Hopefully this is the right forum for this question. I have a 92 RS 305 TBI. the oil pressure guage reads from start up to when the engine warms up then hangs out in the red zone. Every now and then it will jump up to 45-50 psi while driving so I know I don't have low oil pressure. I replaced the OPSU and have the same problem. Next thing I should look at? Maybe its a cluster problem?

 

I'm not sure what you mean by that.

The only sure way to know you have oil pressure is to hook up (at least temporarily) a mechanical gage.

To check the circuit, take the oil pressure sending wire off and ground it. Turn on the key, the oil pressure gage should go to max. If it doesn't, you've got a circuit problem. If it does, something isn't right with the signal the gage is getting.

If you replaced the sending unit and nothing changed with the way the gage operated, it would tend to indicate it's not a sending unit problem.

 

Just something relatively simple to try- yank the gague cluster, clean the contacts on the quick disconnect at the back of it (and the other side of the connector in the dash) and shove it back in, making sure the connector lines up properly. I had one that was flaky and a good clean-and-reseat of the gague cluster cleared it right up.

No guarantees, but something simple before you troubleshoot wiring harnesses or buy and expensive gague cluster to try to solve the problem.

 

Thanks for getting back to me. This car though has a three wire plug on the sending unit so I don't think that will work. I have read some posts that say these cars won't run with the OPSU unplugged, is that true? I also read that the fuel pump run signal is also run through the opsu. So can I still hook up a mechanical guage or will the car not start? I know the car has good oil pressure its just one of those things I hate to see as I am always checking guages as I drive.

 

I don't think that's true, it may have been true for some older models.
My '88 has separate switch and sender units. I've unplugged the switch before startup with no problems just to test the story.
If yours will startup without it plugged in then all you need is to know which wire is for the pressure sending part of the sensor and ground it. If it's accessable you can also put in a Tee to keep the sensor connected and read the mechanical gauge pressure off the other leg.

FYI
During my cam break in the pressure would hold rock steady at 40 psi at all rpms. My builder told me to change the oil filter, problem solved, because he knew exactly what to do, I assume it had something to do with a bad batch of filters he had in stock.

 

yes, my car also has the three wire opsu...did you figure out what goes where? the tan wire goes to the instrument panel and i guess you ground one of the other tabs.....? let me know what you know

 

The body of the 3-wire sensor provides ground for the oil sender part of the sensor, grounding the tan wire to test is what I was referring to.

One (org) of the other 2 wires goes to a junction powered by the inline fuse next to the battery, the other wire (tan/wht) goes to fuel pump relay circuit. So there can be 12v on both of those wires because they feed the pump, ECM, and the MAF among other things.
If you read 12v across the org & tan/wht wires the switch is open, 0v the switch is closed.

 

I just installed a mechanical guage. I put the sender in on the top of the motor (where all the older cars had the stock sender, back of the block behind the intake, next to the dist.) The car has great oil pressure for a higher mileage motor. I didn't touch the factory sender at all, it can read if and when to wants to and I don't care now.

 

So, where did you put the mechanical gauge?

I put all the oil sensors and mechanical gauge on a manifold where it's easy to get to: