This from an earlier thread:
Here's from an old post. I did this a while back and it still works fine. May help some or at least give you some ideas.
"NO/LOW oil pressure warning system on 300SD 1984. This is what I did, you may have another way.
Usual cleanliness, safety warnings apply.
If you do this wrong, your oil WILL run out of your engine and ruin your day!!!! Check all joints and connections on the bench FIRST!!!
Obviously, YOU need to verify ALL before proceeding. I hope this helps, but I can't be responsible. (yada, yada yada, lawyer, yada, atty., yada, yada).
Parts:
Radio Shack 12v. buzzer (cheap)
Radio Shack red LED (with built in resistor)
12mm X 1.5 male X 1/8" NPT female adapter (steel)
1/8" NPT street "T" (has a male with two females)(brass)
1/8" male X 1/4" female adapter(brass)
mid '80's Audi or VW (water cooled) oil pressure sender for idiot light) (~6 psi. ?) (cheap)
Tools:
1/8" NPT tap
1/8" NPT die
12mm.X1.5 tap
Try here for fittings
http://www.fittingsandadapters.com/
If you have access to machining tools or can find other appropriate METRIC fittings (I looked everywhere for other metric fittigs and found nothing that fit this purpose) you can do this much more easily. A metric street "T" would make this nice, but it eluded me. Maybe someone has a source?
1. Remove the oil pressure sending unit from your car with a pan underneath. You will drip out ~ 1/2 pint of oil, then it will stop.
2. Braze (I soldered with a torch and plumber's solder) into the threads (small thickness of braze or solder) of the female side of the 1/8 male X 1/4 female adapter. Now retap that side with the 12mm. X 1.5 tap. You'll be surprised how close they were already. This is where your original Mercedes oil sender will eventually go, so be attentive.
3. Tap and die all 1/8" NPT threads in the "T" and adapters with the 1/8" NPT tap and/or die to clean them up and deepen the amount of "bite" for security. You'll see what I mean when you dry fit them.
4. The Audi/VW sender has a 10mm thread. Take your 1/8" die and run it over that. Yes,... rethread the 10mm. on the Audi sender and make it 1/8" NPT. They are SOOO close to start, after you run the 1/8" NPT die over it, they match quite well.
5. Assembly:
a. 12mm. X 1.5 male X 1/8 NPT female adapter goes into the oil filter housing (use an aluminum crush washer)
b. male of the street "T" goes into female of that adapter.
c. Audi/VW sender (rethreaded to 1/8" NPT) goes into the side branch of the "T".
d. 1/8" male X 1/4" female (brazed and retapped to 12mm. X 1.5 thread pitch) adapter goes into end of the "T"
e. Original Mercedes sender goes into the retapped end of that adapter (use an aluminum crush washer). Reconnect original wire.
f. Put a new wire (with spade connector) to the connector on the VW sender and run it to the buzzer. Connect other side of buzzer to a "start/on" hot lead. Connect the LED in parallel with the buzzer and place buzzer and LED where you want to.
I have all of the hardware done on my car and am running around to check and recheck stability and for any leaks. I have not hooked up the buzzer yet, since I have a few things to do at the same time to my instrument cluster. (my modified gas engine inst. panel with economy gauge as a boost gauge for the turbodiesel needs calibrating). The buzzer may get annoying since I expect it to buzz whenever the key is on without the engine running(oil pressure less than 6 psi). We will see if I go with only the light long term. Since 6psi is the same as zero, as far as I'm concerned, it should work to warn of a pesky ruptured oil cooler hose, broken filter canister bolt, road rage bullet through oil cooler (this is SoCal after all) or what not.
FWIW that's what I've done.
Edit: Hooked it all up with 0.3 bar VW switch and it works perfectly. LED right in between gauges and buzzer is unmistakeable.