M103 Oil pressure sender

A few weeks ago my 300SE started to show low oil pressure on the gauge.  It happened all of a sudden while on a drive.   At the time, I surmised that I had a dead M103 Oil pressure sender.   They are apparently a common fault.   I ordered a new one and fitted it today.

The M103 oil pressure sender can be found on the side of the oil filter housing at the back of the engine.   Most of the threads you’ll find on the various forums are referred to the W124.   Apparently on the W124 space is tight and it is a paint to remove the sender.   I can only imagine how hard it must be on the W201 then.     On the W126 there is loads of space, as can be seen in the photo.

With the right spanner, you could probably remove it with the oil filter in place.   I elected to remove the oil filter first.   It made it dead simple to disconnect the wire, and then unscrew the sender unit with a 17mm spanner.    As can be seen in the photo below, there is loads of room without the oil filter in the way.

I went with a new VDO sender.  VDO are the OEM brand.   There are also Meyle sensors availabe, which are much cheaper.  Meyle is a brand I try to avoid.   There was somebody on one of the facebook groups complaining they have replaced two sensors in as many years.  I bet they used Meyle.   Comparing the new M103 oil pressure sender, the original one was still on the car.   It was dated 7/86.   I put a new 9/19 unit on.   Still made in Germany.

Re-installation is even easier than removal.   There is a little washer that goes back on.  In my case it had remained stuck to the housing.   The new one is certainly a lot cleaner!

I then fired the car up.  I expected to see the oil pressure reading jump to three.   But it didn’t.   Instead, the pressure was the same as before.   Around the middle of the gauge after a cold start.

Clearly there was nothing wrong with my older sender.   I think my next step will be to take the car to a mechanic to hook up a mechanical oil pressure gauge.   This will eliminate the wiring, gauge etc and check the actual oil pressure of the car.

I can think of a few possibilities in order of likelihood.

• Oil pressure relief valve stuck
• Faulty Gauge
• Bad wiring to gauge or sender
• Worn Oil pump

I think the oil pressure relief valve is built into the motor on the M103, so not a quick change to see what happens.  The mechanical gauge will at least determine if there is a problem.