250SE Alternator replacement

On the way back from the September night drive my 250SE stopped charging properly.   I was able to make it back home about an hour on the battery.   Even using the headlights the 250SE does not use a lot of power.   As it happened, a friend of mine had a rebuilt alternator that was correct for my car.   He drives a 280SE 3.5 W108 which uses a 55A Bosch alternator.   The earlier cars (to 1969) used a 35A alternator which is what he had.   This is the correct 250SE Alternator.

Initially I assumed I would replace the alternator myself.   It didn’t take long to change my mind.   The alternator is inaccessible from the top due to the A/C compressor.   It is hard to get from the bottom due to the air filter box too.   I decided that this was a job for the professionals and I am glad I did as it turned out to be a 7 hour job.  The 250SE alternator is just visible underneath the compressor.

Additionally, with the alternator out, it was apparent that a nearby coolant pipe had seen better days.   It was a good opportunity to have it replaced.  Its amazing it wasn’t leaking like crazy given its state.  There was also some wiring replaced at the same time.

I kept the old alternator.  Parts are getting hard to find for these cars now and it is likely rebuildable.

As well as replacing the Alternator, I also replaced the regulator.  I had hoped to buy one of the Bosch solid state regulators, but I think they are no longer available.  I could only find one vendor selling them and they wanted almost $500USD for it.   In the end I got a Beru unit.   The voltage regulator in the car was a Bosch points type, but was not the original one.   It looks like it was originally for another car as a small separate adaptor was needed to plug it into the cars wiring harness.

The old one was still working, but I am getting better voltage out of the new one.   Curiously, one of the pins was not connected on the old regulator.   I’ve had the car for more than 8 years and it was working, so not sure the impact.   I’ve been able to plug the new regulator into the main plug.   As with the 250SE alternator, I will be keeping the old regulator and that extra harness as a spare.

The rebuilt alternator and new regulator make good voltage and 35A seems sufficient to power the car – even with the A/C which is not original to the car.   There are no ECUs or other power hungry accessories on this car.   The fuel injection is mechanical, so the only power draw required to run the car is the fuel pump and ignition system.   The other main accessories are the car’s lights, the radio and the A/C.