Fuel Gauge Information and Notes
first published: Apr 2015  R. Kwas  Revisions On-Going

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Fuel Sender and Gauge Electrical Calculations
Failure Conditions and Troubleshooting 
Sender Information
Fuel Gauge Generosity or Non-Linearity Considered
Reference Data, Drehspulinstrument 

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The Fuel Gauge is an important indicator, giving the driver critical information.  The system Volvo used is simple, accurate, repeatable, and reliable, based on dashboard Gauge and tank Sending Unit, both supplied by VDO.  The information presented here applies, with model variations, to 6V or 12V versions of the 444, 544 and 122 models.  I have not studied the Smith's supplied Fuel Gauge as fitted in the 1800 models, but electrically, I expect the function is similar.    


122 Fuel Gauge showing gray 82 Ohm Dropping Resistor used in 12V service.

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Gauge Response:  Vintage Volvo Fuel Gauges do have some personality which one needs to get used to...

Non-linearity.  Non-linearity of indication is quite common...mine indicates Full, Full, Full, Full, Half, Half, Quarter, Empty.  This is caused by the geometry of the Float Pivot, and it's lever back to the resistive element of the Sender, with respect to surface level of fuel...after running out of fuel a couple of times and recognizing this non-linearity as the reason (in addition to the fact that I am a male), I just got used to it!  I suppose one could do a little scientific test, and start bending the float support wire to vary this geometry and try to linearize the indication...then again, maybe it is as good as its going to get, and I have more important things to do...! 

A certain non-linearity is typical, because of the geometry of the float in the tank.  However, because of the inverted relationship of the Sensor resistance to Gauge indication, if a poor connection occurs for any reason from Sensor to Chassis, this will result in the Gauge becoming "generous" (indicating more fuel than is actually in the tank), and the non-linearity becoming worse...when and if this connection eventually completely opens, Gauge will show Full...more on this below in Failure Conditions and Troubleshooting

Fuel Gauge is not dampened.  Unlike modern gauges which have a computer between the Sender and Gauge, giving the indicator needle lots of filtering, and a looooooooong time constant, the vintage Volvo gauge has an instantaneous response!  The indicating instrument is directly connected to the Sender, with no computer in sight, so it will instantly indicate whatever the tank float happens to be doing, including bouncing around on curves or while braking/accelerating, along with the sloshing surface of fuel in the tank (tanks do have internal baffling to minimize sloshing and transfer, but some still occurs and it will certainly be apparent at Gauge).  A Fuel Gauge needle with a slight case of Parkinson's Disease therefore does not indicate a problem (a bouncing needle which baps the FULL stop on the other hand might, *see Failure Conditions and Troubleshooting below!)...it is up to driver to get used to this action of the Gauge, and integrate it into a long-term average representative of the actual tank level...another thing you just get used to! 

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Theory of Operation:  Fuel Level indication is by way of a Moving Coil, Balanced Magnet indicator (see Reference Data, Drehspulinstrument) directly connected to a Sensor(Sender) with approximately 10 ohms at empty and 180 ohms at full.  Fuel Tank Indicating system is active when Ignition Power is ON and is on the Fuse1 circuit, as can be seen in this detailed wiring diagram extract. 


Fuel Gauge circuit, 12V showing circuit details. 

Fuel Sender and Gauge Electrical Calculations:  

Full Tank:

I tot = 12 / R tot = 12 / (9.68 + 142) 151.6Ohms = 79.2mA
V sens = I tot x R sens = .0792 x 9.68Ohms = 0.766V
I sens = V sens / R sens = 0.766 / 10 = 0.0766A = 76.6mA

Empty Tank:

I tot = 12 / R tot = 12 / (112.5 + 142) 254.5 = 47.2mA
V sens = I tot x R sens = 0.0472 x 180 = 5.3V
I sens = V sens / R sens = 5.3 / 180 = 0.0294A = 29.4mA

Maximum Values: 

Maximum Sensor Voltage occurs at Empty Tank condition:  5.3V  
Maximum Sensor Current occurs at Full tank condition:  76.6mA

With both Voltage and Current at the Fuel Sender being highly limited to very low levels, there exists no danger of spark ignition. 

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Failure Conditions and Troubleshooting: 

Simple and reliable as the fuel level indicating system is, they do occasionally develop problems, resulting in the gauge not working as expected.  Most common problems involve loss of connection in the resistive path through Sender and finally to chassis, and because of the manner in which the indicating instrument functions, any open in the sender connection will result in a FULL indication...not intuitive, but that's the way it is! 

Typical causes of OPEN Sender Connections which will result in a FULL Gauge Indication: 
 (Gauge Needle quickly jumps to FULL, when Ignition is powered ON) 

1.  Loose or poor connection, at G terminal of Gauge or Sender terminal.
2.  Loose or poor connection, including intermittent, of Wiper contact to resistive element within the Sensor back at the tank (* an intermittent connection, for instance as Sender wiper sweeps across an oxidized or gummed-up area of the resistance element, would cause the pointer to strike the FULL peg!).
3.  Loose or poor connection of Sender to vehicle chassis at the tank itself (tank is bedded with nonconductive putty, and secured with many screws, but which can all eventually loose their good connections).  

Note therefore that before the chassis connection by way of the Tank securing screws opens up completely, the deteriorating connection will typically first get slowly worse and worse.  Since this connection resistance is effectively in-line with resistance of the Sender, this will manifest itself as indicating more fuel than one really has ("generous" Fuel Gauge!), and in an increased non-linearity of the Gauge.  See also Concept of Fuel Sender Function graphic below!   So if it seems that your Fuel Gauge Indication is a bit optimistic, or getting more and more non-linear, you might try removing a few tank securing screws, cleaning them and their contact areas to shiny metal, and replacing them with a dab of ACZP and an internal star lockwasher!  [Comment:  You might even use this as an excuse to your wife for having run out of fuel...]

Typical causes of Other Problems which will result in an EMPTY Gauge Indication
(Gauge Needle doesn't move, when Ignition is powered ON)

1.  Loss of power to the Fuel Gauge Circuit (such as an open Fuse 1).
2.  Loss of power to Fuel Gauge due to loose (or corroded) connection to + terminal of Gauge. 
3.  Loss of chassis connection provided by Instrument cluster (central Gauge mounting screws connect a ground strap of the gauge to the metal instrument cluster which is connected to chassis , see below). 


Fuel Gauge rear view, three connection identifiers at Yellow,
A chassis connection strap runs from Gauge connection to one of its securing screws at Red. 

[Reference Picture]  Note surface corrosion on copper strap (at Red) making its way from edge to the hole at center, where nut is supposed to provide good chassis contact for the gauge.  Corrosion will continue and eventually, this connection will open resulting in symptoms of an open gauge (not open Sensor!).  It is likely that strap under the central nut is in a similar condition.  I'll allow only one guess as to what author's recommendation would be for both locations...that's right, cleaning to shiny metal and application of ACZP

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Sender Information:

Normal Sender Function:


 

When installed, resistance of the sender unit is typically ~10 ohms empty, to ~180 ohms full . The resistance should sweep smoothly and without dead spots as the float arm is moved .

Here are excerpts from an e-mail exchange including a quick conceptual graphic I put together:

Robert A.:  "My fuel gauge works but in reverse.  I took out the fuel sender  (have new tank and sender).  When the float is in down position (empty), gauge shows full.  When float is up (tank full) gauge shows empty."

My Response:  Inverted fuel Gauge action is intriguing...I have not heard of this before, so I am not reminded of any previous experiences.  I expect the Sender resistance is opposite from what it should be (correct sender?)  I suggest you divide and conquer...that is, connect a 200 Ohm potentiometer to simulate sender, apply 10 and 200 Ohms to chassis...Gauge should read empty and full respectively proving gauge is working correctly, and again pointing to Sender.  See also:  http://www.sw-em.com/Fuel%20Gauge%20Notes.htm

Actually, now that I think about it, I have an idea, and it (still) points to Sender!  Is Sender a "universal replacement" possibly connected wrong?  A potentiometer as you know has two ends and a Slider...and the Slider (Chassis Connection) contacts the resistance wire, and is connected to the Float by way of a pivoting rod...so IF the wrong end of the resistance was connected to the Gauge, resistance action would be the exact opposite with Float positions, and opposite reading would result.

Below is a graphic showing correct operation which should make this clear.

...but IF connection to gauge is made at the "wrong" end of resistance wire, opposite resistance action would occur and opposite action would result at gauge! 

Actually Sender mounted "upside-down" (if that's possible) might also give such a result! 


Fuel Sender Function.  Correct action is shown in Black.  Possible explanation for "inverted" gauge action
is shown in Red, since gauge display is a function of resistance to chassis.  Also shown is effective series resistance in Blue,
of poor Sender-to-Tank, or Tank-to-Chassis connection, which will result in a "generous" Gauge and increased Gauge non-linearity.

Fuel Gauge Generosity or Non-Linearity Considered

Sender is electrically connected to Fuel Tank which is electrically connected to Chassis with a direct connections by way of their mounting hardware...or is supposed to be!  If any of these connections are less than perfect, the resulting Resistance is effectively added to the Sender Resistance as seen in Blue above, and this results in a higher indication at the Gauge.  If gauge is generous, or exhibits extraordinary non-linearity, it may be due to this Resistance being added to the actual Sender Resistance.  The Gauge certainly has no way of knowing the difference between intended Resistance of the Sender, and unintended Resistance at the mounting of Sender or Tank!  Sender and Tank mounting screws should be removed, cleaned and replaced with a dab of ACZP...and an external star lockwasher on one or two wouldn't hurt to make this repair permanent! 

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Additional: 

Replacement Sending Units:

VDO.  PN 226001  Last reported cost is less than $30 from Summit Racing.

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A reasonably priced equivalent part is also available from NAPA:  

http://www.volvoforums.org.uk/showthread.php?t=233932

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(Extract from the Tech Article:  6V to 12V conversion )

The Fuel Gauge of 444/544/122 vehicles consists of an indicator based on a VDO supplied precision "balanced magnetic indicator movement" (see below), and a variable resistance Sending unit.  The "balanced magnetic indicator movement" is highly accurate and quite insensitive to variations in supply voltage, a very good design and execution by VDO.  

Here is circuit of what it looks like when an 82 Ohm Dropping Resistor (RV = Vorwiederstand) as recommended in the data sheet, is added for 12V service:


Gauge internal details, including results when checking gauge with an Ohm meter.

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PLACEHOLDER for comparison of pointer position with 12V with 82Ohm dropping resistor to 6V without.

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Links:

Brickboard Thread:  Fuel gauge erratic movement 120-130  https://www.brickboard.com/RWD/index.htm?id=1520929&show_all=1

Sweedspeed Forum, Thread:  Fuel Gauge not working - fixed:  http://forums.swedespeed.com/showthread.php?173692-Fuel-Gauge-not-working-fixed

Brickboard Thread about 445 Fuel Gauge:  https://www.brickboard.com/RWD/index.htm?id=978149&show_all=1

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Reference Data:  

VDO's Component Data and Application sheet for the Drehspulinstrument (Moving Coil Indicator, originally posted on the German Volvoniacs Forum, my translation):

Translation (R. Kwas 2015):

1.  Pointer
2.  Disc
3.  Damping Chamber
4.  Face
5.  Base
6.  Circuit Board
7.  Axle
8.  Coil , upper part
9.  Coils
10. Movable Magnet
11.  Tracking plate
12.  Socket
13.  Limiting Pin
14.  Coilform, lower part
15.  Zeroing magnet
16.  Compensation Resistor
17.  Dropping Resistor
18.  Hollow Rivet
19.  Shielded Housing
Translation (R. Kwas 2015): 
(Anwendung) Application 

Gauge for electrical indication when used in combination with an electrical sender.

(Einbaumaβe) Installation Dimensions

Steckbuchsen = Socket

(Wirkungsweise) Theory of Operation

Measured value is converted to resistance value by way of a Sender. To indicate, a moving magnet multiplying mechanism is used in conjunction with a Face appropriate for the measuring function.  Indicated value is independent of supply voltage.

(Zu Abbildung 1)  Ref Drawing 1.

The gauge consists of three perpendicularly wound stationary coils, and one pivoting coil located on an Axle within these, and equipped with a Permanent Magnet and indicator Pointer.  Number of turns and Winding orientation varies for particular application. 

1.  Pointer
2.  Axle
3.  Permanent Magnet
4.  Coil, upper part
5.  Coil, lower part
6.  Beginning, Measurement Branch 1
7.  End, Measurement Branch 1
8.  Beginning, Measurement Branch 2
9.  End, Measurement Branch 2

Translation (R. Kwas 2015): 

(Zu Abbildung 2)  Ref Drawing 2. 

The three Coils are arranged in two Branches, where Coil III in Branch 1 and and Coil II are oriented at a right angle to Coil I of Branch 2.  Because of the differing orientation of the Coils, the effective magnetic  fields of the two Branches are in opposing directions and combine to give a Resultant field (see Vector Diagram) which has a Quotient relationship between the two fields.  What controls the Relationship of the fields is the splitting of the Current in Branches 1 and 2.  The splitting of the current is determined by the Sensor Resistance (RG) which is in parallel with Branch 2.          

(Zu Abbildung 3)  Ref Drawing 3. 

The Vector diagram shows the magnetic fields of Branch 1and 2 as a vector with  beginning value (RG = 0), ending value (RG = max.), and any possible intermediate value of the Sensor Resistor RG.  In this manner, the Vector of Branch 2 the Resultant field of magnetic fields of Coils I and II.  At RG = 0 (beginning value), no current flows in Branch 2, such that vector VA2 becomes zero in this case.       

With an increasing Sensor resistance RG, the current in Branch 2 increases, and with it, vector V2, which reaches its limit of VE2 with a maximum sensor resistance value.  Simultaneously, current in Branch 1 decreases, and vector V1 returns to its beginning value of VE1.

Vector VR, determining the orientation of the Permanent Magnet sweeps 90º, such that the Pointer, fastened to the Permanent Magnet, also sweeps 90º.  When removing power from the instrument, the weak Zeroing Magnet overwhelms the Resultant field and thereby returns the Permanent Magnet to the zero position from anywhere along its sweep.

Translation (R. Kwas 2015): 

VA1 = Beginning Vector for  RG = 0   Measurement Branch 1
VAR = Resultant Vector RG = 0
V1 = Intermediate Vector Measurement Branch 1
VR = Resultant Intermediate Vector
V2 = Intermediate Vector Measurement Branch 2
VE1 = Resultant Vector for RG = max   Measurement Branch 1
VER = Resultant Vector RG = max
VE2 = Final Resultant Vector RG = max   Measurement Branch 2
β = 90º Angle of Indication

(Zu Abbildung 4)  Ref Drawing 4. 

Since the temperature caused resistance value change of copper wire results in variation of resistance of the measuring coils, a small error occurs as a result.  This error is compensated for by a Resistor in parallel with Coil III.  When using instrument at higher supply voltages, a dropping Resistor RV can be added in the indicator supply line.

RG = Sender Resistance

RT = Temperature Compensation Resistor

RV = Dropping Resistor

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External material sources are attributed.  Otherwise, this article is Copyright © 2001-2015.  Ronald Kwas.The terms Volvo, VDO and NAPA are used for reference only.  I have no affiliation with any of these companies other than to present my experience, and highly opinionated results of the use and care of their products, for the purpose of helping other owners keep their vehicles on the road, safe and reliable.  The information presented is my own, 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…so The New York Times 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 maybe wise-a** comment. 

 

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