Hey guys, the car is a 90 GTU, originally auto. I did a five speed swap and did not have this problem at that point. I recently finished installing a S5 Jspec motor and tranny, retaining the NA harness. Car has an N370 ecu and matching boost sensor as well.

Now, the second you move the key from OFF the oil pressure gauge immediately spikes to max and stays there, even with the car off. On my last FC this happened because the tiny black box bolted to the slave broke.

It does this with or without the wire connected to the oil pressure sender. Normally id check the black box on the slave..but this car was originally auto, so its not there.

Anyone have any advice on where I can begin troubleshooting? Never had the issue after the five speed swap, but after the Jspec went in and i turned the key (never even started it yet) this problem arose. Any help would be appreciated!

 

check the single wire that goes to your pressure sensor (under the oil filter..it looks like a bell).That connector is known to get loose or corrode...or break,.

 

In addition to styx's suggestion...
The oil pressure gauge circuit also has a condensor, which should be bolted to the slave cylinder but migrates around as people work on the car over the years.
This condensor has a single wire connected to the harness and if that wire is unplugged and the harness side touches ground, you get the result you describe.

The gauge works fine without this condensor as long as its connection is isolated from ground.

 

the gauge will peg when shorted to ground also, so make sure it isn't rubbing on any metal or got pinched in the transmission during a clutch job.

 

Oh ya..I forgot about that..and Did it!..had to loosen the freakin bolts and pull it up!

 

That was my first thoughts. But in my first post I mentioned I checked those already. Being that the car was originally an automatic, there was no condenser on the slave..well, because there was no slave lol.

 

The condensor should still be SOMEWHERE, they all had them whether they were auto or manual. I bet you crushed or pinched it without noticing it and thats why your having the problem now

 

My car was also an automatic and it still had it, was located on the transmission top cover by where a slave cylinder would be. It's a little black box with a metal ring sticking out of it for a bolt to go through, one connector from the little black box to the harness. It's gotta be around there somewhere, or it's gone and you're touching ground as already described.

 

its probably inside the bellhousing now, ripped off with the wire pinched to the block.

 

Harsh, but potentially accurate.

 

The black box is not on the automatic transmission from the factory. Wouldn't be possible for it to be pinched there because the wire doesn't run that direction.

I pulled the NA motor and five speed tranny out together, disconnected the wiring harness beforehand. I'll have to double check the area near the slave and see.

 

pull the wire off the pressure sensor, if it drops to zero then you have a faulty sender, if it stays pegged you have a faulty condensor or the wire is shorting to ground somewhere.