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  #1  
Old 10-27-2005, 01:34 AM
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Low oil pressure indicator

I just got an 81 300sd and I was wondering if there is any kind of aftermarket low oil pressure light or buzzer I can put in it? My dad toasted his sd after he broke oil line and now I'm all paranoid about it. I spend half my time driving looking at my oil pressure.

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  #2  
Old 10-27-2005, 02:17 AM
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You should be able to pull off the electric oil pressure sensor and replace it with a T-fitting so you can also attach a warning light sensor from something like a VW Diesel. There are a couple of blank warning light positions in an SD gauge cluster. Hook up an audible warning as well.

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Old 10-27-2005, 02:22 AM
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It shouldn't be that hard if you don't mind cutting a hole somewhere for a warning light.
I'm sure newer MB's have low oil pressure indicators, and the sender for one of those could likely be used, or else one from most any other manufacturer. You would just need a brass 'T' fitting to put inbetween the factory line for the pressure gauge and the block. Finding fittings with the correct threading will be the only difficulty here. Also, ensure that the fittings will be able to seal with the factory ports.
Once the low pressure sender is in, all the ones I have seen work very simply. They just use a single terminal and serve as a switched ground for a wire from the warning light, so you supply +12V to the light, and run the other wire for the light to the sender. When oil pressure falls, the sender will ground the wire, and complete the circuit.

edit: I wouldn't use an audible warning - it would also be on whenever the key is in the run position but the engine is off. Like when you are preglowing, or waiting in the car and listening to the radio. Could be really annoying.
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Last edited by BluEyes; 10-27-2005 at 03:10 AM.
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  #4  
Old 10-27-2005, 03:27 AM
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thanks, I'll hit the junkyard tomorrow to try to find that vw part
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  #5  
Old 10-27-2005, 12:24 PM
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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.
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  #6  
Old 10-27-2005, 01:26 PM
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thanks for the help, I'll go hunt down those fittings.
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  #7  
Old 10-27-2005, 05:23 PM
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jbaj, does it buzz while you're waiting for the glow light to go out as the author expected?

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  #8  
Old 11-26-2005, 05:18 PM
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Late reply, sorry.

Yes, it buzzes quite annoyingly at glow time, or any time oil pressure isn't up.
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  #9  
Old 11-26-2005, 06:42 PM
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Oil cooler lines don't just blow, they leak for a very long time before they actually give way. If you just inspected them once in awhile and changed them when you see them starting to leak you won't have a problem.
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  #10  
Old 11-27-2005, 06:13 PM
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the line itself did not break or leak, it was apperantly shook loose from the metal fitting.
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  #11  
Old 11-27-2005, 06:17 PM
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Perhaps bad motor mounts resulting in excessive vibrations transmitted to the car that caused it?
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  #12  
Old 11-27-2005, 06:20 PM
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The mounts weren't too bad, but I think a previous owner might have cheaped out and had a hydraulic shop make him a new line with the same fittings instead of buying the OEM lines.
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  #13  
Old 11-27-2005, 06:27 PM
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Oh... that's bad. Indeed, the OEM lines aren't very cheap, but I guess people shouldn't expect parts from a Benz to be cheap as these weren't cheap cars when new. I too share your fear/concern for toasting my engine and would also like to see if I can get a low oil pressure warning of some sort.
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  #14  
Old 11-27-2005, 08:30 PM
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Yeah well if fixed properly the oil cooler lines should never blow. Neglect them or cheap out and well, roll the dice.

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