Oil Pressure Light 1981 US 320i

[flyingbrick13]

Greetings from Portland Oregon USA!
I searched for old posts about this but did not quite find what I was looking for.
Also, as a new user, I'm excited to interact more directly with this forum and decided to make a new post.

The car:
1981 US 320i with the stock oil pressure idiot light, no oil pressure gauge.
275k miles
no major modifications
no recent work done

I bought this car a few weeks ago and am waiting for parts in the mail for a thorough tuneup and cooling system refresh.
In the meantime, driving it has been a real treat.
As a BMW motorcycle guy, this is my first BMW car and REALLY like it.

Starting the car the other day I was alarmed to see the oil pressure light settle to dim after being bright at startup.
I was concerned that the light was indicating actual low oil pressure since it did not turn off with revving the motor.
I turned off the car as a precaution.
The oil level was fine and the oil itself looked ok.
I suspected the sensor and unplugged it to clean the connections.
After restart,the cause appears to be the light circuit grounding out at some point because the light stays dim even with the sensor unplugged!
That felt like a huge relief!

Do you agree with the above diagnosis?
If so, here are the questions:
1.) Where does the oil pressure light circuit ground to?
2.) Any common grounding problem points for the oil pressure light circuit?

At 275k miles, I'd prefer a mechanical oil pressure gauge to the idiot light
3.) Any hope of sharing an existing hole through the firewall for a mechanical gauge hose?
4.) If sharing a hole is not realistic, any recommendations on where to drill one?

Thanks much for any input or comments!
Cheers and happy driving
-Marcus

[uwbuurman]

Hi Marcus! Welcome here! Care to share some pics of the new adition?

I ran the cable through the rubber cable sleeve behind the fuse box, in fact along with the wire harnas.

My guess is that the idiot light is shorting in the gauge cluster.

[flyingbrick13]

Thanks for the input, uwbuurman
Appreciate the welcome.
I put some pics in the registry:
viewtopic.php?f=53&t=27971

Perfect, behind the fuse box with the harness, I'll try routing the gauge there.
I'd like to forget about the idiot light but will need to eliminate the unintended ground, even with a gauge.
It'd be silly to have the light on dim with a gauge operational.
I've got some books on order for servicing the e21; however, what's a good way to get to the gauge cluster?
Pull the panel under the steering column?
Then what?

Thanks again, appreciate any direction you can provide
-Marcus

[BertjeConti]

Welcome here,

The oil pressure circuit grounds in the oil pressure switch self, you have only one wire to the switch.

If the idiot light stays dimm there must be a resistance to ground in the wiring from pressure switch to the light.

In fact if the light stays dimm when the wire is disconnected from the switch you won't have to worry about low oil pressure or defective switch.
The idiot light goes on when the engine isn't running, thats also a control mechanisch to check if the light and switch are OK.

[flyingbrick13]

Thanks for the replies. The pressure gauge wire fit easily with the main wiring harness, excellent advice uwbuurman . It's nice to have actual data. ~45psi (~3.1 bar) normal when running?

[uwbuurman]

Great! Happy to help!