J.C. Denton

iTrader: (1)

OK, I have a steady problem with my oil pressure gage and I am absolutely lost now.

Car: 1992 Firebird, 3.1 Liter LH0, stock.

Problem: My oil pressure gage shows always the same. No matter what temperature the engine or the oil has, it ALWAYS shows around 65 PSI. That drives me crazy...
When I turn the ignition on, the needle climbs up to 0 PSI. Fine... When I start the engine, it climbs up to about 65 PSI and stays there all the time. It doesn't move when I hit the gas pedal, it doesn't move when the engine gets hot and so on.
I hooked up a manual pressure gage, here are the readings:

Cold engine
Idle: 63 PSI
2000 rpm: 67 PSI
4000 rpm: 80 PSI
6000 rpm: 83 PSI

Warm engine (around 190°F)
Idle: 45 PSI
2000 rpm: 63 PSI
4000 rpm: 72 PSI
6000 rpm: 80 PSI

Hot engine (around 230°F)
Idle: 35 PSI
2000 rpm: 60 PSI
4000 rpm: 68 PSI
6000 rpm: 72 PSI

So, oil pressure is definitely ok and the oil pump does it's job well.
After the measurement I bought a new oil pressure sending unit. Nothing changed. I bougt 4 (!) other sending units from different manufacturers, like Delco, Standard and so on. Nothing ever changend. So I swapped in a whole Instrument cluster from another car where the gage definitely works. Nothing changed. I swapped just the pressure gage into my cluster from a third car. Nothing changed.
I checked the pins at the instrument cluster and the connectors. Everything fine.

After that I went into electronics, here is the simple wiring scheme:





I checked the voltage at terminal D, orange wire -> ok
I disconnected my fuel pump relay. After few seconds, engine starts, so it seems that the backup works after reading oil pressure.
I checked the grey wire to the FP relay -> ok
After that, I checked the TAN wire, terminal A to ground and to the instrument cluster. Everything seems to be fine here. I could even measure the mysterious 68 Ohms resistor. I spliced into the wiring harness and found the little fellow, everything seems fine, reads axactly 68 Ohms.
What is this resistor for?

After all this, I just couldn't find anything that is not as it should be. Im really out of knowledge here what the problem could be.
I would be happy if anyone here ever had the same or even a similar problem.
I know, the gage is just an indicator and a manual gage is alway better, but I want to keep my car original and just want the damn thing to do what it is supposed to do.
Anyone here who can help?

Thanks guys!

Oliver

SouthSide2

Not sure this is the problem. There are different oil sending units.

3.1 engine- standard #PS236.

5.0 & 5.7 engine- standard #PS228.

The PS228 Has 90 OMMS AT MAXIMUM PRESSURE. 0 OHMS AT 0 PRESSURE.

My 1990 Camaro with the PS228 has no 68 OHM resistor in the circuit. Works fine. If it had that 68 OHMS it would read high all the time I think.
I don't know the specs on the PS236 just that its different.
Some thing to look at.
Good Luck.

J.C. Denton

iTrader: (1)

Thank you for your answer, but I already tried the PS236, T-Series and Standard Series. No difference.