Well given that soldering seems to fix so many VDO issues, I took my dash out today and re-soldered every connection I could on the side with fuel, economy, etc. The tach side has nothing to solder really.

Anyway. My fuel gauge is responding MUCH better. I think its still off though. I was getting a fluctuation of about 1/16th of a tank. About a needle's width. But it no longer jumped from over a half tank to a quarter and back again. Next time I fill it up I will get a better idea of where it stands. I was just wondering if the fluctuation is normal or if I should go in and solder again. AC was not on, incase someone asks about power draw.

Also I re-soldered every connection on the light dimmer switch. The dash and in car switches sure seem brighter. I can actually see they are on in the day light. At night I will have a much better idea on how much/if they are indeed brighter.

Overall an easy job.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Well the dash and such is brighter. But I am still getting a little jump in fuel of about an 8th of a tank. Which is NOTHING like before. But I can still watch the needle do an occasional jump an eighth up then back down.

I am going to re-solder again and then run a separate ground wire to the cluster.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

A 1991 car has a control unit which regulates the dash lights, all the power doesn't go through the rheostat like it did in the older models.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

fuel sender unit needs a good clean on the resistance wires inside , once done no more flickering needle.
mak

Ya.... been there done that. Was the first thing I cleaned. Although the tank and wires were clean at the start by appearance. After cleaning I had the same issue. Showing over half a tank yet the reserve light would kick in. No change.

But after this solder work, its behaving MUCH more accurately. And its not jumping nearly as much. So something with the soldering sure had an effect on it.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Thanks for posting!!

I have been battling my fuel gauge for over two years!

I looked at all the solder joints under a microscope and found the connections at the calibration pot were cracked. I remelted all the connections and I am back in business!!

Cool! Like I said, mine is much better but I need to dice back in and re-solder again. I have a new rule with my Benz. If the part is stamped VDO then re-solder is a must!

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

I spoke too soon. It was good for about a day. It is better, only jumping 1/4 tank vrs 1/2 tank.

I did notice that when the car is started, (warm or cold), it is good, and after a consistent period of time it will start to get jumpy. (something warming up?)

I found that if I push the stud for the fuel gauge around, (carefully), I can get it to jump and then go back depending on how I apply pressure to the stud.

I have previously tightened up the connector sleeves in the gauge so I do not think it is a contact issue. I am thinking it might be a cracked trace or an iffy wire in the gauge its self.

Very frustrating!!!

I hear you! But I am glad mine is a little more accurate. So soldering the cluster did something.

I read that some, to fix other issues, run a separate ground from the back to a location in the dash. I am going to do that just to see if it helps at all.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

I think I finely got it.

I got to thinking after I noticed that it took a cretin amount of time before the gauge would start acting up like something was warming/heating up and its resistance was changing or making and braking contact from expansion.

The Pot to adjust the gauge 0 is right next to the instrument lights and the board was dark brown like it had been cooking. I removed the pot and attached wires through from the back to the pot connections, and relocated the pot outside of the cluster so I can adjust it with it all together and to keep it away from the heat. In doing so, I found that the trace at the pot was broken. My resolder job helped, but because the brake was under the masking, it did not get soldered.

I took small wire and created a parallel circuit to the entire trace. I went from the socket pin to the top gauge pin, then to one side of the pot, from the other side of the pot to the resister.

The gauge behaves just like both my 123s, moving with the sloshing fuel as I make turns, but no jumping!! Before I could push on the back of the circuit board and the gauge would jump, but it does not do anything when I push on it now.

I do not know if relocating the pot was worth anything, but I would highly recommend running wires over the traces.

I will look into that. I notice they painted the board surface white. Makes it hard to see the traces of copper. Its worth a shot.

If you still have issues, you may want to look at the two actuator mounts for the needle "motor". Mine looked bad.

Also I know what you mean by it looking cooked. Only the fuel gauge components has some ear-wax like substance at the solder points. As if heat was baking off any of the flux and such.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car