How do I read my fuel gauge?

[ykobayashi]

Hi,

I’ve been working on my gauge sender and I’m realizing I don’t really know how to read the gauge. I took my sender apart, cleaned it and started testing it out while it was installed in the car. No jumps, needle goes up and down.

But...when I slide the float all the way down to the reserve shorting bar, the needle drops to what appears as the 1/4 tank mark. It goes no lower. I would think running the float down to the shorting bar would make the needle go to R.

My R light comes on. The needle reads full when the float is all the way up. And it gradually moves down without jumps as I slide the float down by hand. But it stops at the 1/4 marker.

I don’t know if I’ve ever run it lower than a quarter. I usually fill before that.

Is this normal for the needle not to read empty on R? Is it supposed to stop at 1/4 or has my zero drifted off after 35 years?

There’s a fixed resistor on the gauge. Perhaps I need to trim residual value and add some resistance to it to make my gauge read empty (at R).

I read my manual and it didn’t say anything about the needle going all the way down to R.

Just curious if your gauges go down to R. I recall my 240D did. My contacts at the sender look clean. Perhaps I have some residual resistance in the circuit or the needle has just drifted.

[Clemson88]

Is there a film on the wires which run from the top to the bottom of the sending unit? It seems I read here just last week that such conditions would change the resistance value and affect the accuracy of the gauge.

I might suggest you search this section of the forum for 'gauge.' The thread shouldn't be hard to find. Perhaps it will be on the first page of search results.

[ykobayashi]

I cleaned the nichrome wires with emery cloth. Got them really shiny. I was thinking if the problem is on the wires it would cause jumps which I don’t have. Hmmm...dirty wipers on the float would cause a steady offset. I was afraid to clean those because they looked too delicate.

The contacts in the plug were clean.

Perhaps my gauge has drifted. It is a meter of sorts.

I guess I can recalibrate the thing with resistors, or perhaps never let it go below 1/4.

[Clemson88]

If you're the only one to drive it that's a good solution. To be safe you might store a gallon of cooking oil in the trunk for emergencies or in case you get distracted and run out.

[tdoublenastywitit]

Why cooking oil and not just diesel??

Safety? From explosion or fire? I assume the assumption that I've assumed is correct?

[Clemson88]

I"d worry more about the smell than as explosion. It would take one hell of a lick to set off a gallon of diesel and I don't think a spark would start an ignition if the diesel was contained without oxygen.

[Rogviler]

I carry emergency canola oil. For me it's just the fact that I'm not sure what I would want to store the diesel in. One of the leaky EPA fuel cans? Or a non-regulation jug that someone might throw a fit over (you never know these days)? It's just easier to have a bottle of something I know isn't going to leak and if it does, no big.

Remember, fresh cooking oil is NOT the same as WVO littered with chicken junk and who knows what.

-Rog

[ykobayashi]

Hey that’s a good idea. I used to burn WVO in the 240d. It lasts a long time even if it is no longer edible. Just keep it sealed.

I need to look at the sender/gauge circuit.

My guess is that it takes 12v and just runs it through a known load in the gauge and hen through the resistive potentiometer in the sender. There is a calibration resistor in parallel inside the sender that bypasses the nichrome wires. I think this is the resistance it sees when the tank is full when the slider is all the way at the top. Sooo...that is the full setting which is pretty close.

My problem is there is too little resistance in the empty position when the slider is all the way down. So it isn’t like I have a bad contact, the resistance of the nichrome seems to have decreased or the gauge resistance is too low. I think this can be solved with some resistors. Probably just old.

I was just asking if the needle is supposed to hit R when empty and I guess the answer is yes.

[ykobayashi]

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).