Today I tore down my cluster and looked around inside it. W126 fuel gauge is pretty much like all the others, just an armature needle, a coil and a resistor. Probably hooked up in Wheatstone bridge form with the resistive wire in the sender.
It is very much like the high school kids science project of how to measure the resistance of a wire using a cheap voltmeter circuit. Shown here.
https://www.wikihow.com/Calculate-Unknown-Resistance-Using-Meter-Bridge
Basically you have some known loads in bridge configuration and you can find the slider ("jockey") position. I tested my gauge with and without the engine running and it doesn't care if the charge voltage is 14v or 12v.
Ok, so lets start off with some photos of the gauge for those interested. Same old same old as in other MB cars. I thought it was solid state. Funny how you can imagine all kinds of ways it works but just going out to the garage and taking a look closes the book.
My Gauge:
Basically 3 parts. A coil, an needle armature and a 68 ohm resistor. Nothing looked wrong so I just put it back in unchanged. What is there to drift? Maybe the hair spring in the needle gets weak?
I tore down the sensor again. This is a pain because it requires the removal of the back seat in the 126. I found this resistor in parallel with the nichrome wires. At least I think it was a resistor...a small valued one like 2 or 3 ohms *. I noticed when I moved my slider up and down, if I just had a little more resistance, the needle would read lower. As I said earlier empty was at 1/4 and the needle would never hit R even if the slider was all the way down. This thing was effectively lowering my sensor resistance and hence making my measured values of fuel higher.
So I cut it. I figured the resistor was making the resistance value lower by bypassing the current around the nichrome sense wire. So I figured cutting it would increase my resistance across the board full to empty. And it did. The full was still about 3 ohms. Was it even a resistor? I think so. I think before the cut it was 2.8. The empty was 80 before the cut and 86 after the cut. Or something like that give or take.
Not sure what the consequences of taking this "shunt" out will do. Maybe I'll burn up my gauge. It seems to have been acting to slightly lower the resistance through the sensor. A trim perhaps?
I don't know. Why did it need it and why does it zero better now without it? I don't know. Something changed. All I know is when I slide my sender all the way up my needle reads Full and when I slide it all the way down my needle goes down almost to R.
In the meantime, I'm going to be driving around with a gallon of some kind of oil in the trunk to get myself to the nearest station if the car stalls again.
*Edited
(Edit - I don’t know what the value was...I just calculated it and 2 -3 ohms makes no sense. I must have misread my meter...should be like 1k).