I just replaced my oil pressure sending unit and now my gauge reads correct but my fuel pump remains on even after the key is removed from the ignition.

Two questions here. One, I read somewhere that the OPSU powers the FP so how/why did my car run before when I never heard the whirring?

And secondly, is my assumption correct if I think it's a relay issue?

 

Your fuel pump has two parallel power supplies. The pump relay cycles the pump on, and off after a few seconds if the key is only turned to the on position. Once you start the engine, an ignition pulse tells the computer to activate the relay to run the pump constantly. The secondary circuit through the oil pressure switch is a fail-safe, to keep the fuel pump running in case the relay fails. Contrary to some popular belief, the oil pressure switch isn't a safety device to kill the fuel pump should the engine lose oil pressure. Since you weren't getting a fuel pump run-on issue before, I'd say the oil pressure switch must be faulty, or you got the wrong one. Unplug the switch and see if the engine doesn't still run like it should.

 

Yeah that's where we're at now. After I installed the switch, hooked it up, and heard the constant whining, we temporarily disconnected it so it wouldn't run all night in the garage.

I think the switch is right. I bought it from Summit, and pound for pound it looks like the exact same thing. Harness hooks up to it just like the old one as well.

 

Try touching a ground wire from a good frame source (or negative battery) to the body of the oil pressure switch. Don't know if or why that would make a difference, but it's the only other thing I could think of checking before figuring the switch is goofy. If it then works right, you have a bad, or no engine ground.

 

Very confused to say the least!!!!!

1.) New OPSU. Makes the gauge work, but also starts the FP whining which continues even after key is removed.

2.) New FP Relay. Does NOTHING for this situation. Tried the new relay in both spots (since I didn't know which of the two was for the FP) and the damn pump still runs after the key is out.

3.) Tried above said wire test from negative on battery to OPSU case and it STILL IS WHINING!!!

I'm out of ideas here. I bought the OPSU from Summit so I'm assuming it's a good unit, but what in the heck did I open up here? I mean, the car ran fine (and quietly) before I installed this damn switch. I hate to leave it unconnected just to shut the pump up but I very well can't risk frying the damn thing by letting it whine 24/7.

 

Then I go back to my first thought and say your switch is screwy, or not the right one for your application. Do you have a link to the one you got at Summit?

 

Yeah, here she is: Standard Motor Products PS228 - Standard Motor Oil Pressure Gauge Sending Units - Overview - SummitRacing.com

 

I thought you had mentioned it, but guess not....what year is your car? That switch is for 89-93. Also, "new" doesn't always mean it's good or defect free.

 

It's a 91 with a 305. And I hear ya, just because I bought new doesn't mean it doesn't possess a manufacturers defect but I haven't had a problem with them to date.

So far.