w140 oil pressure indication question
 

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

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?

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

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.

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

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.

Thanks for the response moneypit SEL,

I'm starting to think that I may have a loose wire somewhere.
Any ideas from this point? - like helping me trace the wire to where ever it goes?
:confused:

Don't make it hard on yourself. Disconnect the wire at the oil pressure sender. Turn the key on. The gauge will peg at 3 bar if it's like mine. With the key still on, ground the wire you removed from the sender. The gauge will drop to 0. You've just vetted the gauge and wiring. The only thing left is the sender, which I'm guessing has an open circuit somewhere between 0 and 1 bar.

I'll have to wait to Sunday to find out. My only question is why would the gauge read zero with the ignition on (connected to the sensor). With this in mind, why would shorting the wire yield a different result?

could be the spring in the sender
 

I had a sender unit that failed. The internal spring must have broken. It would stay at 3 when running, would sometimes go to 0 when turned off. If you have a mechanical guage it should be an easy check. I don't think the sender was expensive, that could be another easy thing to try.

Chuck

Mebby because you have zero oil pressure with the key on and the engine not running.

I believe your car has good oil pressure and is reading correctly.
140 chassis used two different OP senders [ B/5].. If you have the single terminal one , the sensors resistance increases as pressure increases . No connection will result in Zero pressure.
There is a micro processor [A] at the gauge for the 2 terminal set up that is in conjuntion with the oil level warning system . If the oil pressure drops to zero and the lamp lights, the is no pressure . If zero and no lamp, there is a wire disconnect.
Do you have the single wire sensor?

Yes, if you are referring to the single pole on the end of the pressure sensor (called pressure switch in the parts catalog)
Thanks for the info-

Pardon the interuption, but I am now confused. If the resistance increases as the pressure increases, wouldn't no connection = infinite resistance = max pressure on the gauge?
Not to completely hijack the thread, but do you know why MB made the oil pressure and level sensor so complex? On my old sled, they are seperate, and appear to work fine. What benifit is there for combining level and pressure and hanging a computer on the whole thing?

Good question! I am a bit confused as well. As soon as I get home tomorrow I'll start trouble shoothing the system and post the results.