Oil Pressure Guage pegged
 

Here's a problem I've had since I got my 300SD and thought I'd see if I could diagnose and fix. My oil pressure guage pegs to the top pressure as soon as I turn the key. It sits at 0 when the key is off. When I got my car, I had a mechanic look at it and he said that he checked the guage and the sending unit and they were fine. Is this a wire replace job (of course, I'm sure, made more fun by the harness and plug arrangement there) or should I recheck everything there? I just thought it would be fun to be able to see what the REAL oil pressure is... Thanks for any help/advice ahead of time!

In the case of my 1987 300D with the exact same symptoms, it was the sending unit.

It is pretty easy to test and isolate the problem to the sending unit. Disconnect the wire that is plugged onto the oil pressure sending unit. The gauge will either peg or read zero (I forget now which is which). Now ground the wire and the gauge should read the full opposite of what it showed in the disconnected situation. This lets you know the gauge and electrical harnes wire is OK.

Nothing left but the sensor at that point. They are available for a good price here in FastLane.

I sure felt MUCH better being able to read the real oil pressure. What a relief to know it was OK.

For my situation with the 300D, it was easiest to reach the oil pressure sending unit from under the car. While I do have ramps, it was only necessary to get the car up just enough so I could slide under - so I used a jackstand.

Ken300D

TxBill and KenD,

Thanks, I'll give that a try. I'll let you know how it goes. I agree- I have been going on faith that I'm getting good oil pressure, but I'd feel much better actually seeing it on the gauge.

- Ted

oil pressure gage
 

Had the same symptom on my 300SD. I crawled under the car to find the pressure sening unit to check it out, and found the wire had somehow gotten pulled loose. I re-hooked it up and that fixed the problem. So you may want to check for loose connections, broken wires, ect. before replacing the sending unit.

Mea culpa - I just went out to drive my car and found that the oil pressure gauge is pegged all the time, even with the key off. That's what I get for remembering the issue at night. Is the same diagnosis appropriate or am I resigned to tracing wire at this point? Again, thanks for the help.