Just got done swapping chassis harness and having same issue as we were having before we swapped it. When I got the car the dash harness was hacked up because the previous owner tried to do a bullshit obd0-0bd1 swap. Problem we were having before was when turning the turn signals on or headlights the CEL would come on and would go off when turning the headlights or turn signals off. RPMs also weren't working & the temp gauge and gas gauge would go up with the headlights also. And now after the swap, the cluster acts fine without the headlights being on but when you turn them on the oil light comes on the RPMs drop all the way down and the temp gauge goes up. The only thing we have not swapped over is the fuse box. The car it came out of was a 90 si and we took everything including the light harness.

If anybody has any ideas feel free to throw them at me. Ready to get this car on the road and wiring issues have been holding me back the whole time.

 

When one circuit has an effect on another unrelated circuit it is a grounding issue.

Although circuits can be unrelated, eg. 12V constant circuit like head lights and switched circuits like signal lights they all share one thing in common, grounds, all grounds are common.94