FredR

Had the car in the last couple of days to have the a/c comrpessor r/r'd. Took the car out this afternoon in 35C heat and the a/c was colder than I have ever experienced at such ambient. I then noticed that the oil pressure readout on the dash was a full scale. when the engine is switched off it zeroes.

To me this suggests a problem with the sender unit or it is reading correctly and something wrong with the pressure relief valve. I suspect the former although must say I felt uncomfortable not having the analogue gauge behaving as normal. I have noted earlier that the pressure gauge needle could sometimes be a little unstable just above tickover.

Do these symptoms sound familiar to anyone?

Regards

Fred

Maybach_Man

Have you checked the wires are the right way round??

FredR

Have not checked anything as yet- no logical reason I can think of why any wires on the oil pressure sensor should have been disturbed yet alone re-installed wrong way round- interesting thought.

Regards

Fred

Speedtoys

Ya..mines at 5 all the time.

Key on or off. Fun times.

Dictys

Fred, if the a/c wires have been messed around near the compressor, then this could have moved the front harness.

The oil pressure wires run through this harness and having just dissected a s4 harness which has been out here for the last 10 years there is a good chance the wire jackets within the sleeving have crumbled and there may be a short. If there is any damage to the ends of the front harness then there is a good chance that the center of the harness where the wires double back on themselves is shot.

Just a thought and can be checked by checking the resistance between the oil sender plug and the 14 pin connector.

depami

Check that the wire is still connected to the sending unit. Open circuit = max pressure reading.

FredR

I was thinking of something along these lines- scary thought if and when these things start crumbling. Apart from the shrink sleeve stuff that petrifies my wiring in geenral has not fared too badly to date. However, this theory conflicts with Denny's later comment that if the terminal is disconnected it will drive to full scale.

Dictys

To be fair I am not sure if open is max or not, however there are two oil connectors, at least on my 90. There is a single wire which goes to the oil level sender, then three wires which go to the oil pressure sender. I have no idea what the effect of the failure of one of these three wires would be, someone wiser than me would need to answer that.

928 at last

Mine had the same symptoms.
New sender unit fixed the issue.

cfc928gt

Same thing on my '89 GT, needed a new sender.

FredR

Thanks for the various inputs- does sound rather like the sender unit- will be asking Roger to put one in with my engine mounts

regards

Fred

Bill Ball

It could be anything that breaks the electrical path, but the sender is the most likely culprit if it is still connected. The sender provides a variable resistance signal over a fairly small range, higher for higher oil pressure, so if it fails open circuit, that pegs the gauge.

Rexxus

Hiya,

I have the same issue. Oil gauge reads full scale all the time. BUT, I disconnected the negative terminal on the battery and noticed the gauge went to zero. Does that mean anything to anyone?

FredR

The sender was shot however that was not the full story. Turned out the wiring in the front of engine harness was shot and even that was not the end of it. Not sure what the transition year was but at some stage the S4 went from a two pin sender to a three pin sender [mine is MY90]- the difference being the latter used a hard wired earth rather than use the chassis as an earth. The saga was eventually resolved when I changed to the earlier two pin type sender with simple spade type connectors.

Never managed to fathom out why the three pin sender seemingly had the problems it exhibited. In your case I would check the state of the wiring first by taking a look at the 14 pin connector next to the hot post and the state of the wires as they enter the sender next to the oil filter.

You did not say what model year you have.

ramcram

Fred, when I rewired the whole front of an 86 32V, I was surprised to see the oil pressure sender worked backwards to most others. That is to say, open circuit from the sender gave full scale deflection, closed circuit [grounded] gave '0' deflection.
Your GTS will have a 3 wire sender though and I don't know if they work the same?