fuel gauge & light not working - PeachParts Mercedes-Benz Forum

Admiral Ahani · #1 03-06-2004, 11:47 AM

I did a lot of searching and didn't find information specific to my problem. Other threads discuss jumping or dancing fuel gauge needles - my needle is always steady but reads the wrong amount.
My gauge works, but it reads full when the tank is full and it reads 3/8 when it's completely empty. It's as though it needs to be recalibrated. Also, the fuel reserve light never works, not when I turn the key to position 2 or 3, nor when I actually run out of fuel (which I did on Wednesday). I recently took out the fuel level sender and cleaned the gunk out, and cleaned the contacts too. This problem has been there for about 10 years, and my cleaning didn't help. I took the bulb out of the cluster and tested it and it works. Any Ideas? The car in question is a 1981 300D. Thanks!

dmorrison · #2 03-10-2004, 08:26 PM

Well I need the exact same answer. I have just finished rebuilding and painting the instrument cluster. All lights were replaced but my reserve light does not test. I have followed all the service manual tests. I do know that my reserve light contacts in the sending unit are bad. I cannot get them to work. So I guess its time for a new sending unit, crap $231 for the 240D.
But the light is supposed to test even if the sending unit is bad. The light, in fact all the lights that test when you turn on the key get there power from The cable in the back of hte instrument cluster.
Now where in the cable the lines go that I will have to find out. I tested the resistor and diode, they all checked ok. So there must be something else that is tied to the reserve light test.

Dave

dmorrison · #3 03-11-2004, 01:07 AM

Ok someone must know why the reserve light is not testing.
The sending unit reserve light switch does not work . I tried to get it to work but it would not. But the test circuit is completely seperate to my knowledge.

What I do know.
The bulb is good.
The dash circuit from the bulb to the cable is good, pin 4 and 5
The resistor on the circuit/dash board is fine 17.7 ohms.
The diode on the circuit/dash board is fine correct in one direction.
The continuity from the lamp to the #7 pin ( #7 provides the ground) shows the normal resistance.
The test circuit power comes from pin 5 and shows normal voltage when the key is in run.
The ground for the test circuit goes out pin 7 which also ground the battery/charge light.
The battery charge light does come on with the key in run and goes out when the car is started.
The #7 pin goes to the alternator for its ground so I may have something wrong in the voltage regulator or alternator.
The charge light operates normally and the battery has not died due to lack of charge.
If I have a problem with the #7 pin wire to the alternator why is it comming on in "run " and going out when the car starts?
The #7 pin has a second wire that goes to the warning circuit for the brakes. It has diodes so it should not be the normal ground.

Tomorrow I will check the alternator and voltage regulator.
Otherwise I am completely stumped with the reserve light not testing

Dave

Theurig · #4 03-11-2004, 01:05 PM

I believe if your car is pre- Sept 1983 the test light does not come on when you turn the key to any position. this was changed with later models.

dmorrison · #5 03-11-2004, 01:24 PM

The reserve light test mode was installed in cars AFTER Aug 1982.

For Admiral Ahani, The light will not test but should work in the nomal operation of the car. So his sender/circuit is faulty just like mine. But the 83 240D should have the test mode work. The circuit is in the instrument cluster. I have tested it, but not the ground. so I will continue seaerching, like I don't have other things to do

.

Dave
7709

dmorrison · #6 03-14-2004, 05:27 PM

Well go figure
I cleaned and recleaned the sending unit trying to get the reserve light to work and it just would not test correctly. I decided to go with Rick Mileys advise (from another post) and just have her refuel when the guage got to 1/4 full, which is not a bad idea anyway, keep all the crud away form the screen. My daughter and son went to a friends house last night and took the 240D. Just a few minutes ago my son asked how much fuel the car has when the reserve light comes on. I told him its broken and won't come on. He told me it came on last night. So the Texas A&M Gods fixed the sending unit ( For those who don't know. The car was bought new by an Aggie, his son drove it at A&M and then his brother drove it at A&M) . That's OK with me. Now if I could just get the reserve test to come on.
However I realized while talking to my son about the light. the ground for the test goes to the Alternator and the warning circuit for brakes. The brake wear light is on and will not extinguish. I'm now thinking that the warning circuit may have something to do with the reserve light and brake wear warning light.

This just in!!!!!

I swapped out the Warning circuit, which all the online sites call the "seat belt warning relay" with the one in the 82 300TD. The units are the same, and the brake wear warning light now goes out after the car is started. So some success. However the fuel reserve light still does not test. I will get a new Seat belt warning relay ( since I know mine is screwed up , the seat belt buzzer does not sound) and try it again, I have a feeling that the new relay will fix all the problems.
I'll let you know when get the new relay

Dave
7709

Admiral Ahani · #7 03-21-2004, 03:00 AM

Well it would seem that letting my tank run dry that last time has loosened some gunk and now the car stalls when it gets warm or hot. The engine just bogs down (especially uphill) and dies. I have to clean out the lines and change the filters. I've seen things fix themselves if something related is replaced, so I'm going to wait until after this clean up job until I replace my sending unit. I'll post my results when they happen.

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