Don't remember the size. It wasn't in my kit. Think I found it up the street at NAPA. Just take your sender and your gauge connector to a real parts house so they can match up what you need.

Hmmm
 

I have seen a two Bar variation between new and no O-rings.

I just took a look under the oil fill cap and watched the cam oil squirters and it's getting plenty of flow. That's at idle where the pressure gauge reads zero or close to it.

So I take it that the gauge reading is off or I wouldn't be seeing that.

But it'd be a good thing to fix that anyways. I noticed the tach isn't working either so I may have gauge electrical issues.

Those (85's) have an OVP relay, I believe it's with the rest of 'em under the fuse cover. Check the one with a red top, there is a fuse that may have blown. You might get your tach back.

The tach inop is most likely due to the tach amp issue. The fix involves unscrewing the amp cap (located on drivers side inner fender)-pull the plug out of the cap and place a shim between the cap and the plug. Then reinstall.

^ MB changed that system in 1984, no more amp.

The 85 also has a KLIMA relay in the mix, but I do not know it's location.

Answer
 

1985 300SD
In the rear of the fuse box. :D

Also chasing zero oil pressure at idle issue with OM617 in W123. Mechanical gauge with oil feed line to gauge. Oil pressure hits full off idle above 1000rpms.

I believe I replaced the o-rings in question at last oil service a few months ago. I can replace them again just for kicks.

Any other suggestions? oil pump? drive chain? (I cant see how the chain would cause this.)

To the OP with 300SD, if indeed you have an electronic sender, replace that. When they age, they read lower and lower.

Renntag, you'll want to "second opinion" the oil pressure gauge as well, feed comes from the oil filter housing. If the pressure is at 0.5 bar at idle (is your idle speed too low?) and reaches 3 bar by 2000 RPM, you're OK.

Could still be sub standard gauge in dash. Remember the normal pressure above mid rpms is always pinned. That is caused by pressure higher than the gauge can read.

Just because your gauge is touching the third mark at higher speed does not mean that it is not still reading low.

As mentioned before by Hunter you have already changed the old oil rings on the filter stem. Or at hot idle have a quick look to see if oil is still being supplied at pressure through the oil filler cap hole. I went back and see you have done this.

The electrical oil pressure sensor is highly suspect from your description as already mentioned I believe.

what size o-rings are on your filter?
should be 6mmid, 2mm wall, and 7mm id, 2mm wall.
EVERYBODY keeps saying 6mm is right for both. I disagree. the 6mm is stretched really thin on the upper groove.

Listen to VsTech first... be sure those are correct AND that the tiny hole down the center of that tube is clean and open.... critical to oil everything in these engines...
Then if there is still a problem one thing the FSM says is that the screw in oil pressure valve on the back side of some of our engines can fail or even fall off... this might allow the symptoms you mention.. 2 at high rpm.. but 0 at idle....
When you do replace it be sure you have the latest recommendation.. there are at least 3 designs or pressure changes over the years... all shown in the FSM of course..

I have only used the o-rings that come with my filter. Where can I source just the two you mention?

The sender thread is actually M10 straight threads (like a machine screw) with a crush washer for the sender but 1/8-28 pipe thread will fit, it's almost the same (nice coincidence), use Teflon tape and don't screw it in too tight.

If there is room, you can tee the line and have the electric gauge plus a mechanical back-up gauge. I did that in my '87 300D (W124, OM603). Different chassis and engine but the electric gauge sender is the same as the 300SD w/617 engine.

Jeremy