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Skeleton Potentiometer

Discussion in 'Troubleshooting and Repair' started by stormtigers, Jan 15, 2013.

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  1. stormtigers


    Jan 15, 2013
    Hi Folks,
    This is a car related problem, specifically Mercedes Benz circa 1980/90's.
    MB had a circuit board made by VDO Siemmens for the W124 cars (and other series).
    There is one item that is cursed the world over and that is the fuel gauge.

    The sender in the tank is a "tube type" which essentially is a metal rod with a float and two resistance wires on the outside. As the float rises or falls the amount of resistance being sent to the gauge up front varies.
    This variations are
    3.3+/- 0.7 ohm when the tank is full
    85.6+/-2.5 ohm when the tank is empty (also activates a low fuel warning light at certain level)

    The problem is over time the damn potentiometer loses the conductiveness ? of the surface that the wiper travels across and consequently you end up with a fuel gauge that does the cha-cha.

    There is some suggestion that stabilant-22 can be used to restore some of the "performance" but that product is almost exclusively sold int he USA now and is grossly overpriced when all you need is a few drops.
    Below is a picture of the errant piece - I can't see any part numbers on it.

    So my questions are is to all you learned people,
    1. Does anyone know where such a product can still be sourced because VDO don;t make them anymore OR
    2. Can someone suggest how the use of two 0.5W resistors of varying rating could be used as a substitute as one guy in Europe suggested. However he said he used 68 & 82 ohm fixed resistors OR
    3. Suggest a modern substitute that could be easily impemented.

    This is a world wide problem for the W124 Car community so any help will be gratefully acknowledged and attributed to you

  2. pwdixon


    Oct 14, 2012
    Unless I've misread this completely this looks like a pretty standard preset pot, I would imagine almost any pot of similar resistance would do the job and nowerdays you can get multi-turn precision sealed pots pretty cheap just about anywhere.

    You will need to remove the original pot and measure the resistance across the non-wiper teminals unless the part has the resistance value written on the other side.

    You will need some idea of the power to rate the pot correctly but hopefully you can work that out from the battery voltage and the pot resistance but I wouldn't expect that to be too much of an issue though you do say someone else used 0.5W fixed resistors which would be right if the pot adds up to 150ohms and the battery is 12V.

    Calibration after the replacement would be by forcing the fuel sensor end to end while setting the pot I guess.
  3. duke37


    Jan 9, 2011
    If it sometimes works, try a squirt with some switch cleaner.
  4. stormtigers


    Jan 15, 2013

    Thanks for the reply.
    Please excuse my totally ignorance on such issues. If I may pose a couple of questions..

    As mentioned the "MB manual" defines the upper limit as being
    3.3+/- 0.7 ohm when the tank is full
    85.6+/-2.5 ohm when the tank is empty

    How then does a fixed resistance component which totals 150 ohms suit this application.
    The way he has connected them they share a common solder joint.
    The other end of each then goes to one of the two other solder points.
    If I am understanding correctly :-
    1. the common solder point is the resistance signal from sensor in the tank
    2. the 82 ohm resistor is watching for the higher resistance reading and when reached activates the fuel low warning light
    3. the 68 ohm resister is adjusting the fuel gauge needle based on the amount of resistance from the sender in the tank
    4. does a fixed resistor allow current to pass through hence adjusting the needle until such time as it's load limit if reached at which point it no longer passes current through ?

    This may be all very basic info to a lot of you guys but I am a total novice so please excuse my suppostions.

    If I have understood it sorrectly then it means the "guy who did this mod" has measured the resistance at a point where he believed the fuel tank only had "X amount of fuel" remaining and has set it up so that when "X" is reached he will start getting a low fuel warning light (resistance signal >68 ohms).

    Now if I have it right then X would appear to be in the region of a just under a quarter full tank (supposing that 3.3 +/- is a full tank)

    Would fixed resistors work then such that once the 68 ohm's is achieved and passed (i.e low fuel) that resistor stops working and the other resistor continues adjusting the needle until such time as the resistance reaches 82 ohm at which point the warning light will come on or does it start the instant 68 ohm is passed ?

    What I am having trouble coming to grips with is how combing two fixed resistors can be made to do the job of the trim pot ?
    Perhaps it's that side more than anything that is confusing.
  5. stormtigers


    Jan 15, 2013
    Maybe the easiest solution - any particular product you can suggest ?
    I have read about lubricants and I'm a little worried that if a film is left on the surface it will further inhibit the conductiveness of the pot ??
  6. duke37


    Jan 9, 2011
    I have used Maplin switch cleaner, others recommend Servisol.

    The skeleton pot is likely used for calibration and could be replaced by two fixed resistors of the correct value. I presume that if the readings are unreliable then this is due to the moving potentiometer controlled by the fuel level.

    Have you managed to deduce the circuit?
  7. stormtigers


    Jan 15, 2013
    I hate to admit it but I haven't even worked out how to use a multimeter properly yet so I am very much flying blind.
    Below is the excerpt from the MB manual on this issue.
    Maybe the diagram along with the specifications I listed in my OP might help somewone decipher what I have said ?
    Last edited: Jan 15, 2013
  8. stormtigers


    Jan 15, 2013
    I hear you.
    But it's not the sender giving us issues its the potentiometer which has lost it's ability to register the resistance signal. I have replaced the sender already and the output is fine.
    It's the damned little junky thing at the back of a PCB in the instrument cluster (see picture in the original post).
    The problem with sourcing another is they yards only sell the full cluster for anywhere between $80-$100 which irks me because
    (a) you are unable to determine whether it too had the problem which is very common
    (b) it's a $5 electronic component with 5 minutes soldering time !

    Hence my attempt to understand what I could buy from the local electronics store to fix the issue !
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi Ray. Let's start from the beginning.

    Correct me if I'm wrong on any of these. Your fuel gauge (and the related low fuel light) are erratic because of intermittent behaviour of the trimpot you've shown in the photo. The tank level is sensed by a tubular sender with a float and resistance wire, which provides a variable resistance to some circuitry in the dashboard. This circuitry is where the trimpot is located. The problem is with the trimpot, not the sender. You want to replace the trimpot with a suitable new part, or possibly with two resistors of the appropriate values to simulate the trimpot being set at the right setting. The trimpot setting is used to calibrate the fuel indicator so that it indicates full and empty when the tank is actually full and empty.

    Assuming I've got all that right, here is some more basic information.

    The trimpot's resistance does not _necessarily_ relate directly to the resistance of the sender in the fuel tank. You need to find out the resistance of the trimpot.

    A trimpot has three terminals. The two end terminals connect to the ends of a "track", made from carbon, resistance wire, or something similar. The middle terminal connects to the "wiper", which makes contact with the track at a position that is set by rotating the middle part of the trimpot.

    The important specification for a trimpot is the end-to-end resistance, that is, the resistance of the whole track. You measure this with a multimeter connected to the two end pins and set for resistance (the Omega symbol). Usually though, you cannot measure this resistance with the trimpot in-circuit; you need to remove it from the circuit board, otherwise other components on the circuit board will affect the reading.

    Are you able to do that? You'll need a soldering iron and some pliers so you can pull it out of the board while the solder on the pins is melted. Once you've done that, you can measure its resistance.

    In this case, it appears that this trimpot may be a high power type. If that's the case, if you replace it with a low power trimpot, the new trimpot may get too hot and fail. You may be able to buy a high power replacement trimpot though. I suggest you take photos of the original trimpot from different angles and include a ruler in the photo so we can gauge how big it is. We should be able to guess whether it's a high power trimpot or not.

    The best option would be to replace the trimpot with a suitable new trimpot. To do this, we need to choose a replacement that will physically fit into the old position, so we need a picture of the holes that the trimpot terminals fit into, preferably also with a ruler in the shot.

    It may work out best to replace the trimpot with resistors. A trimpot at a fixed position can be replaced with two resistors; one takes the place of the track from one end to the wiper position, and the other takes the place of the track from the wiper to the other end. However, sometimes only one end of the track is connected; in this case, a single resistor is enough to replace the trimpot. Can you tell which pins of the trimpot are used by looking at the copper tracks on the circuit board that it's soldered into? That might make things simpler.

    You should also be careful not to change the setting of the trimpot as you remove it. You should be able to measure the resistance between one end and the wiper, and between the wiper and the other end, to determine the setting of the trimpot and the correct resistor values to replace it with, if you decide to do that. You should also find that the two resistance values you measure that way will add up to the total track resistance you measured. If they don't, that could be because the wiper isn't making proper contact with the track, because of failure of the trimpot as you mentioned. You might have to estimate the resistances from the position of the wiper, assuming you can see it clearly.

    But your first option should be to replace the trimpot with another one.

    I hope this all makes sense.
  10. stormtigers


    Jan 15, 2013
    First off thanks everyone for the suggestion.
    KrisBlueNZ has nailed it exactly as the problem/issue.

    Absolutely spot on ! Sure you don't own a vintage Mercedes ?

    Nothing revealed on this image.

    Nothing revealed on this image really .

    I think if the functionality is retained then "the fixed resistor option" is the easier option and incidentally is the one option that has been photo documented (see below).
    As I mentioned earlier it is this method and my total lack of electronic knowledge that made me ask whether this is a suitable route to take. I assume they used a variable potentiometer it was so they could "fine tune" the specifics of the vehicle/fuel tank combination ? Otherwise why not used fixed resistors.


    I think we can get some good info from this modded gauge image.
    The guy who did this asked all to bear in mind that this is from a 50 litre tank on a Mercedes 190.
    I suspect the specs are the same for his potentiometer as they are on the 70 litre tank in the Mercedes 300CE (based on what resistors he has used and the specs from the MB page I posted).
    I don't think fuel capacity has any relevance unless the resistance figures expected are different between the cars.

    He has on the right an 82 or 83 ohm resistor and on the left a 68 ohm resistor if my color reading is correct !
    These have been co-joined I believe into the spot where the "wiper" leg was connected.
    And to the left position the 68 ohm and the right the 82/83 ohm.

    Regarding the measurements the diagram shows checking the readings at the "main artery" power plug that goes into the instrument cluster.
    That doesn't measure the trimpot BUT it does give the readings being received from the other end.
    As I am leaning towards the fixed resistor solution if you endorse that it will substitute properly, what I will do is take the sender from the tank (empty=float down) take a reading and then invert the sender (full-float up) and take a reading to see what my car is sending through.

    Will that be beneficial in determining the correctness of the resistors to be used ? :confused:

    And might I add I cannot thank you enough for taking the time out tohelp me with this. It is a most infuriating problem and paying $150 to a instrument cluster repair shop just irks me no end when the components are sub $10 !!!!!! :eek:
    Last edited: Jan 16, 2013
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Nice pictures Ray, but as you said, they don't reveal much, except that the instrument cluster is beautifully made and in very good condition.

    OK, so you want to go for the fixed resistor option. That should be OK. Personally I would have tried to replace the trimpot first, and only use the fixed resistor option as a last resort.

    Yes, the trimpot is provided so you can calibrate the fuel gauge, taking into account any variation in the resistance of the sender and the fuel tank size.

    Can you take a picture of the OTHER side of the board that the trimpot mounts onto? I would like to see whether there are copper tracks running to all three trimpot pins. If only two pins are used, you will only need one resistor.

    The easiest way to determine the resistor values is to measure the trimpot resistances. I assume the trimpot is set to give the correct display on the fuel gauge?

    Can you remove the trimpot using a soldering iron? Heat up the connections on the back of the board, and pull the trimpot out using pliers. This will be a bit fiddly. You will need to try to melt the solder on all three pins simultaneously, by moving the soldering iron quickly between them. Or you may be able to wiggle the trimpot from side to side, by heating one or two pins, pulling the trimpot out on one side, then heating the other pins, and pulling it the other way, until it eventually comes free.

    Once you've got the trimpot out, you need to measure resistance between the wiper (middle pin) and each of the end pins, and also resistance between the two end pins. The first two resistances you measure should add up to the end-to-end resistance, if the wiper is making proper contact.

    Then you need to fit two fixed resistors as shown in that photo. That is, commoned ("co-joined", you said) at the wiper connection.

    The values of these resistors should match the resistances you measured on the trimpot, between the wiper and the two ends. Resistors aren't available in all possible values though; you'll need to find the closest "preferred" values. 68 ohms and 82 ohms are both preferred values, but they may not be right for your case.

    The best way to determine the right resistances to use is to remove and measure the trimpot. (Assuming it is set correctly to give a full tank indication when the tank is full.)
  12. nepow


    Jul 18, 2011
    I can't see this either... basically a pot is a mechanical device of which it's resistance is proportional to it's rotation linked to the float sensor. Unless I'm missing the point here!! Peraps a rotary switch with several ways is used, in which case each switching position can then be set up by indiviidual resistors in series. Switch cleaner has already been mentioned, WD40 also works well unless the carbon track is worn. A mechanical single pole multi way rotary switch is a possibility perhaps, providing the position selector tensioner is removed to allow free travel. Just a thought Regards Nepow
  13. davenn

    davenn Moderator

    Sep 5, 2009
    yup you are missing the point,
    if you read through the thread ... the pot being referred to is the trimpot on the board
    NOT the float sensor ( which may or may not be some sort of pot)

    As Kris has been saying several times to stormtigers....

    REMOVE the trimpot CAREFULLY dont wreck it else you wont be able to do resistance measurements on it
    Choice 1 .... measure the full value of the trimpot end terminal to end terminal
    ... say it measures 220 k Ohms (220,000 Ohms) Plus or minus a few a couple of k Ohms for tolerance .... Go buy another 220 k Ohm pot

    Choice 2 .... measure from wiper to each end, as Kris has said and select 2 resistors that as closely as possible equal those 2 values

    Replaceing the trimpot with another is definately the easiest way

    This is not difficult ... its a 5 minute job ... seriously it should have been all done and completed by now ;)

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