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.

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I find that whenever I buy one of these older cars, chances are the distributor cap needs changing.   It is not generally an expensive part on the mainstream engines, and can make a big difference to the smooth and reliable running of the engine.  An old and pitted cap is going to produce a weak, inconsistent spark.    I normally just purchase a cap and rotor when I buy the car.  On the off chance it doesn’t need replacing it is good to have a spare.

My 86 300SE is no exception.   The cap wasn’t in terrible shape but it was clearly worn and ready for replacement.  The contacts were worn and so was the rotor.

The M103 distributor cap differs from the other cars because of how it connects to the camshaft.  To remove the cap, there is a cover for the plug wires that snaps off.   Next, three 5mm allen bolts hold the cap to the engine.   Two of them are easy to see but the third one is underneath the cap and only just visible from the left hand side.   I found it easiest to remove it with an allen socket and a small extension.   Using a 1/4″ ratchet made it easier to clear the Serpentine belt.   I used an adapter for my 3/8″ socket into the smaller ratchet for this purpose.

It is easier to remove the plug wires first.  I found there wasn’t clearance to remove the cap until the plug wires were removed anyway – the fan gets in the way.    The actual distributor cap has a black cover that provides the mounting points for the plastic cover and splash protection for the ignition wires.   On the surface, mine looked ok but on closer inspection I would have preferred to change it.   It has a couple of small cracks in it.    In the picture above, the old M103 distributor cap on the right still has the cover attached.

Once the cap is removed, the rotor requires 3x3mm allen bolts to remove it.   In my case, one of the bolts was rounded off.   It was quick easy to remove with vice grips, at the expense of breaking the old rotor.

I gave the inner plate as good a clean as I could.   Ideally I would have replaced this too.  As can be seen in the picture above it has seen better days.

In total, the job took about an hour to do, including the rounded off bolt.   I had not changed an M103 distributor cap before, so it would likely be faster next time.      Note, that the M104 engine is very similar.   The M104 is basically a DOHC version of the M103.  More power, but also even more prone to eating head gaskets.

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