An Automotive Electrical Primer ...and remember:  Please don't call it juice!   
R. Kwas 10/02  changes on-going
(mostly when the spirit moves me!) [Comments Added!]


Proving Everything Bad you Ever Heard About Lucas
Heat Due to Resistance


So your thinking of working on the electrical system of your car, but you're not so confident...or maybe it's been a while since you had that electrical chapter in your high school physics 101 course...a looong while, and you have to admit, you weren't so interested at the time, preferring to make time instead with Alice Poobats during class...not to worry, I was paying attention...(and made time with Alice after school, in private).

In response to requests from a few vintage Volvo owners who asked for a little help which would help them find their way through their electrical systems here is my electrical primer for the non-engineer, containing practical applications, not too many big words and only as much math as absolutely necessary. 

Before starting, let me start by assuring you of two things:

1.  With very few exceptions, the Voltage found in an automotive electrical system is not high enough to harm or kill a human being!  Even the high sparking voltages of the ignition system do not contain sufficient energy to harm a person...wake you up with a jolt yes, yeow!, and likely put you into what I call a "state-of-higher-awareness", but not injure (unless you have a pacemaker...and even these are protected against getting zapped...but you wouldn't want to do the test, would you?)!  We'll stay away from the ignition wires, since this is a course dealing with electricity and not the ignition anyway.

2.  Fuses are wonderful things!  They are electrical links which are sized based on a few factors and placed in a circuit to stop the circuit from passing way more current than what it is supposed to under normal conditions.  Unlike Hollywood might have you believe, cars do not just explode when they drive of the road (it just looks impressive on film!), and electrical things only (very rarely) explode in firework electrical sparks the way the dashboard of the Enterprise did, and still does just about once a week!  [Exception and preventative reminder note to self:  Do not drop wrench across battery terminals...ever!

Only in Hollywood!

First, some Definitions:  In order to speak about electrical things, and have someone appreciate what you're trying to tell them, a correct understanding of the terms is called for.  It is also important to use these terms properly when communicating about electrical issues. 

Voltage   - The force which causes electrical current to flow.  Sometimes also called EMF for Electromotive Force.  In vintage Volvos this is a mere 12Volts, with the 444 and older models using even half of that!  Measured in:  Volts

Voltage is always measured across two points.  That is, from one point, typically (but not always) the chassis of the vehicle, to some other point in question...and since the negative side of the battery is connected to the chassis with a heavy cable, these measurements are effectively to the battery negative. 

In the same way one can do a quick tongue check of a 9V transistor battery, you could conceivably also stick your tongue across a car battery and not be injured (and you'd have to have a pretty big tongue)...I also don't know why you would mind you (I recommend using a Voltmeter instead!)...the point is:  Don't let the size of the car battery scare you.  It's Voltage which makes electricity dangerous!  A battery's size analogous to the size of the bucket which is storing the electricity (hey, it IS a bucket storing electricity!)...that car battery could run your transistor radio for...oh...about two years...get it?

Current   - The measure of the amount of electricity flowing in a circuit.  Measured in:  Amps. 

Current is what does the actual work.  If no current flows, no work gets done!  Your transistor radio runs just fine on a tiny amount of starter on the other hand needs quite a lot's also doing a lot more! 

Resistance  As the name suggests, the opposition to flow of current.  Measured in:  Ohms. 

Researchers are very seriously working on it, and it will be one of those technological achievements right up there with credit-card-paid-for, home delivered, General Tsou's chicken, complete your own fortune cookie, but until they come up with "superconductors" all electrical conductors oppose the flow of electricity to a certain extent at "normal" temperatures, that is, not those of liquid nitrogen.             

How is this of interest to us?  ...well, it must be taken into account when deciding what size wire to use when we want to hook up a circuit.  A fat wire has lots of metal in which current can flow, and so because it has less resistance than a tiny little wire must be used when connecting up a "heavy user" of electricity.  For example, a starter has to do a lot more work than the courtesy lamp, that's why it is connected to the battery with a honkin' wire! 

A poor electrical connection in a circuit can be due to a wire of too small a gauge (poor design), or a connection which is loose or corroded (problems which can develop due to vibration or over time, especially where there is also a strong acid in use). 

Poor connections consist of high resistance where we don't want it.  These oppose or even totally prevent the flow of electricity when they open up altogether (like at corrosion, see Tech Article:  Gas-Tight-Joint).  An unintentionally good connection on the other hand is a low resistance connection where we don't want it...also known as a short circuit.  A short circuit allows current to flow where its not supposed to.

Most of the time, resistance is an unavoidable side effect in a circuit, and presuming the designer of the circuit has picked the wire gauges, connectors, and other details correctly, it can pretty much be ignored.  There is however one place where we intentionally place a poor conductor in the current path.  Its a fuse! 

Definition:  A fuse is an intentionally included sacrificial weak link current conductor, carefully sized (unless you're Lucas, in which case who the hell knows what guidelines they used for sizing...see below!) such that under normal circuit operation it will pass current just fine...however exceeding its current rating will cause it to heat up to a point where it will melt, opening the circuit, causing the current to stop flowing, thereby saving other conductors in the circuit from the same fate.  You've got to admit, that's pretty might be inclined to ask:  How come they didn't ever have any fuses in the circuits supplying power to the dashboard of the Starship Enterprise?  How come Kirk's dashboard exploded in the most spectacular way, almost weekly, and with showers of sparks?  Because it's Hollywood in action again, that's why!   (Reference Information:  How to size a Fuse)

Power   -   Measured in:  Watts.  Once the current actually starts doing some useful work, this is measured in Watts.  We can express the work a lamp does in Watts, heat in watts, we can also the work a motor does in Watts. 

Ohms Law is a mathematical relationship which tie the four previous items together using strictly algebra - nothing more painful, and allows us to understand the way an electrical circuit will act before we hook it up.  Ohms Law is very useful in that it allows us to figure out what size wire we need, because we can calculate how much current will flow in a circuit, or how much heat to expect, or even help us explain why a failed circuit is behaving as it is during troubleshooting.  In a new circuit, it is better to consider a circuit from its Ohms Law standpoint, rather than hooking it up and just trying it...electricity is lightning-fast and unforgiving of errors, but emphatically NOT unpredictable!

Ohms Law:  V(voltage) = I(current in Amps) x R(resistance)

...or after some algebraic manipulation (I'll spare you the mathematical Kung-Fu), the derived equation for Power:

P(Power in Watts) = V(voltage) x I(current in Amps)   

With this equation, we will be able to (sometimes it is useful to) calculate the amount of work being done in electrical terms. 

As a reference, one horsepower is equal to 746 Watts.  Dividing those 746Watts by the system voltage of 12V, give about 62Amps.  In other words, your 62Amp Alternator, when fully loaded, takes about one HP from the engine...actually a bit's not perfectly efficient in turning all of the mechanical energy from the motor into electricity.

Practical applications of Ohms Law: 

Problem:  Calculate the amount of current in the headlight circuit, and what size (wire gauge, or AWG) should be used for hooking up this circuit, and what size, if any, fuse should be used to protect this circuit. 

Answer:  Checking a 122 wiring diagram, we see a total of 2, 40Watt headlight lamps.  Using the Power equation solved for current, we see that the answer is: 80W/12V = 6.6Amps.   A 15A fuse (about twice the expected current as a rule of thumb) would be a good value if we were going to fuse this circuit, but since it is an important circuit, which he wouldn't want to do without unnecessarily (in the case of a nuisance blow blowing), designers typically accept a bit of risk, and headlights are not fused.

Proving Everything Bad you Ever Heard About Lucas (insert choking sounds here!) 

[This Tech Article is also available in German for my Volvoniac friends:  Beweis alles böses was mann über Lucas hört stimmt! ]

...they were either drunk and disorderly, OR intentionally or unintentionally incompetent and reckless OR just plain stupid, when they designed and specified the Electrical System for the P1800!  Besides the practical aspects and question of why a P1800 only has 3 fuses total to begin with(?!), and common knowledge of their components crumbling because of poor materials quality, Lucas also got the engineering wrong theoretically! 

In the following Wiring Diagram Excerpt of the carbureted 1800, the Marker (also "Parking") Light circuit is shown alone, and below, Ohms Law is used to prove another instance of the bad things we've heard about that company...I don't want it to seem like I'm jumping on the bandwagon, but if they went to the trouble of adding a Fuse on the Marker Light circuit, why would they specify a ridiculously high value which wont give any protection....what were they thinking??


Volvo 1800 Wiring Diagram Excerpt, showing only Marker Light Circuit, including an inappropriate 35A fuse
Note also that Lightswitch representation shown is oversimplified, but that is a separate issue, which does not affect information presented here! 
(See: )

Calculation of Total Wattage on this circuit:
                                                                           6W (Corner Marker Lamps) X 4           24W
                                                                           4W (License Plate Lamps) X 2         +   8W
                                                                           2W (Instrumentation Lamps) X 9      +  18W *
                                                                                                                         W Total   =   50W 

*  Note:  Maximum possible Wattage/Current is shown, and this occurs for Instrumentation Lamps when Variable Instrument Lighting Control is set to brightest setting.

Expected maximum steady state Current at 50W is therefore:   50W/12V = 4.17A

Multiplying that by 2 (typical factor for Incandescent Lamp circuits in automotive service to accommodate operating conditions and prevent nuisance fuse blowing) = 8.34A
...and applying the ("Use Nearest Standard Value") rule to this Current for the recommended Fuse Current value, that would be 8 or 10Amps.  See also:  Fusing Rules

So it can be seen that for Lucas to specify a 35A Fuse value, is about 4 times higher than it should be and just wrong (even if that was using the British Fuse rating, which specifies the opening current, not the maximum allowed steady state current value, see:,%20Allocation%20and%20Troubleshooting.htm#British_vs._American_Fuse_Rating ), and so this is highly questionable!  What is certain, is that having a fuse of such a high rating installed on a circuit wired with 18ga. wires (rated for 16A max in free air, and less in sleeving), an electrical overload would result in wiring damage before the fuse actually cleared the overcurrent fault! 

Besides confirming all of the bad things we've heard about Lucas with this objective, theoretical proof, the author therefore strongly recommends that all (carbureted) P1800 owners replace their Fuse 3 with one rated a much more appropriate 8 or 10A Type 3AG.   See also: 

Later (fuel-injected) 1800s were no longer assembled in Britain, so benefited from the later (non-Lucas) harness, with a more appropriate, multi-fuseblock, similar to those also used in the 140s, located in the driver's footwell, with more, and more reasonable sized fuses, so this does not apply to those vehicles. 

In studying the 1800 E, ES Fuse Allocation, Parking light circuits are split into Left (F11) and Right (F12) circuits, with 5A Fuses specified for each...a more reasonable value, and consistent with the author's finding and recommendation! 

See also:!.htm#Reference_Information


Heat Due to Resistance in the current path

Looking at the P = V x I equation again, we can see that any time we pass current through a conductor, and there is a voltage across that conductor we will have a power generated.  This is also called resistance heating, or I²R loss.  If we have a solid connection through wire and connections of adequate size, the resistance is so low, that we can pretty much call it zero, hence no (significant) voltage is across the connection and we have no (appreciable) heat generated. 

If however we continue to pass current through a conductor when that conductors' cross-sectional area has been diminished in some way (a nick in a wire, broken wire strands, or strands that didn't make it into a crimp connector, a loose screw on a terminal, a worn contact in a switch, or through years worth of corrosion), at some point, we do start generating a (significant) voltage across the conductor and the resulting heat at the poor connection.  See below!  

Front side.  This would normally be hidden under the terminals!

Back Side.  This would be hidden also, unless Fuseblock was demounted

Perfect proof!  A fuseblock returned to SwEm as a core, showing thermal damage caused by heat generated by hidden step: Fire in the Hole!  I bet this heating would have shown up on an Infra-Red scan!  See also Link:




Getting the most out of your $29 Multimeter. 

Measuring Volts 

A voltmeter will measure voltage across an open fuse, since the open fuse will act exactly like an open switch (that's in fact what it I've-detected-an-overcurrent-so-I've-sacrificed-myself-and-opened-the-circuit, switch!).

Measuring Resistance  You can also use your Multimeter set to measure resistance to check is a fuse or lenght of wire are intact.  The one thing to remember is:  If the meter is set to check resistance, it will not take kindly to having a voltage applied across its leads. 

A brief and very good video on a multimeter: 


Water Analogies: 


A circuit must consists of at least the following three things:  Power source, control, and a load.  And even though without the wire to connect them together, nothing would work, the wire itself doesn't even get a separate billing...wire connections are presumed...or else we would just have a box full of parts and pieces.  A fuse, additional switches, power sources and multiple loads just complicate things...but when searching for a trouble cause, it is always advantages to "boil everything down" to the three things. 


What is a relay? 

If a relay is rated for 30Amps, it does NOT mean that the circuit it is hooked into will draw 30A!  That would be incorrectly presumptuous! Recall that the amount of current is determined by the circuit numbers plugged into Ohms law, not what is printed on the side of the relay!  The rating on the relay simply refers to the amount of current the relay is CAPABLE of safely passing.  The actual magnitude of current which will be passing will ALWAYS be determined by Ohms law...that is I (Current) = V (Voltage) / R(Resistance)

Practical example:  Installing fog lights - why do we need a relay? Because a relay has a much bigger contact than the dainty little control switch, and is therefore better suited for carrying the significant current it takes to fire up those lights. 

A few numbers and calculations...typical driving lights are rated at 55Watts each.  Using the I=P/V, this works out to a total of about 9Amps of current.  Looking on the side of our little switch, we might find that it is rated only would not last long, probably heat up, and shortly fail.  It's time to use a relay...we will use the switch to control the comparatively miniscule current required to close a beefy relay contact rated to easily carry as much as 30Amps, but in this case will carry just those 9Amps...easy!        

Some common problems explained: 

A lamp is off when it should be on.  If we can remove the lamp and inspect the filament through clear glass, great.  Sometimes though, we might find, that the filament seems to look intact...time to press that trusty little multi-meter into service again and check the actual resistance.  If the meter finds that there is no connection (at all) there may very well be a tiny, invisible open circuit in the filament, but if the meter shows the circuit has a (low) resistance when measured across the filament, its probably just time to look for and clean away any white or green corrosion at the connection points.  


Why is the wire (or connector) hot?...because it is passing more current than its resistance will safely allow!  If this is an original wire, maybe some strands have broken from vibration (making it effectively a smaller wire), or someone has added another load to the circuit (like more lights), increasing the current.  If it is a connector, maybe it has loosened over time and some "green stuff" has gotten between the contacts increasing the in-line resistance.  In both cases when current flows, more heat then normal is generated across the increased resistance in the path...better check it out, and decrease the resistance (or current) before the connector does its impression of a fuse, heating up and melting open!

An an example, see:  TechBulletin3.htm #fusebox fire! 

See also:  I2R or Resistance Heating above!


Your following a car at night whose taillights are on.  As the driver steps on the brake, the brake lights come on on both sides, but the parking light goes out on one side.  What gives?  Is the driver tying to impress you or police with a light show?  Is the car haunted or possessed?  This is an interesting slightly complicated failure likely involving the chassis connection of the parking light/brake light on the side which is exhibiting the strange flashing. 

See also:  Voltage Drops Cause Inadvertent Circuit Interactions


Grounding or earthing or return or chassis...the terms can pretty much be used interchangeably when it comes to automotive electrical systems.

Grounding or earthing does not mean you should drive a stake into the earth where you park your car, and connect the car to it.  This term is a holdover from house electrical wiring where this IS done to give lightning a conductive path to the earth and also prevent a voltage potential from forming on metal objects.  

All mean the same thing:  Having an electrically conductive path to the negative side of the vehicle’s battery.

In some cases there may be actual negative side wiring, but in many cases (i.e. lighting assemblies, horns for loads [for outputs], and oil pressure, fuel senders [for inputs]), a short wire is attached to the electrically conductive sheetmetal of the vehicle, and at the front of the vehicle a short wire connects the negative side of the battery to the sheetmetal of the vehicle to complete the path.  Result:  15 feet of wire saved!  …multiply that times the number of loads and sensors in just one vehicle, then times the number of vehicles produced, it starts to make sense for the huge savings which a manufacturer can realize.


[From]... in response to fuse blowing questions.  Link to Thread: "fuse-blowing woes"


(Normal) Fuse rating is 8A. Have additional loads been hooked up to this fuse?

If that fuse blows as described, it could be difficult to locate the problem, because the conditions which cause it to blow may be intermittent...the best thing in that case is to make mental notes of ALL conditions preceding the blown fuse, so that its' blowing can (hopefully) be associated with something you can check once you get home...even something as seemingly unassociated may play a role...For instance: Like braking hard while turning left - [a wire with compromised insulation touches the chassis, or is touched by the tire iron in the trunk etc.]

Yes there is a "reversing light relay"...located on the (distributor side of engine) inner fender...but intermittent failures of these relays are rare. If you really get stuck, disconnect Brown wire (at fuseblock right side terminals of Fuse2, determined by process of illumination) supplying power, if problem does not reoccur for a few weeks, that sure suggests it was on that load circuit, and gives you more (slightly) specific things to investigate...

Other hints:

Using an Ampmeter, connected in series with one load (horn, reverse light, brake light) at a time (Brown wires on right side terminals of Fuse2), and measure their current as a reference...

Horns, which are under constant moisture attack, and are only occasionally exercised, can draw more current with age.

The manner in which a fuse blows, also gives information...completely vaporized indicates a very high fault current or "dead short" open fuse with much of its' element intact, just a tiny section "blown open" suggests a mild, sustained overcurrent...sometimes this can be useful in locating the fault...but the most useful thing is correlating it with a particular load, or some action...Amazon electrical systems are wonderfully simple, so it hopefully wont take too long to locate and correct the fault, and restore reliable operation.

Good Hunting...please let us know how you make out.


Reference Information:

How to size a Fuse (from 1965, so must be old, outdated information right...wrong!):


External material sources are attributed.  Otherwise, this article is Copyright © 2002-2012.  Ronald Kwas.   The term Volvo and Lucas are used for reference only.  I am not affiliated with either company 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, including giving Lucas all the grief they earned (see above!).  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! 

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.


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