So, troubleshooting the Oil pressure idiot light.
Its supposed to be lit upon key on pwr.
once the eng. is fired the pressure in the galley causes the switch to ground out, extinguishing the light.
or some such....
Anyway the 74 2.0L Im working on , indicator does not illuminate when key on.
Easy to figure out you'd think.
1st pull the gauge to check if the bulb is burned ..nope.
Test the source pwr (white w/red wires)... yup
Test continuity end to end of the sender wire....yup
So I wired up the circuit with independent pwr. ... works like it supposed to
Put it back together... it starts working
Ok great next up ...wait ..couple of ignition cycles & it stops again

Check fuse 9, replace for good measure & clean the terminals.
Im thinking there is a ground issue somewhere ... so I check ground point A, loose.
Ground point B, loose.. check all the rests clean & tighten just to be sure.
No change..
So this car still has it seat belt interlock & fasten seatbelt light ..that work.
But I check the ground spade there & clean, it all looks surprisingly tidy.
But still no joy!
I need some BTDT help...anyone?

Did you replace the bulb with a new one? (if not just do that to eliminate the chance of an old wonky bulb)

I think you meant when the engine fires the oil pressure switch opens breaking the loop and the light goes out.

Below is the simplified schematic of the OIL light and the oil pressure switch. If the light isn't turning on with key on/engine off, pull the OIL light and measure voltage on both terminals. One should be +12V and one should be zero. The one which is zero is the one to the oil pressure switch.

The OIL light terminal with zero volts goes to the oil pressure switch. Measure resistance to chassis on this terminal. Then start the car. The resistance should go from zero to infinite when the engine fires.

Let us know what you find.

Remove the terminal on the OP sensors/switch and scratch it to the engine block. If it lights up then the wiring circuit is OK; replace the OP sensor/switch.

If it doesn't light up, start tracing wiring.

Thanks Spoke that is what I implied.
My comment above glossed over the detail you list, I did measure the positive voltage on the supply side but didnt check the sender wire for the change to open upon starting. Ill be checking this.

If you are starting and stopping the engine repeatedly, there is enough pressure in the system left over to keep the lamp out when you put the key in.

The stock sender has a really low threshold for "low oil." 2-4 PSI depending. It does not take much for the engine to be retaining that pressure if you restart the engine right after shutting it down. Especially if the oil is still cold and thick.

zach

Thanks zach but that didnt turn out to be the problem ..
SOLVED it was a bad sender unit.
got a two'fer as it was loose in the case so it may have been an known issue.
Appeared to be causing an oil leak as well, the top molded piece with the spade connector
could spin in the housing ...DOH