My Oil Pressure gauge has been "stuck" at 3 bar for about six months now. Sort of a curious problem. When the engine is off but the ignition is set to position "2"(on) , the gauge will read zero, as it should. When you start the car, the gauge has about a half second delay and then shoots to the top of the scale and stays there until the car is turned off.
I've read that this is a classic indication of the oil pressure switch is bad. Well I finally got around to ordering the replacement switch. Put it in with relative ease. Only problem is the this didn't cure the problem.

A couple of notes: The old switch would allow the needle to move ever so slightly -maybe 3.0 to 3.1 bar as the engine reved up from idle. The new switch makes the needle stick to the top (approx. 3.1 bar) with no needle waver at all.
I cross referenced the part numbers and the new part matches the old one. (004-542-94-17)
I get about 81 ohms on the old switch sitting on the bench.

Could my pressure really be that high?? At idle??


cc: benzworld.org

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Paul Hansen

1997 S500 (100,000mi)

My W140 pegs the oil pressure gauge with ease at idle when the engine is cold. It doesn't drop down at idle until the engine is warmed up... then it drops to about 2.
What's your reading when the engine is hot?

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KenP

1996 S500 W140 Coupe

Hot as defined by 90c on the coolant gauge and about 15 minutes if driving - still pegged.

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Paul Hansen

1997 S500 (100,000mi)

Well, it wouldn't be the first time that a new, replacement part was as bad as the part being replaced. or, you may have a problem in the harness or cluster (ouch).
Only way to be really sure of what's going on is to remove the electrical sensor and put in a temporary mechanical pressure gauge to see the actual pressure... and then go from there.

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KenP

1996 S500 W140 Coupe

As soon as I get home (Sunday), I'll run a gauge cluster test and do exactly what you say- verify the pressure with a mechanical gauge. Hopefully, it's not the cluster.
Also, does anyone know if an open circuit will cause zero deflection or full deflection of the gauge?

Thanks again everyone

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Paul Hansen

1997 S500 (100,000mi)

On mine, an open circuit caused the gauge to peg at 3 bar. For a while, I just thought I had Really Good oil pressure.

You can check yours by disconnecting the oil pressure sender (thus creating an open circuit) and watching what the gauge does then.

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1989 300 SEL that mostly works, but needs TLC