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Discussion Starter #1
I've seen enough topics on this to start a thread about a common problem many forum members have experienced:

Dancing fuel gauge.

About 8 times out of 10 it can be attributed to a faulty fuel sender which you can clean or replace quite easily but for those times its not the fuel sender the only other component it can be is the fuel gauge itself.

TYPICAL SYMPTOMS:

1. Fuel gauge dances from 1/2 to empty but not when near full

2. Fuel sender replaced does not address problem

This was my experience and I ended up sending it to Palo Alto Speedo so they could remedy the situation. Turns out a small trimming pot that is part of the cluster is the culprit and all Palo Alto does is replace the pot with one of identical value. The manufacturing tolerances on these pots results in some age-related slop between the wipers that only manifests itself as a problem when very little current (1/2 full to empty) is travelling through the wipers. When the tank is full the fuel gauge is recieving enough current to overcome the slop in the wipers. The problem is VDO no longer stocks these pots and you have to get creative in obtaining them (you probably have to write VDO for the pot parameters as well since I don't think its in Palo Alto Speedo's interests to give that information out for free).

There is another alternative... alot cheaper than sending your cluster in for repair.

There is a product available on the market called Stabilant 22. Its a special block polymer that is normally non-conductive except in the presence of an electric field. OEMs have been using this for years on autos to enhance the stability of electrical contacts since you can't make the area surrounding the contact conductive. Its the next best thing to having a selective hard solder joint... that you can easily reverse.

Anyways, the stuff is expensive but you can buy small quantities of it and use at the recommended dilutions. I use 25% v/v diluted in ethanol. For fixing the pot with this compound all you need to do is put a drop on the pot (make sure it wicks inside where the wipers are), let it dry, repeat about 4 or 5 times. What will happen is the slop between the wipers will be filled with a thin (microns) residue of the compound and the pot will essentially be fixed (for this application).

I've included 2 photos of what I'm talking about... the pot is directly above my thumb, looks like a white ceramic disc with a base soldered to the board. There is a metal face which is the factory fixed wiper. Don't try to jiggle the wiper to clean it as its value is fixed.

As far as I know, the only compound that will work for this fix is Stabilant 22. You can buy 2 mL for about $30 online... expensive, but not as bad as paying $200 to send into a speedo shop. Once the pot has been treated it should be good to go for the life of the car.

Additional questions are welcome.
 

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Discussion Starter #3
I put it in the gap between the metal ring and inner assembly. Make sure you shake the 25% solution well before applying as the polymer really isn't soluble ( it's more like a colliodial suspension ). It is good to repeat the application several times to build up enough residue to close the slop between the wipers. Just make sure you allow for the ethanol to evaporate between applications.
 

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So, I have a dancing Temp Gauge. It is not the sending unit. The gauge will dance up to 220. I can bang on the dash and it will come back down. Is there an adjusting pot for the temp gauge?
 

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Great info, thanks!
Earlier today I just replaced my tank sending unit (1989 W124 200TD) without succes (it started to bounce until settled at 1/2 no matter how low my fuel level is). Then I replaced the fuel gauge. Nothing! Gauge is stuck at 1/2 but my tank has not enough fuel to turn off the warning light...

Sender was cleaned, lines are not broken, turning it upside down makes the gauge shifts well from full to empty and back. Put the sender back in the tank... Gauge reads 1/2.

Well that is not funny at all.

I did not push the instrument cluster all the way in ;) , and I will try your solution!
 

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I've done this, worked a treat. I was fairly liberal with the Stabliant, just got a good drip on the little brush they give you with it and slopped it on until it wicked down around the pictured green bit (which was off-white on my 1990 300CE). Left it to dry for ten minutes or so then put on another coat, repeated four times.
 

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Hello,
thank you for the nice write-up.

I have measured the resistence of the trimmer as a whole, it was about 125 omh, then to each individual leg to see what was the current situation( before chaning the trimmer position). One side the resistence was about 90 omh, on the other about 40 omh.

I managed to fix mine by:
1. Using compressed air to spray onto the trimming pot, especially the wiper hands.
2. Dropping some small quantites of ethanol 95% on the resistence field and moving the wiper hands all around a few times.
3. Then compressed air again.
4. Finnaly returned the reistence indication on both sides to about the same values: 90 and 40 omh .

I measured the resistence from the fuel sensor and it was 61 omh, then connected the Instrument Cluster and the gauge showed just a bit above 1/4 tank full. Which should be alright. These days I will further monitor it, hope it will stop jumping around and showing 1/4 when it is near reserve...

In regards to the compressed air, I used that in the first place as I found a nice article on the net which says normally only compressed air should suffice to get a pot working OK. (ref.: How to Clean Mixing Board Faders and Trim Pots | eHow.com ).

Good luck to all who are about to fix this problem themselves.

Here is a photo of my trimmer pot on the Instrument Cluster from the side with the wiper blades:
http://www.oldum.net/~hijacker/mb/remonti/snimki/Instrument_Cluster/FILE0005.JPG

Thank you,
-Nikolay
 

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Hi all,
It seems that if not applying any contact spray on the trimmer the effect is only temporary. The fuel gauge started playing around not a month after it was cleaned!
So I ended up replacing the trimmer with 2 fixed resistors of 0,5W power.
One I chose was 68 omh and the other one 82 omh. Those were approximate values I measured after I removed the trimmer(measure the resistence only after you have removed the trimmer, otherwise different values may be observed ...). Right now the gauge shows slightly less fuel, but at least it is constant.

Here are two pictures of how it looks now with the resistors on board:
http://www.oldum.net/~hijacker/mb/remonti/snimki/Instrument_Cluster/chast2_rezistori/FILE0003.JPG
and
http://www.oldum.net/~hijacker/mb/remonti/snimki/Instrument_Cluster/chast2_rezistori/FILE0004.JPG

Best of luck to all and happy fixing.
Cheers,
-Nik
 

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Hey Nik,

That's a nice solution you came up with. Let us know if using 68ohm and 82ohm (0.5w) resistors does the trick long term. I think it should be fine but you'll be able to tell us after few tank full of fuel has gone through the car.

Cheers.

Ali
 

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Hello Ali,
yes, those fixed resistors are just the ideal values for the car. Please note it is a W201 190E 1.8 1990 with a fuel tank of 55 liters + reserve. Those values will not be the same for fellows that have 70 liters tanks etc.

Cheers,
-Nik
 

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Hi all,
It seems that if not applying any contact spray on the trimmer the effect is only temporary. The fuel gauge started playing around not a month after it was cleaned!
So I ended up replacing the trimmer with 2 fixed resistors of 0,5W power.
One I chose was 68 omh and the other one 82 omh. Those were approximate values I measured after I removed the trimmer(measure the resistence only after you have removed the trimmer, otherwise different values may be observed ...). Right now the gauge shows slightly less fuel, but at least it is constant.

Here are two pictures of how it looks now with the resistors on board:
http://www.oldum.net/~hijacker/mb/remonti/snimki/Instrument_Cluster/chast2_rezistori/FILE0003.JPG
and
http://www.oldum.net/~hijacker/mb/remonti/snimki/Instrument_Cluster/chast2_rezistori/FILE0004.JPG

Best of luck to all and happy fixing.
Cheers,
-Nik
I am an electronic dufus. How does sticking two resistors both that high resolve the variable resistance when the lower resistance is down around 3.3 ohm when you have a full tank ?
I am desperate for a solution but I don't understand enough of what you did to tell me what I should be looking for with a W124 70 litre tank !

:surrender:
 

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I am an electronic dufus. How does sticking two resistors both that high resolve the variable resistance when the lower resistance is down around 3.3 ohm when you have a full tank ?
I am desperate for a solution but I don't understand enough of what you did to tell me what I should be looking for with a W124 70 litre tank !

:surrender:
:dunno:
 
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