I have a 94 S500 with 173K miles on it. I changed the oil tubes last year. This summer I have noticed on longer trips at highway speeds over 75 MPH, with the AC on, and the engine temp over 95C, my oil pressure gauge goes to 0 and the red light comes on. When this happens, I take my foot off the gas, the needle jumps up to around 3. The engine purrs like a kitten, so I assume it is a bad oil pressure sending unit, or bad wiring when it gets hot. Oil pressure readings are normal for other everyday driving. Can anyone offer any different thoughts, and a procedure to change the sending unit. I know where it is. I assume I need to get under the car to do it.

Thanks

The 140 oil pressure gauge is partially just an idiot light. You might wish to review the operating theory. I think that if the oil press isn't to a certain level at a certain rpms then it does what you are seeing. This could be a sender.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Also the wires that connect to the sending unit are part of the lower wiring harness...failure of that harness can cause bad readings of oil pressure & OR oil level.

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Thanks guys. I will check the wiring, but I almost hate to touch it.