+1

Wow, lots of ideas here, I too have been considering creating such a device for my beast.

Someone mentioned amperage concerns, that can be solved by using relay contacts, operated by the sensor.

Someone mentioned wanting it to be a normally open switch that closes on fail, that's not a good setup, a broken wire or corroded contact will let you down. I deal with a lot of safety sensors and switches at my job, when designing the setups, you want them "failsafe", which means you want to supply power to the PLC input (or whatever device you use) until a failure. This is because almost every circuit failure you run across is a loss of voltage to the device, it is extremely rare that you might run across a stuck on relay contact or the like. You want to have a false alarm rather than not having the alarm when you need it.

On the keeping it quiet at startup issue, you can use a timer relay inline with the sensor circuit, preset for the amount of time it takes to build oil pressure, then closing a NC contact across the sensor circuit (you want to get a relay that times "off", meaning it is powered on to hold open and de energizes to close, remember "failsafe"). use the other NO contact to power a spare dash light in a blank idiot light hole, to let you know the system is working (It will light until the timer cycles off).

My only contemplation is whether or not to include something to shut down the engine after the alarm has been on for a given number of seconds, I could see this saving the engine, but also could see it becoming a safety concern to the vehicle/driver in certain road/traffic conditions. I think this is why the automakers do not have that feature.

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.

Here's a low oil pressure/high coolant temperature kit. Do people think it could be adapted to our engines?

http://www.moyermarine.com/cgi-bin/shopper.cgi?search=action&category=KTAS&keywords=all&template=Templates/B000_storebuilder.html

I will be having a custom turbo oil supply line made for my new turbo, I will see about having an extra T for an oil pressure sender added.

Surely we can find a 12 volt buzzer which does not require a relay.
A timer is just another thing to fail.

If you are referring to my post about the switches I was talking about two switches... the oil pressure switch is on with no oil pressure... but is hooked in line with a switch at the operator station who presses a normally ON switch to OFF to keep the buzzer from buzzing until the engine provides enough oil pressure for the oil pressure switch itself to take away electricity from the buzzer... this is the simplest system I know of which causes the operator to either hear or act to not hear the buzzer each time they start the car... a fail safe system as far as causing the operator to test AND fail safe as far as the switch configuration.

Indeed a buzzer can be had, some had posted however that they didn't like buzzers. It's a matter of preference.

I see your line of thinking with your setup, it seems failsafe, but the circuit still relies on the integrity of it to conduct and the contacts to make for it to sound the alarm, so what happens if it works @ startup, and 2 miles down the road you hit that little road crack, where that frayed to one or two last strands wire fails completely? Fail safe means it alarms or halts a machine with a loss of power to a holding circuit or PLC input (safety circuit would fail into a safety mode), so if the circuit failed it would falsely alarm, but would never fail to warn you in a time of need. There's really several ways to do this, pending what you want to spend and what your preferences are, I was just simply putting in my views to add to the idea pool.

Engine shut-down seems foolish to me. One part goes bad, one wire gets eaten by the squirrels, etc... you're in traffic, barely staying alive on the Atlanta interstate, your OP gauge is reading 3+ and you're happily cruising along, the wire falls apart, ten seconds later you're a sitting duck trying not to get run over.... eek.

The warning signal needs to be audio in nature.. that is why planes have speakers saying ' pull up'... instead of a light on the dash...

I have no idea what you are objecting to .... no system will work if the wires are cut. At least my suggestion causes the system to be checked regularly... any safety person knows that getting people to do the right and sometimes boring thing can not be counted on. My design makes a noise until the engine gets the oil pressure up. This causes the person to know it is working at that time... a vicious squirrel under the hood that intentionally disconnects it will certainly defeat the system.
To have it fail safe as far as warning that the electricity has failed to make it to the warning system might include a magnet which holds a steel ball off a contact... then if that squirrel is successful taking electricity from the oil pressure circuit you will know it... but if that squirrel attacks the backup system you are still out of luck.

I do not suggest that the warning system turn off the engine by itself...
Even that Perkins example earlier assumes the vacuum pump/lines/diaphram are working at the time the oil pressure goes down.

Physically low oil level is way harder to design and build and install compared to oil pressure... which can be tapped at several places....
Low oil level almost has to be inside the pan... :eek:

What about the already built-in "low oil level" dash light?

If you already have one you don't need another one....
LOL
If the factory put one in...then you have already paid for it...and they made provisions for it fitting into some space...
After the fact working is very different...
And inside the pan is a very harsh working environment.... for something you really want to be able to count on...

Just realized I was assuming too much. :sleep1:

My 2nd gen w126 came from the factory with an oil level sending unit built right into the oil pan, and an orange light beside the "low coolant" light for "low oil" - I forgot that w123 and first gen w126 might not have it.

Yes, you must keep in mind that you are mostly watching your ' poor cousins' talking amongst themselves .... :D

*sigh* you just get used to what you're used to. No other implications intended.