Okay, so I just installed my procharger and everything was fine up until a week ago.. Now my oil pressure gauge is showing 60 psi. This is the factory gauge on the dash. So I decided, maybe the sending unit went bad again so I got a new one.. No change. So I checked the resistance over the terminals on the sending unit. It's showing open circuit between any of the terminals. I checked this against the new sending unit and it too has open circuit. So I put the old sending unit back in. I checked the harness and one wire had the insulation burned off so I cut and spliced a new wire into that. So that's fine. I disconnected the battery just incase the oil gauge was a function of the computer. No change. So now when I turn the key to on, the oil pressure gauge moves up to 60 PSI and stays there.. I know there's pressure building so it's not a mechanical problem. and I know it's not generating 60 PSI in my engine.. When I take the key out, it will stay at 60 psi for a bit then it'll finally drop to 0. Anyone have any ideas? Please advise and thanks in advance.

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Bruce Chang
ICQ #2992488
89 Firebird Trans Am
305 TPI, ATI Procharger Supercharger, Ported SLP Siamesed Runners, Siamesed Intake and plenum, Edelbrock TES, Hotchkis LCA's and Panhard Rod, GMPP 1.6 Roller rockers, B&M Shift Kit, Spohn Subframe connectors and driveshaft loop, Flowmaster Muffler, Accel Super Coil, Crane Hi-6 Ignition, Energy Suspension motor, tranny and torque arm bushings

 

Oh yeah.. I checked the wiring and it shows, 12V, ~6V and ground so it would appear the wires are correct, no? Any takers?

 

Bruce,

Check the wiring at the instrument cluster. If the sender is new, and the tan wiring in the engine compartment is correct, the I/P may have a problem. If the tam wire is grounded somewhere between the sender and the I/P, the gauge will peg full scale as you describe.

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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
Adobe Acrobat Reader

 

An open in that circuit will cause the gauge to go to full scale, not a short. A short will cause it to go to 0.

Are you sure your sending unit is for a gauge, and not a warning light? Your description of it makes it sound like that. You can tell if you turn the key on with engine not running, and it goes to 0; then as soon as you start it up, it goes to full scale.

There are 2 OPSUs on most of these cars. One is a switch that works the fuel pump. The other is a variable resistor that works the gauge. I'd be inclined to suspect that you have been working on the fuel pump one, not the gauge one. One is behind the distributor, the other is in the side of the block just above the oil filter. The gauge one is a large can with one terminal, and should show a low resistance (< 100 ohms) with no pressure, like sitting in your hand; the FP one is smaller and has 2 terminals.

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"So many Mustangs, so little time..."
ICON Motorsports

 

Yeah.. I'm sure that I'm working on the correct oil pressure sending unit. I had problems with it two years ago and replaced it and it's been working fine since. Also the port on the backside of the engine has a plug in it and there is no sending unit there.. The sending unit I'm using is for both the fuel pump and the instrument panel. I know it's not for a warning light because there is a gauge on the dash so I'd be inclined to think the sending unit is for a gauge. Unless there's a warning light in there too.. =o)

 

I could be wrong, but I have never seen or heard of a unit with both the variable resistor and the switch in it...

The correct thing should be a can about 2" in diameter and 2½" long, with a single terminal, and a dark blue wire with a big round rubber boot on its connector. If you aren't seeing any continuity between any terminal and the shell, or if it's zero ohms, then you don't have the right thing, and it will never work right. Keep in mind, I'm talking about whether you have the sensor for a warning light, not an actual warning light.

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"So many Mustangs, so little time..."
ICON Motorsports

 

Yeah.. I'm sure it's both the gauge and the fuel pump sensor. It's got three prongs. My 84 Firebird had the pressure sensor on the backside of the engine so I know what you're talking about.. My 89 has only one oil pressure sending unit and I even compared a new one to the one I have now and the resistance between any of the three terminals shows open circuit so I'm almost certain it's not the sending unit, but who's to say.. i should ground it out and see if the gauge falls to 0. Hopefuly it's not the gauge itself.... Thanks for the advice.

 

Okay.. so I've checked the wiring.. we have continuity from the wiring of the sender to the instrument panel. I'm going to check the voltage when I start the car and see if it rises with engine speed. Then I'll know it's the actual gauge in the instrument cluster.. Has anyone had a problem like that and what did you do to fix it? Thanks in advance.