveling along at 40mph, it calls for some immediate investigation!

Condition 1 Normal, Non-Failure Mode
Condition 2 Failure of Charging System
Condition 3 Fuse 1 Open
Condition 4 The “Other” Condition
Regulator Bypass Test
GEN Indicator in the 1800
    Troubleshooting the Generator based Charging System of Earlier Cars
AMP Indicator in the 1800E/ES
    Troubleshooting an 1800E Charging Sys
Generator Checks
    Generator Internal Circuit
Detailed Explanation of the Lightly Glowing Amp Indicator Condition
Brushes

Reference Information

----------------------------------------------------

The info presented here applies to unmodified Volvo 122 (and substantially 544), and 1800 (Bosch generator) 12V electrical systems, and is intended to help in the understanding of these systems for the purpose of trouble locating and repair after the AMP indicator comes ON.  I have long ago upgraded just about all of my vintage vehicles to modern, high output alternators with fully electronic internal regulators, and I’m a big advocate of this, but this article is intended for those who prefer to remain original, as locating the troubles once the AMP indicator comes ON can be tricky for qualified individuals and totally baffling for the inexperienced.  The article is under constant amendment and improvement... 

Notice:  I stand behind all the information presented here, having put it together with the utmost care, but in the end, the reader is responsible for its use, the consequences of their actions and their own knuckles.  The information presented should be used in conjunction with normal, cautious shop practice.  I will not be responsible for all your actions, especially those which cause injury or damage!  Work safely!

 ----------------------------------------

Wiring excerpt of 122 Wiring Diagram with Charging (AMP) Indicator wiring highlighted.

Since the deceivingly simple AMP indicator is actually employed in a slightly complicated manner (not to mention that is not even indicating AMPs - that was just a handy - and short, electrical sounding word that would fit on the tiny indicator**), a complete understanding of this indicator’s function is necessary.  It may, in some circles, be called an “Idiot Light”, but the manner in which it was designed into the electrical system is anything but idiotic…its actually rather ingenious…in a do-the-maximum-with-a-minimum kind-of way! 

**  If the AMP lettering is no longer present on the indicator, see also:  https://www.sw-em.com/122Instrument_Panel_Indicator_Lens_Graphics_Repair.htm

The AMP light itself is a dirt-simple (2 Watt) incandescent lamp, but the manner in which is cleverly worked into the electrical system allows it to light up and indicate two very different failure conditions.  Herein lies the complication.  In the first place, in the normal condition (IGNition ON, engine running / all-things-fine), the lamp actually has voltage applied to both sides, and so does not light!  Understanding this simple fact is essential to appreciating what is happening here, and unraveling the complication.  Reminder:  For any electrical device to function it requires voltage across its terminals.  This voltage serves to encourage the flow of electrical current through the load, powering it up…and indeed, the AMP indicator as well, must have voltage across it to light! 

Reviewing, a complete 122S wiring diagram, or FIGURE 1. below, it can be seen that, one side of the AMP indicator is connected to Fuse 1 (IGNition Power).  The other side is connected to, and monitors the Charging System (Generator) Output at, the Voltage Regulator (VR) D+ terminal.  The reader will note and should understand, that in this situation, with voltage on both sides of the lamp, no current flows, and the indicator remains unlit!  

FIGURE 1.  AMP Indicator and related circuitry for a 122, showing wire gauges. 
Numbers associated with wiring are wire cross-sectional area in mm².  O.C. denotes “Other Connections”.
I have now added the three important nodes involved:  Generator Output, Battery Power, and Ignition Power. 

Again, in order to light the lamp, the voltage-across condition must be satisfied.  Only this will allow current to flow.  The really observant or electrically enlightened reader will also notice, that this condition may be met by tying either side to ground.  [See:  Idea 1 below]

Condition 1:  Normal, non-failure mode.  AMP Indicator is OFF.

The two conditions, which will cause the AMP indicator to light while the engine is running, are therefore:

Condition 2:  Failure of Charging System (…the armature of generator does not produce voltage, but instead, completes circuit to chassis allowing lamp current to flow…for example…when IGN key is turned ON, but before starting the engine). 

Condition 3:  Fuse 1 open (…at which time, the other [unpowered!] Fuse 1 loads complete the circuit to chassis allowing AMP Indicator lamp current to flow). 

Condition 4:  The “Other” Condition:  AMP Indicator glows dimly while engine is running.  This is a frequently occurring condition known to just about all owners of these vehicles, discussed separately below.

 ----------------------------------------------------------

Conditions Explained:  

Condition 1  is the normal non-failure condition of Ignition ON and Charging system output normal.  Since AMP Ind is electrically connected across Ign Pwr and Gen Output, this results in voltage on both sides of the Indicator, and it remains OFF! 

Condition 3 [...placed here, before Condition 2, because it is simpler and can be performed "on the road"]  Checkout of open Fuse 1 causing AMP indicator to be ON. 

An open Fuse 1 is quickly verified, as it is accompanied by the inability to turn ON other Fuse 1 loads…specifically, these are:  Directional indicators, Fuel gauge, Blower, and Wipers.  If the AMP Indicator ever comes on full, while the engine is running, or does not extinguish shortly after starting, this is the first thing, which should be checked for, as it is the simplest.  The driver, even a technical layman can, and should do this…as soon as practical. 

Functional test when AMP indicator comes ON with engine running:  Turn any of the other Fuse 1 loads ON.  If loads do not function, Fuse 1 should be suspect.  Open hood at the next opportunity, and with IGN ON and engine running, activate left directional indicators (these are easily seen while standing next to the vehicle on the drivers side).  Remove the fuseblock cover (you should know where it is!), and visually check the state of Fuse 1.  If the fuse is visually OK (Suggestion:  Always install Euro-fuses with element visible to allow for an easy inspection!), spin it in its holder…if this restores the connections so that the directional indicators start blinking (and causes the AMP light to extinguish), corrosion may have been present and breaking the connection.  Spinning the fuse has cut through this corrosion at the conical fuse ends, and reestablished a connection…  This is actually a rather common failure…on the injected 140s and 1800s, this failure occurring on the fuel-pump relay fuse is also well-known…

…if, on the other hand, the wiper worked when the switch was tried, Condition 2 may exist, or the classic “Other” Condition caused by long-term corrosion.  Checkout and correction of Condition 2 will require a more careful investigation of voltages under the hood using a multimeter.  This is down-and-dirty technical stuff, and should be performed by someone who is at least familiar with the operation of a multimeter.  

Verification of open Fuse 1 (or associated connections) condition:  Shut  IGNition OFF, allowing motor to come to a stop.  Turn IGN to the ON position, but do not START.  If AMP indicator does not light, Fuse 1 should be suspect.  Explanation:  When IGN power is applied, but before starting, the AMP indicator is supplied on one side with IGN power and on the other with a connection to chassis through the non-electric producing armature of the generator.  The AMP indicator should light…if it does not, it must be assumed that it is not being supplied with power, and therefore Fuse 1, or associated connections are suspect. 

If replacing a (blown) Fuse 1 remedies the condition …it’s a mixed blessing.  This is the simplest failure to fix, but one has to wonder why the fuse actually blew in the first place…because actually blowing the 25A rated Fuse 1 is fairly unusual. 

If on the other hand, just the action of moving the fuse in its holder (or spinning it) is enough to restore function, that suggests the more common poor connection due to corrosion condition. 

Consider yourself lucky for not having a dreaded charging system failure , but deduct two points for neglect, and, at earliest possible opportunity, clean fuse-ends and the circular connection holes in the clips properly with abrasive, apply Zinc Anti-Corrosive Electrical Paste, and reinstall fuse.  Suggested reading:  Gas-Tight-Joint Tech article. 

Having checked Fuse 1 and found it to be intact, and spinning it to try to cut through any corrosion did not fix the symptoms, another check, requiring a multimeter should be performed before moving on to checks of the charging system.  With the IGN ON and engine running, the IGN power should be measured at the Fuse 1 and Fuse 2 (input) terminals on left (Black wires), as well as (output) terminals (Red wires), on right side.  If voltage is measured at the input but not the output, an open connection at the fuseblock exists…this may be caused by a poor connection due to corrosion at the push-on terminals, corrosion between the riveted contact plates or conical fuse-ends (as both discussed in the Gas-Tight-Joint Tech article), or a blown or fatigued fuse which was previously missed. 

If checks around the fuseblock reveal no problems areas, a failed charging system must be considered to be causing the AMP light to be ON.. 

Condition 2  Failure of Charging System, causing AMP light to be ON.  This is the next thing, which should be checked.  It is down and dirty technical stuff, requiring tools, a multimeter and at least a basic understanding of how to correctly put these to use without hurting oneself (knuckles not withstanding!). 

With engine running, measure voltage from D+ terminal (Generator output) of VR to chassis.  If this voltage is less than 2 Volts, Charging System (which is a combination of the Generator and VRegulator), has no output. 

VRegulator Bypass Test:  [As the generator is not regulated during this test it should only be performed momentarily...that is, just long enough to perform the test].  While continuing to measure the output voltage at D+, short the DF terminal of the VR to chassis (all wiring normally in place), using a screwdriver or similar at the regulator DF terminal.  This takes the place of the VReg and completes the path to ground, allowing field current to flow (see:   Simplified Charging System Wiring below).  An output in excess of 12V should now be read on the D+ terminal, and this voltage should also be measured, without a drop, on the battery plus terminal.  Raising the RPMs should raise this voltage, as the generator output comes up.  This suggests a Voltage Regulator problem.  Replace VR.  Note:  Successful repair of the VReg is beyond the scope of this article and without calibrated test equipment and setup, likely beyond the abilities of even the most experienced backyard wrencher!  There is a lot happening in the VReg!  (Link:  Bosch Generator based Charging System in Detail)

If bypassing the VReg does not result in an output, the Generator itself must be suspect.  Link to:  Generator Checks

See also: Troubleshooting the Generator based Charging System of Earlier Cars 

Condition 4 The "Other" Condition:  A softly glowing AMP Indicator is actually an absolutely normal and typical condition in the 122 (and 544) vehicles!  It is not an indication of issues or a problem  It occurs because of the Resistance of Fuse 1, and subtleties and wire gauges used in the harness, which result in a minor voltage drop, and because of the sensitivity of the AMP Ind lamp, this VDrop is able to gently cause the Lamp to glow.  See also:  Detailed Explanation of the Lightly Glowing Amp Indicator Condition 

What owners may (or may not!) take notice of when operating these cars, it that the lightly glowing AMP Ind condition actually slowly increases with time.  This increase is very slow, and so may not even be noticeable, but it is due to the increase in surface and contact corrosion occurring which increases the slight Vdrop mentioned above...it never decrease...but since we are aware of it, we can take measures to minimize it.  These include treating Fuse-contact ends and FuseBlock terminals superficially with ACZP for all its benefits, and eventually, when the the FB internal Resistance caused by the stacked and riveted construction gets so significant it can't be ignored, the FB must be replaced/reworked.  See also:  https://www.sw-em.com/gastight.htm          

-----------------------

GEN Indicator in the 1800:

The beauty of a night-time view of the early 1800 Instrument panel photographed by Andrew Garn...but with the GEN Indicator shining bright like
that, I guess he was parked, and had the Lights and Ignition ON at the time!  A normal condition, because the Charging System is not contributing! 
Gorgeous picture shown with Andrew's kind permission.  See also:  https://www.andrewgarnphoto.com/

In the P1800, the Indicator called AMP in the 122,
is labeled more correctly GEN! 

They may be labeled differently, but the way they're connected, with voltage on both sides of the Indicator under normal conditions, is absolutely the same and typical.  There is a bit of a difference in the information it gives a driver though...read on... 

It can be seen in this 1800 Wiring Diagram Excerpt, that GEN (LADDNING) INDICATOR of the 1800 is connected across IGN Power and GEN Output.  In this way, under normal conditions of engine running and CHG SYS having an output, there is 12V power on both sides of the Indicator and it remains unlit, just as in an Amazon!  In the event that CHG SYS does not produce an output (for instance when Ign is turned ON, but before engine is started), 0V then occurring at the D+ terminal of GEN, would "sink", or provide the negative side return current path, to the GEN IND, causing it to light, and thus advising driver of the Chg Sys having no output.  Reference also:   https://www.sw-em.com/Cut-Out-Contact_Notes.htm#COC_with_High_Gen_Output 

The observant reader will also notice when comparing Indicator implementation between the 122/544 and 1800 wiring, that in the 1800, the GEN IND is powered directly by Ign Sw Term 54 (Ign Pwr) and not by way of a fuse as in the 122.  This means that when a GEN IND ON condition occurs in the 1800, a blown Ign Pwr Fuse2 is not a possible cause, as it might be in the 122/544 (Condition 3, above).  This is a minor difference only noticeable to the most careful observers among us, but it needed to be mentioned as it is a significant difference when troubleshooting...indeed if the GEN IND comes ON in an 1800, failure of the CHG SYS is more of a distinct possibility, whereas in the 122, it might just be due to a blown Fuse1 or simply open associated elec connections due to corrosion of contacts in the area of the fuse! 

See:  https://www.sw-em.com/gastight.htm  and:  https://www.sw-em.com/anti_corrosive_paste.htm#ACZP_on_Eurofuse_ends  

Also, the "Daisy Chain" wiring implementation is different in the 1800, so the [Lucas automotive electrical aberration] FB is involved in the connection of power to the IGN SW Term 30 [OH NO!] from the wiring found in a 122 where there is one, uninterrupted wire and no "Daisy Chain" to introduce more connection points and decrease ignition reliability!  This is a philosophical difference between British and Swedish wiring implementation, which the author has previously written about.  See: https://www.sw-em.com/1800Ignition_Wiring_Swedisch_vs_British.htm

See also:  Troubleshooting the Generator based Charging System of Earlier Cars

-----------------------

AMP Indicator in the 1800E/ES: 

In these vehicles, the Indicator is once again named poorly, with a term which suggest it is somehow measuring current (Amps), when it is not...!  The AMP Ind has Ign power on one side, and on the other side, monitors the Alt output.  

AMP Indicator in an Injected 1800.

My e-mail response to a SW-EM Alt Kit Installer asking if [it is] "possible to get faulty "Amp light" condition when there is really nothing wrong with the alternator but light gets turned on?"

"The AMP Indicator is a simple little 3 or 4 Watt lamp, but wired in a somewhat clever manner in the ES (and most other vehicles also actually) to monitor the output of the Charging System, and to light and alert the driver when it fails and no longer has an output.  It does this by having Ign 12V on one side, and the Chg Sys output on the other...and under this "normal" condition, the Ind remains unlit!  If the Chg Sys were ever to not to have an output while Ign is ON, the "Voltage across to light lamp" conditions would be met (for instance when driver turns ON Ign, but before starting engine), to light the Ind. 
 

See also:  https://www.sw-em.com/123GT_Charging_System_Notes.htm#1800E_ES_Charging_System_Notes   Refer to E/ES Wiring Diagram Excerpt I have there.
 

[Is it] "possible to get faulty "Amp light" condition when there is really nothing wrong with the alternator but light gets turned on?" 
 

Answer:  Yes! 
 

Explanation:  Recall that for the Ind (or ANY lamp really!) to light, the condition of Voltage across must be met...but this can happen in TWO distinctly different manners:
 

1.  Failure of the Chg Sys to have an output while Ign is ON (the primary and intended function of the AMP Ind)! [In these Alt based Chg Systems, it does this by monitoring the Alt output not directly at the output B+ Terminal, but indirectly by way of the D+/61 Term.  Refer to internal wiring of Alts here: https://www.sw-em.com/123GT_Charging_System_Notes.htm#bosch_motorola_similarities_and_differences  ]  

...but also: 
 

2.   Notice on the WD Excerpt below, that the source of Ign power for the AMP Ind is from Ign Sw term 15, by way of Fuse4.  With Ign ON and engine running and Chg Sys putting out just fine (verified by measuring the system V across Batt when it occurs, and this measures higher than 12.6V, meaning Chg Sys is pulling it up with its output), IF this fuse were to blow open (OR, a corroded contact at the conical ends of the fuse were to make the fuse ACT LIKE it was open, a condition known to happen, for those who have not preventively treated their fuse with ACZP...!), the condition of "Voltage across to light lamp" would also be met!!  I suspect that is the condition you have experienced! 
 

That's the long of it!  The short of it is:  Remove ALL of your fuses (inspect and assure they are not open or excessively corroded) and lightly abrade their conical contact ends, as well as the (inverse) conical contacts in the Fuseblock where they normally reside when installed, then reinstall with a dab of ACZP (see:  https://www.sw-em.com/anti_corrosive_paste.htm#ACZP_on_Eurofuse_ends   ) to prevent future corrosion. 
 

I fully expect that will remedy the AMP Ind ON, when Chg Sys OK condition you may have experienced and are asking about...so says:  The Volvo Electrical Whisperer!

BTW...there will be a generous sample of ACZP included with your SW-EM Alt Kit!

Let me know how you make out, Regards, "

1800E/ES Wiring Diagram, Charging Circuit with AMP Indicator Excerpt:

The observent reader will notice that unlike in the 122 or 1800 Generator based Charging Systems, the AMP Indicator of the Alt based systems is not directly connected to the Alt output (which would be the B+ Term of Alt), but it is connected instead to the D+/61 Term.  This teminal serves the purpose of being a combination of an Alt Exitaction and also for the Output Monitoring because of the connections internal to the Alt.  See here: https://www.sw-em.com/123GT_Charging_System_Notes.htm#bosch_motorola_similarities_and_differences  

Troubleshooting an 1800E [OE Alternator based] Charging Sys:

My response to a posting asking how to tell if Alt or VReg is bad when measuring 12V as Sys V (and helpful poster suggesting to install a Voltmeter): 

"Voltmeter is going to tell you what you already know: That Chg Sys has no output!

What is state of AMP Indicator? See: https://www.sw-em.com/AMP_Indicator_ON.htm#AMP_Indicator_in_the_1800E-ES

Jacnar has given good info! ...which is to separate the VReg from Alt, and MANUALLY apply Field Current, by connecting Alt DF to Battery, while monitoring system Voltage (obviously while engine is running and turning Alt). If V rises when Revs are above Idle, it means Sys V is being pulled up by Alt output contributing output and charging Bat, that means VReg (or associated connections) is suspect. If Alt does not pull up Sys V, Alt itself is suspect. Remove Fanbelt and measure Field Resistance with turning Alt Pulley...(from memory, it should be under 10 Ohms with little variation)...if higher, or R bounces significantly, Brushes may be excessively worn and not making contact, or field is open.
...but BEFORE taking things apart, assure all Chg Sys associated connections are clean metal and tight (and protected with ACZP can't hurt!). This includes chassis connections!

See also: https://www.sw-em.com/123GT_Charging_System_Notes.htm#bosch_motorola_similarities_and_differences Good Hunting, and let us know what you find!"

-----------------------

Troubleshooting the Generator based Charging System of Earlier Volvo Cars:  

My answer from e-mail exchange with Taylor F.:  8/26/2013  Subject:  Re: ..... and alternator conversion kit?

> "Are you certain the Generator is the problem?
>
> If you are certain the problem lies with the Generator and have spares and or a competent shop which can repair/rebuild it, or can do it yourself, that certainly is one alternative (check sw-em site for hints and troubleshooting info) but you or the shop you entrust the project to would have to be one familiar with the Bosch Generator based charging system, or the project has only a small chance of having a successful outcome.
>
> Often however, if the problem lies with the Charging Control...better known as the Voltage Regulator, which has mechanical contacts and which is therefore subject to wear...repair could be even trickier...(Link:  Bosch Generator based Charging System in Detail)
>
> The Alt upgrade on the other hand has a lot going for it, which you are undoubtedly aware of...and reliability [and margin of output] is  definitely one of them...  "

"A simple test to bypass the VReg and manually call for Generator output while a Voltmeter is connected to battery.  Raise engine RPMs to about 2K, then for purposes of test, use a test wire to connect Df terminal of VReg to chassis (this bypasses VReg and applies Field current manually, by completing connection to chassis, which Vreg would normally)... monitor system voltage.  If it noticeably rises, and system/battery voltage starts climbing, indicating Battery is charging, it suggests Gen is OK and putting out, and VReg is not working as expected.  Testing VReg is beyond the casual mechanic as it requires special equipment, techniques and experience. 

------------------------------

Generator Checks.

In-Situ Reciprocity Test.  This is a test, helpful for determining the state of health of the Generator.  It is performed with the IGNition OFF, Generator installed and wired normally, but with the Fanbelt removed.  A side-effect here is that the wiring from the VR is checked at the same time.  A successful test, gives a certain level of confidence in the Generator, harness and connections. 

Performing the Reciprocity Test is simple…after loosening and removing the Fanbelt, use two temporary test jumper wires to:

1. Connect the DF terminal to chassis, and
2. Connect the B+ to the D+ terminals at the VR. 

This applies Battery power and Field current to the generator, and by applying a marvelous principle of science, which states that many processes run backwards as well as forwards, the Generator should spin as a motor.  If it does not, Generator is suspect.  Remove test wires and proceed to…

Passive Resistance Tests.  With IGNition OFF, Battery plus disconnected, and loose fanbelt, disconnect the D+ and DF wires from the Generator.  With a Multimeter in the Ohms function, verify the internal connections as shown in the graphic following.  The D+ terminal (Armature output, which runs through the brushes) to chassis reading should read about:  50 Ohms, remaining fairly steady and without a lot of fluctuation when spinning the generator pulley in the normal running direction.  The DF to D+ terminal should read about 4.8Ohms, unaffected by a spinning armature.  This is a basic check on internals of the generator as shown below.  Note that the power connection for the Field is permanently made, within to the generator, but requires an external connection to chassis in order to allow field current to flow.  This in fact is, what the VR does as it monitors the system voltage against an internal reference, and what the troubleshooter has done, manually, when performing the previous Regulator Bypass Test. 

Generator Internal Circuit:  

Note that only one of multiple Armature Windings is shown for simplicity.  

Poor Connections!  Don’t discount these, since poor connections are a definite possible cause of electrical problems, especially on a 40 year old vehicle, it wouldn’t be such a bad idea to assure all connections are clean and tight before starting to replace expensive electrical components.  One would feel pretty silly if hours of troubleshooting and expensive replacement work were all for naught, if a problem turns out to really have been caused by a corroded or loose connection! 

Zinc anti-corrosive electrical paste is the Anti-Seize of the electrical world!  Use this paste after disconnecting and cleaning electrical connections of the vehicle.  The connections critical to the charging system are, voltage regulator (including its mounting bolts which make the connection to the vehicle chassis!), generator, battery, starter, IGN switch, fuseblock, groundstrap (connecting chassis and engine). 

Detailed Explanation of the Lightly Glowing Amp Indicator Condition (for engineers!): 

Condition 4 The “Other” Condition:  AMP indicator glows slightly.  The well-known condition of a dimly lit AMP light in the 122 or 544 vehicles, is not a separate condition, but a variation of Condition 1.  It is caused by the slight voltage drop, which occurs, by design, across the Fuse 1**, as the indicator lamp is a small, sensitive lamp, even a slight voltage across it will satisfy the voltage across rule, and cause a slight glow of the lamp.  This is absolutely normal, and even occurred when the car was new!  If however, additional voltage drop occurs due to poor connections, the voltage across the lamp increases, and the lamp glows more brightly. 

The effect of a dimly lit AMP Indicator (particularly evident when turning ON Fan Switch* in dark conditions) is a direct result of voltage drops due to current passed (IR Drop **).  Specifically, as can be seen in FIGURE 1. , Voltage Drop across Fuse1 (which is inherent for any fuse and cannot be avoided) plus associated connections in Fuseblock (which can and should be avoided), is caused by passing the currents of the downstream loads. 

*  The observant reader may have noticed that the dimly glowing AMP Indicator effect is definately more pronounced when turning ON Fan Switch than when turning ON Wipers, although both are Fuse1 loads and the Wiper current is higher in magnitude.  This seemingly inconsistent action can be explained by the fact that the Voltage Drop resulting from Fan current is actual higher than the Voltage Drop resulting from Wiper Motor, and this occurs because of the wire gauges involved…a heavy wire gauge (4mm²) runs between Fuse1 and Wiper Switch and a much lighter gauge wire (1.5mm²) connects Wiper Switch to the Fuel Gauge...and since voltage drop on a wire for a given current is less on a heavy wire than a thin wire, it should be clear why the AMP Indicator typically gets brighter when setting the Fan Switch to the High position while being largely unaffected by the Wiper Switch

**  IR Drop Explained:  For the non-engineer reader, the IR Drop is simply the Voltage which develops across any resistance in the path of current.  The term comes from Ohm’s Law.  Normally, the IR Drop is ignored and not spoken of because components and conductors in a circuit are intentionally sized when designing the circuit, so that these Voltages are miniscule, because any voltage dropped along the way is not available at the load to drive current through it and do the intended work.  In other words, when things were new, things were simple:  Voltage at the Source was controlled by some sort of Switch and it was either ALL across the Switch (when Switch is open) OR ALL across the Load (when Switch is closed).  In the vintage vehicle world however, corrosion and loose or otherwise less-than-good connections can, and often do, become significant contributors to circuit action.  This must be carefully considered and taken into account when troubleshooting issues that arise! 

My response to a Swedespeed posting:  Alternator light glows  [Its actually not an Alt Light...its the AMP Indicator...and that is even a poor name, because it doesn't even indicated current...that was just a handy electrical term which would fit on the small lens...I suppose it could be referred to as an Alt Light after a conversion to Alt, because then, when ON while engine is running, it does indicate a problem with the Alt/Chg Sys!]

https://www.swedespeed.com/threads/alternator-light-glows.637942/#post-7747162

"Resistance of the relatively small gauge 1.5sq cm between Fan and Wiper Switches is actually a major contributor to the glowing AMP Indicator effect (notice it is the smallest gauge in the Ign power current path of the Fan, but it is located in the relatively cozy passenger compartment, so this path has unlikely increased in R)...the R of Fuse 1 itself is inherent, as you know, necessary, and some surface corrosion notwithstanding, can also not be eliminated or changed...what typically gets worse with age/corrosion is what I call the associated connections on the F1 circuit, such as conical fuse-ends [superficially accessible, and without disassembly!], and interconnector stack corrosion  of the Fuseblock [not accessible without disassembly!  See:  https://www.sw-em.com/gastight.htm and https://www.sw-em.com/123GT_FuseBlock_Rework.htm ], which are notably located in the significantly less cozy engine compartment...when these increase, and they will, and do, they also manifest themselves as VDrops in the current path, which get added to the V across the AMP Ind, contributing to the effect ...refer to https://www.sw-em.com/AMP_Indicator.JPG (the current drawn by Fan causes the VDrop on this wire, and this VDrop increases V across Lamp of Indicator also...this is why the effect is more pronounced when turning Fan ON).

This gets into the finer points of VDrops across current carrying conductors, and interactions of multiple VDrops of differing causes, so is more of a theoretical discussion between us engineers...the practical one-liner explanation and solution and recommendation for non-engineer owners is to have clean fuse-ends and pushon connectors (shiny/snug/ACZP protected), and when eventually the effect gets worse, recommending a FB refurb/replacement.  "

-------------

Excerpt from and e-mail exchange...a simple question, with a complicated answer!:  [Further comments, not in the original thread, added]

  I am going to test the voltage drop across the [Fuseblock] connectors and wonder what you consider to be an acceptable loss. [...a simple question but with a complex answer!...because the voltage drop is a result of Ohms Law derivation equation V=IR, so it is mathematically, a function of BOTH the circuit current which varies with the Load (this is expected) or total resistance at the FuseBlock (some due to the R of the Fuse, intentional [See also:  How does a fuse work  ], and some due to the unwanted and unexpected corrosion/poor contact), so it's tough to give a numerical maximum...but if you measure across the hook-up terminals (not the fuse terminals), while the typical load or even highest magnitude of current is flowing, in other words, the load(s) are ON (suggested for this:  F1 - Wiper,  F2 -  Brake Lights, plus Horn, F3 - Parking Lights, F4 - Courtesy Lighting (obviously less critical)  Reference:  https://www.sw-em.com/Fuses,%20Allocation%20and%20Troubleshooting.htm#122_(12V)  ) I think I would want to see no more than a half of a volt drop...if I did...I'd measure right across the fuse to verify it was only across the R of the fuse, and not across an R of the associated terminals or even the conical fuse contacts...if it was, some abrading to shiny metal and application of ACZP to minimize it was due!  [It is the Vdrop on Fuse 1 and associated connections, which causes the AMP Ind glow!]
...sorry for the electronics lesson, but there is no way of doing this without getting into at least some level of detail...why don't you do this test and report the findings...I may need to consider my recommendations further, based on your findings. 
See also this reference:  https://www.sw-em.com/Corrosion_on_1800E_and_1800ES_Fuseblocks.htm#Four_wire_Low_Resistance_(Kelvin)_Testing  ]

-------------

Summary [for 122/544]

In order to light the AMP indicator, the voltage-across-condition must be satisfied.  The two conditions are:

1.   1.    Fuse 1 open.  [This applies only to 122/544 wiring, because of difference in the wiring of the Indicator in P1800, see also:  GEN Indicator in the 1800  ] 

2.        Failure of charging system.

Given good connections, these are two conditions under which the AMP indicator may light, but add to that, frequently occurring faults due to subtle corrosion and/or poor connections, locating and remedying the trouble can get complicated…fast!

Following an order of Simplest-to-Complicated fault isolation, check Fuse 1 and associated connections first, then Charging system, breaking this into Generator, then Voltage Regulator, components.  

----------------------------------------------------------

Voltage Regulator (VR) function for a generator electrical system explained: 

1.  The VR’s main function is to control the electrical system by monitoring system voltage and activate the generator to contribute electrical power when system voltage (and therefore battery charge) is low. 

3.        A second function is required in generator systems.  That is, to disconnect the generator from the battery when the IGN is OFF and generator is not contributing (in fact, the generator would take [and unnecessarily waste] battery charge if left connected under this condition!).  Update:  The Cut-Out Contact (here:  "Cut-in contacts") function has been documented in detail here:  Cut-Out Contact Notes

Simplified Charging System Wiring Diagram
 

Link to Sw-Em Tech Article:  Bosch Generator based Charging System in Detail

----------------------------------------------------------

**  Review:  How a fuse works: 

A fuse is an intentional weak electrical link, which has a small but important resistance (R - Ohms) to the flow of electrical current (I - Amps).  As current flows to the downstream load, this resistance causes a small voltage drop (due to Ohms Law V=IR), which in turn causes power dissipation (P - Watts) to occur (also known as I2R heating).  As the current increases, so does this heating (exponentially, in fact).  By designing and constructing fuses carefully, these can be used as simple, effective, circuit protective devices, with a predictable (normal and expected) level of current which will be allowed to pass, and a predictable amount of (fault) current which will, stop the fault current from being allowed to pass.  The fuse “blows” (melts, breaking the connection and thereby power delivery to the load), when excessive current is drawn by the circuit it is in series with. 

Link to additional info:  Fuses, Allocation and Troubleshooting.htm#Understanding_the_information_an_open_fuse_can_give

----------------------------------------------------------

Idea 1.  It is easy to see that depending on which side is supplying the power and which side is supplying the ground (return current path to battery negative), determines the direction of current flow through the indicator.  This immediately brings to mind those cute little bi-color LEDs, which have two different color LEDs in one package, back to back, and so light up differently depending on current direction.  These might be nice to use here…Green for Fuse 1 blown, Red for Charging system failure. [I wrote this a while ago, and since then, Red/Blue combination LEDs have become available...using one of those would be better I expect, because lighting an Indicator Green is not generally recognized as an indication of a problem condition...but Red for Fuse 1 open, and Blue for Charge Sys failure might be very effective...] 

----------------------------------------------------------

Links:  

Brushes:  

Thread on British Forum: AMP Light on full red on startup:  http://www.volvoforums.org.uk/showthread.php?t=232356

From that thread, Derek was kind enough to post this picture with measured dimension,
of a new Volvo PN 273500 Generator brush. 

One can see on the picture that once Brush is worn past about the 50% point, its' width will be more than its' height, and the spring force, which holds it pressed to the commutator, will be a lot less, at some point becoming inadequate...that's when it's time to replace them, and this is what Leif Anderson's "road-fix" below is based on...

Excerpt from a post of mine in that thread:

"I specifically recall years ago...Leif Anderson, while visiting the US and joining us for the Mt. Snow Volvo Meet...when someone's Ch Sys failed due to short/worn Brushes, he nonchalantly walked by a bunch of guys up to their elbows in underhood Volvo dirt and Generator dust in the parking lot of an event (when we should have been hobnobbing and drinking Chablis with everyone else) and suggested simply putting the same Brushes back into guide with a 90deg twist, as they were worn shorter than in the wider dimension...this made them taller and able to get more preload force from the Spring...and it worked!...to return function of Ch Sys as a temporary fix...long-term, this leaves a step-wear in commutator so requires proper service including removing any step from commutator ASAP, but is a good emergency road fix in a pinch."

...now that's what I call thinking "outside the herring tin" (Swedish equivalent of a popular saying)!  

Link to good info (VW) on Brushes and changing them:  http://www.reluctantmechanic.com/step-by-step/change_generator_brushes.php

---------------------------------------------------------

Notes / References / Additional: 

Link to Zinc anti-corrosive electrical paste. 

Link to Sw-Em Tech Article:  Bosch Generator based Charging System in Detail

-------------

Additional:  The official green Volvo Service manual calls for a nominal field coil resistance of 4.8 Ohms.  No armature readings are given.  After a call on the Brickboard and Volvoniacs fora, for actual resistance readings on generators made, some helpful owners (Thanks Phil, Joe, McMike ) provided readings on known good units, and also suspected bad units.  The typical good readings are included in the previous text for reference.  The readings from suspected bad units are included below along with comments to help owners troubleshoot these generators.  Finally, service recommendations are made for repairing the non-working units. 

-----------------------

Suspected Unit 1

D+ to D- (Armature, static):  Open (infinite Ohms)

D+ to D- (Armature, while turning):  Open

DF to D+ (Field only):  Open

Comments:  When making resistance measurements on old, possibly less than clean automotive components, its obvious to assure good connections are made…so the first thing I would ask in response to these readings is:  Are your sure you were making a good connection with the meter probes?  If the answer is: “Yes”, the second thing I’d ask is:  Are there any guts in the thing or is it perhaps just an empty shell? 

Service Recommendation:  Open inspection band, retake readings on wires or connections inside to verify, if no change, disassemble totally, and recheck Armature and Field resistances while apart.

-----------------------

Suspected Unit 2

D+ to D- (Armature, static):  48Ohms

D+ to D- (Armature, while turning):  Bouncing from 20 to 200 Ohms

DF to D+ (Field only):  4.8 Ohms

Comments:  Possibly worn brushes, possibly a contaminated commutator, or both.  Field resistance is nominal, so this generator has possibilities!

Service Recommendation:  Open inspection band, check height of brushes and color of commutator, using emery board clean commutator to a shiny copper, clean using carb cleaner or brake cleaner solvent to flush (conductive) dust from brushes, and copper particles from commutator clean-up, then compressed air to dry. 

-----------------------

Notes on Generator Troubleshooting:  

From e-mail exchange with Taylor F.:  8/26/2013  Subject:  Re: ETA on 122 seat suspension kits and alternator conversion kit?

> Are you certain the Generator is the problem?
>
> If you are certain the problem lies with the Generator and have spares and or a competent shop which can repair/rebuild it, or can do it yourself, that certainly is one alternative (check sw-em site for hints and troubleshooting info) but you or the shop you entrust the project to would have to be one familiar with the Bosch Generator based charging system, or the project has only a small chance of having a successful outcome.
>
> Often however, if the problem lies with the Charging Control...better known as the Voltage Regulator, which has mechanical contacts and which is therefore subject to wear...repair could be even trickier...
>
> The Alt upgrade on the other hand has a lot going for it, which you are undoubtedly aware of...and reliability is definitely one of them...

---

---------------------------------

This article is Copyright © 2005 - 2024.  Ronald Kwas.  The terms Volvo and Bosch, are used for reference only.  I have no affiliation with either of these companies other than to try to keep their products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles and future! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you don’t, you’re just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment.