I have a 1984 300SD, the oil pressure has always been at 3 while driving, 2 at idle, now it is at 3 when I first start it and as it warms up drops to 1 1/2 bars. I pulled the wire off the sending unit, grounded it and it pegs the guage. I replaced the sending unit with an unknown one off a parts car that had been run out of oil and water. They both behave the same 3 at startup and 1 1/2 bars when it starts to get warm, the temp guage hasn't even started to register yet.
I measured the resistance of the old sending unit = 8 ohms and the replacement measured 28 ohms. The original one rattles the replacement does not.
Do any of you know what the resistance should be on a NEW one?
Oil is Mobile 1 syn 5w40 and that has been in for 1000 miles behaved OK with 3 bars.
Any Suggestions?
Gary
OBK 37

The performance of the engine.........and the gauge.........is normal.

When you increase rpm's to 1500 or more, the gauge should again peg at 3.0 bars.

You're simply observing the drop in pressure due to the warmer oil at idle. The manual allows idle oil pressure down to .3 bars. You're considerably higher than this.

You'd probably need a new sender to get a completely accurate reading. My old one was reading 0-.3 bar at warm idle and it took over 2,000rpm to get it to 3bar. New one shows 1.9bar at warm idle, and it pegs to 3 instantly with throttle.

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OK here is what I have done. 1: Changed out the sending unit with an unknown replacement ( dead parts car). 2: Tested the Oil Pressure Guage by grounding the wire off the sending unit. The connector felt really loose so I opened it and it was hanging on by only 2 strands of wire. Cleaned it all up and resoldered it. Then the guage would then peg at 3 bars. BUT after the engine ran for two minutes the guage would drop from 3 bars to 1 1/2 bars and would not rise past 1 3/4 bars... I'm not happy! 3: Pulled the instrument cluster out and connected up the parts car instrument cluster. My car is an 84 300SD and the parts car is an 83 300SD, one of the connectors is different on the parts car. Ran the engine for 5 minutes and the Oil Pressure guage did not move off the 3 bar position. Took it for a 15 minute test drive and its back to normal 3 bar + while running and 2 bar at idle. Just thought you might be interested. Grounding the guage is not the final test. This has not been a good 2 weeks for me as my Ford F250 Diesel's injection pump took a vacation and then the Benz strated acting up, but I'm much better now. I no sooner got the Benz working and Seattle Injector called to let me know that my injection pump is rebuilt and ready for pickup. yipee skippy
Gary
OBK #37