WE HAVE A 1986 300SDL AND THERE ARE SOME STRANGE THINGS HAPPENING THAT I DO NOT UNDERSTAND. HERE'S THE PROBLEM:

WHEN THE IGNITION SWITCH IS FIRST TURNED "ON", THE LEFT AND RIGHT SIGNAL ARROWS WILL COME ON TOGETHER AND THE CAR LIGHTS WILL DIM, THE SPEEDOMETER AND OIL GAUGE WILL GO TO ZERO AND ALL DASHBOARD LIGHTS WILL GO OUT FOR A BRIEF PERIOD (TENS OF SECONDS TO A FEW MINUTES). THE CAR ENGINE WILL CONTINUE TO RUN FINE DURING THESE EPISODES. AFTER THE PROBLEM CLEARS ITSELF, EVERY THING WORKS FINE. THIS EFFECT IS TRULY APERIODIC AND IS NOT PREDICTABLE. HELP!

THE NEXT PROBLEM INVOLVING THE DASHBOARD CONTROLS IS ALSO WEIRD. IF I HAVE LESS THAN A FULL TANK OF GAS, THE GAUGE WILL BOUNCE BACK AND FORTH BETWEEN WHAT'S ACTUALLY IN THE TANK AND FULL FOR A SHORT PERIOD AND THEN STABILIZE. THIS MAKES IT DIFFICULT TO KNOW HOW MUCH FUEL I ACTUALLY HAVE IN THE CAR. TO MAKE MATTERS WORSE, THE TEMPERATURE GAUGE ALSO BOUNCES UP AND DOWN AND INDICATES THAT THE CAR IS OVER-HEATING WHEN I KNOW IT'S NOT. IF THE CAR IS DRIVEN FOR A MINUTE OR TWO, BOTH GAUGES GO TO THE NORMAL POSITION.
THE OIL PRESSURE GAUGE INDICATION IS NORMAL DURING THESE EPISODES. WHAT DO YOU THINK IS HAPPENING?

I don't know about your instrument light problem, but your fuel gauge problem is a bad fuel sending unit. This failure is very common, and the gauge bounces just as you described, especially when you are at a half tank of gas.

Also, please refrain from typing in all caps. In internet etiquette, it is the same as yelling.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

Thanks for your response. It happens to be easier for some people to read/type all capital letters- too bad it offends you.

no no, it doesn't offend me.

It's just that anyone who reads your post will think you are yelling.

Everyone that posts in all caps will always be told to the caps lock button off, at least on this site.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

Hi there,
I must agree with Suginami, it is both offensive and harder to read your post with all caps. On to the problem - You might have a poor ground to the dash causing your problems (Mercedes vehicles are notorious for poor solder joints most everywhere on their wiring and printed circuits). This could be the result of a cold solder joint on the printed circuit board on the panel itself, or could be in the wiring from the plug to the ground.
You can easily check this by connecting a multimeter between the ground circuit of the panel and a true body ground somewhere on the sheetmetal of the body. If you measure any voltage between these points, you have isolated your problem to a poor ground circuit to the panel. From there it's just a matter of locating the failure point and correcting the problem. If it turns out to be a cold solder joint, try to remove the old solder first by "wicking" it off the panel, then apply fresh 60/40 rosin core solder. It might also be a good idea to check the integrity of the ground strap from the engine to the chassis.
Good luck chasing it down!

Richard Wooldridge

To: Richard Woolridge et al:

Where is the fuel sending unit located in the car?

Can the fuel sending unit be cleaned by use of diesel fuel additives or must it be replaced?

I will check the grounding system.

thank you

In my car (W124), it is located behind the trunk liner on the right hand side.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

Hi there,
I think it would be wise to fix the grounding problem before worrying about the fuel sender, which is located inside the fuel tank. It's quite possible that both the fuel gage and temperature gage are affected by the grounding problem. Your oil pressure gage is mechanical and doesn't need electricity to operate. On many vehicles the fuel and temperature gages have a voltage regulator mounted on the back of the instrument panel that provides a constant voltage to those instruments so they are more accurate, but I'm not sure that your vehicle has one. If I remember the circuit diagrams correctly, it doesn't. Both instruments need a good ground to work properly, however!!

Good luck with it,

Richard Wooldridge