Hey Guys,

Finally getting the car back together and everything is working great. Except...the oil pressure light. It doesn't illuminate at all. Here's what I did and what I tried. I'm hoping the brain trust can solve this riddle.

1974 2.0 non-appearance group with combo gauge.

What I did...

– The oil light was working when parked back in October.
– I removed the gauge cluster to do the LED upgrade, but did not remove wires to the oil pressure light socket. Just left it dangling with the wires attached.
– Installed new ignition harness from Jeff Bowlsby.
– Installed new oil pressure switch from Pelican

What I tried...

– Removed wire from pressure switch and directly grounded it, bypassing switch. No light.

– Check continuity between oil pressure wire on harness and known ground using multimeter. When ignition is on ACC, continuity is 1, ignition off it's 0.

– Remove sender lead on back of light socket, ground that side of socket and light works, so bulb / socket is good, and power is being fed to it.

– Removed and re-seated ignition harness plug. No change.

Also...on my car, the oil pressure light is fed by a red/white power wire, and what looks to be a solid black wire. From what I can see, the 74 wiring schematic shows that the sender wire is supposed to be red/green, but it doesn't appear as if this black wire has been spliced in. The terminal and boot looks original like all the others.

All other lights (gen, brake, blinkers, etc.) work normally.

Any thoughts or suggestions? I was thinking I might try reattaching the old ignition harness to see if that helps, but the continuity test seems to show the new harness is ok.

Thanks all!

Do you know which wire is the power wire and which is the ground wire? In theory, the power is the red/white and the ground is the green/red.

Check that one wire has +12V when the key is on, and check that the other wire gets grounded when the engine is off. If both of those are present, then the light should light up. If it does not, the problem is in the bulb or the light socket.

--DD

Thanks Dave. Yes, the red/white wire /spade has 12V and I confirmed the bulb/socket works by directly grounding the opposite spade. But when connected normally, the light doesn't illuminate, even when the wire to the pressure switch is directly grounded. Very strange.

I'm not great at reading schematics, but it looks like the top left pin (1) on the ignition harness switch is where the pressure switch wire attaches. And pin 5 on the main harness (top row, 3 from right) is where the wire from the gauge terminates. If someone can confirm...or offer any other suggestions, I'll try tracing those tonight.

TIA

Check continuity of black wire at the light socket all the way back to the other end of the wire harness where it connects to the relay board, then confirm how it travels through the relay board pin to pin for continuity there, and then check the wire that runs all the way to the pressure sender. Must be a break in the wire.... since light is good and test seems good on new pressure switch.

Have you done anything with the seat belt relay under the passenger seat? The oil pressure wire makes a stop there.

In that case, it sounds like your pressure sender wire is not the wire that is plugged into the light.

--DD

Thanks Dave and gang. I never disconnected the wires from the socket when I pulled the (working) light from the gauge. I’m going to try and decipher the schematic and check continuity. Will report back.

Hi Guys,

I was able to trace the problem to a lack of continuity between the seat belt interlock block and the gauge. I'm guessing the PO replaced that part of the circuit with the black wire that failed somewhere. I installed a new wire (with the right color markings) and all is well. thanks so much for the help!