W124-Intermittent Fuel Gauge Problem

[csnow]

W124-Intermittent Fuel Gauge Problem

I have searched past threads to no avail.
Model: 1986 300E

Symptom:
Fuel gauge sometimes reads max full, even when tank is not.
Sometimes it reads accurately. Seems to be no pattern to failure.

Attempts:
Since these symptoms have typically indicated a sending unit failure in the past (other cars), I 'pre-emptively' replaced the sending unit. No improvement. Wires and connectors on tank end are in perfect order.

Questions:
Is this a possible failure mode for the gauge, or is a short in the wiring more likely?
If it is a gauge failure, does anyone have a 'Radio Shack' repair procedure?
I'm good with a soldering iron...

[sunil190e-1.8]

are any other dash gauges or instuments playing up?

if so could be the voltage regulator on the back of the dash display.

or possible lose connection in the wiring.

[csnow]

Thanks for the response.

Everything else in that cluster is working, including the Low fuel warning light.

Could it still be this voltage regulator you speak of?

[sunil190e-1.8]

i can't be sure on the location of the voltage regulator,but this certainly was faulty on a previous audi 5 cylinder car that i owned and on that car it was behind the instument cluster -it was a small electronic fuse like device solder onto the electronic wiring of the board on the audi-do not know if mercs have a similar set up.

[ctaylor738]

Check the battery and alternator. I know this sounds nuts but see my previous posts on this topic.

[csnow]

Thanks Chuck,

I did see your posts with that extraordinary charging system 'failure prediction behavior', but I have replaced my alternator and battery recently, and they test up to spec.

[kanadary]

i use to get the fuel gauge not going to the full position after i refueled. i replaced the fuse that goes to that set of intruments and its working fine now. not sure if its related as the fuel gauge problem was intermittent.

[ctaylor738]

Have you checked the ground at the sending unit?

[csnow]

Chuck,

Perhaps I have the 'fuel gauge wiring concept' wrong in my head.

I would envision ground being at the gauge end of this circuit.

I picture the fuel gauge as being essentially a 'resistance meter'.

I would expect current to flow from the gauge, then through the sending unit, where the sending unit applies a certain amount of resistance (depending upon fuel level), then back to the gauge which measures this resistance, then to ground.

Am I on the wrong track?

[ctaylor738]

You have the right idea, but the wiring diagram shows the ground off the sending unit. The guage gets positive and then I think measures voltage from it through the sending unit to ground. At the sending unit, the blue /black comes from the guage and the black/green from the low fuel warning.

The ground is common to both.

I could fax the diagram to you later today. It is all in the Haynes manual.

[sunil190e-1.8]

sound like the fuel gauge problem is contagious!!

i filled the gas tank on my merc c200 -uk spec-and the fuel gauge stopped working after that.

it worked temporarily once the failed completely,

anyone have an ideas on a similar problem?

need help soon:

[csnow]

Thanks Chuck.
I will checkout that ground.