The oil pressure switch on my 1991 300CE, 178m went out. It started acting like a tachometer and then just sat at zero all the time. It would ocassionally bounce at start up but then right back to zero.

Replacement is pretty routine. Small hands are a help. It screws into the back of the oil filter housing by the fire wall with a crush washer. Only access is from below. You'll need a 17mm stubby wrench. If you are strong enough, a good tug will break it loose, but I put the wrench on it from below and popped it from above with an extension. You can try removing the wire on the back of the sensor first but this is very difficult and you run the risk of breaking the plastic clip. Thankfully the connection was designed so it can spin as you unscrew the sensor. So gently unscrew the sensor as you spin the connector and the wire will not kink. There is enough slack so that after the sensor is loose, you can take a good look at it and pop off the connector with a small screwdriver. Make sure to remove the crush washer as it may be stuck to the oil filter housing. Dry fit the connector to the sensor. My new sensor had an extra tiny bit of solder on the pin and dry fitting it ahead of time made the connectiion go on easier. To tighten I used an extension with a weatherhead socket on the end of the stubby.

After changing my IP delivery valve o-rings, my oil pressure stays at 3 all the time with key on. Is this the default if the wire is knocked loose from the sensor?

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You can't always get what you want, but if you try sometime you just might find you get what you need.

I don't know. Hopefully someone will chime in.