Electrical Problem
 

We love our '85 300SD. I received a telephone call this morning from my other half telling me that on the way to work the Oil Pressure and Temperature gauges quit working. The car has over 300,000 miles on it and we want to keep it going.

Are the gauges on this car fused, and is there a separate Instrument Voltage Regulator on this car?

I have done some work on these cars, but have never gotten into the electrical portion. Never had to do so.

Ideas on where I start to troubleshoot this?

Sincerest Thanks for the help fellows.

I have two Benz autos this 300SD and a '74 450SE that is a project when I have some time to get to it. Love the Benz's...........

Was it all of a sudden? Check the fuses and the ground points first.

More Information
 

My other half just called again on lunch hour. This time I receive more details. It seems that the cluster failed in total. The failure includes the Oil Pressure Gauge, Temperature Gauge, Speedometer and Tachometer. The clock and warning lights apparently are still working. This sounds like some kind of fuse or power supply to me now more than ever. Power to the whole thing........

Where do I look as far as location for this?

I don't know how similar to the 300D your 300SD's electrical system is . . . but in the '85 300D (W123 body) there's an over-voltage protection relay (OVP) that powers all of the gauges and some other electrical things. It has its own 10 Amp fuse. I don't know where it is in your car, possibly in with other relays near the fuse box? In the 300D it is hidden up behind the passenger kick panel.

I want to thank those who tried to help.

The problem was found. When the car arrived home, I opened the fuse box and everything appeared good. I did some testing and found the problem to be a fuse that looked good but had cracked where the filament met the end caps. The fuse fell apart in pieces as it was removed from the fuse block.

A replacement fuse was installed and everything was fine.

Just for the record, many moons ago I worked for Chrysler and in came a 1969 Plymouth with a problem.

When the left turn signal was energized the signals worked perfectly, when the right turn signal was energized the signals would flash (3) times and then on the 4th flash the system would start operating as if the Hazard warning flashers had been turned on.

The owner of the car had gone to several shops, all of which wanted several hundred dollars to chase the "gremlin".

I found the problem to be a new bulb that had been installed in the right front. The filament of that bulb was cut too long and when the bulb flashed multiple times the filament would heat and stretch down in position contacting and energizing the second filament and then the other lights.

Ever since that one, I diligently check bulbs first.

Thanks Again.....

The brake lights on a 300SD are also on that fuse, I had it blow before too and lost all gauges. Kinda weird that they put all gauges + brake lights on the same circuit....oh well.