New 300SD!!!! Oil Pressure gauge reads ZERO at idle and 2 at WOT. Where to start?
 

I just picked up my first W126 for $250. It was purchased "as is" because it wouldn't shift past 1st gear. Unplugged the kickdown switch and that was fixed. then a few other issues (no alternator, etc).

But now that that's all working I finally got it home (1 hour plus drive). Anyways, I noticed the dash gauge says ZERO on the oil pressure at idle and only 2 at 3-4k cruising speeds. So... that's not good. The car was run on SVO (stock fuel system). Anyways, where do I start. My guess is to second opinion the oil pressure (i.e. make sure the gauge is telling me the truth). But not too sure how to go about that.

thanks for any ideas.

I'm sure you've already checked the oil level...

If the pressure really were that low, wouldn't the engine have self-destructed or overheated by now??

As long as the oil level is normal then you probably have a bad oil pressure sender.

X2, I would guess that the thing would have been long gone on a 2 hour ride if there was indeed no oil pressure. Therefore, the problem would most likely lie in the oil pressure sensor and/or the gauge itself. As an extremely cheap car, I doubt you'd want to throw much money at it though so perhaps the best course of action would be to do a compression and leakdown test provided you have the tools to diagnose the health of the engine. If you can grab a used cluster out of the junkyard you could eliminate that variable (and OP sensor too, if you're willing to get dirty at the PNP). Who knows what damage the WVO could have done internally--this is quite a gamble, but what a fun one!

As far as the alternator, if your car still HAS one on and isn't functioning, check the voltage regulator held on by 2 screws in back of the unit--9/10 times that is what goes. If you do have to buy a new alternator, I would recommend upgrading to the bosch AL129X for more amps (search "AL129X" on here for more info on this) and it costs roughly the same.

Answer
 

I assume the 300SD is 1981-1985 OM617...

If that is correct, your starting point is the two small O-rings on the oil filter cover dip tube.

This is a common issue overlooked during oil changes...

When the two small O-rings become rock hard and/or break/crumble = the oil pressure reading drops/reads low.

Seal ring, x2
MB# 015 997 94 48

Fastlane: Oil Filter O-Ring
http://catalog.peachparts.com/item.wws?sku=W0133-1644350





Have a great day.

Well it's not ZERO while driving but it is while idling. It's around 2 at cruising speeds (most of the way home). So it hasn't had zero oil pressure. Idling has only been a second or two.





Thanks whunter and yes this is a 1985 with a 617. So when the o-rings fail to seal, does the actual pressure drop or just the gauge display?

I have to say, I have never seen the little canister o-rings affect the oil pressure by more than .1 bar between being old and brittle and fresh.

When I had the same problem a new pressure sender fixed it.

If it were me, I'd attach my mechanical gauge and see what the actual pressure range is and go from there. Pretty easy to do on these cars.

That's what I was thinking. Where do I tap into the pressure at? I've got gauges for various things so I'd think one of them will have a range that will work.

maybe you can just remove the electronic sensor & connect there.

I thought the pressure gauge has an actual fluid line through the firewall to the oil filter housing (not an electrical sensor)?

I've had oil leaking at the cluster in past MB Diesels.

As far as fittings what am I looking for to connect to a gauge?

The SD set up has an electrical connection, no oil line to the cluster. On the lower back of the oil filter canister is where you'll find the sending unit.
Don't know of the connections necessary.

Yep -- pull the 'lectric sender at the oil canister and attach yer gauge there.

Ah ha... ok, good to know. I am used to the actual line on the 300D models I've owned. I wasn't aware that the SD's used electric gauges for the oil pressure. Thanks!

Lux, any idea what kind of fitting I'll need to check the pressure? Not the standard 1/8" pipe thread fitting most US made cars use I'd guess?