W111 alternator upgrade

This last week my 250SE has been at the workshop having an alternator upgrade.   Back in 2020, my alternator gave up, and I replaced it with a rebuilt 35A alternator.    Ever since then, that alternator has struggled to charge the car.    It would charge the battery when driving along during the day, without any accessories.   But as soon as the A/C was running or the headlights were on, it would barely make enough power.   The charge warning light would come on at idle, and even faintly glow when driving.

The W111 alternator is supposed to be a 35A model, and the previous alternator had a 35A part number.   It kept up much better.     I can only assume that either the previous alternator was rebuilt at some point to make more power, or the new one wasn’t a good rebuild and was making less power than it should.    I had already replaced the external voltage regulator to rule this out.

That W111 alternator barely being able to keep up with the car’s power needs made me reluctant to take the car on long trips.   I also wondered if it had anything to do with the car’s recent running issues.   With the charge warning light on, was it even making enough power at idle for a strong spark?

W111 alternator

I decided to source a better alternator, and purchased a Bosch rebuilt unit from the USA.   Instead of going with the standard w111 alternator of 35A, I went with a 55A alternator.   This is a bolt in replacement that still uses an external voltage regulator.   The one I went with is used on the late 280S/SE W108 and the /8 models.     Other than a slightly different plug, you would hardly notice the difference.   I didn’t want to change the wiring to go with an internally regulated alternator.

The Bosch rebuilt units have a good reputation, and this W111 alternator looked almost new.

W111 alternatorAs can be seen in the picture, the rebuilt alternator didn’t come with a pulley.   When my mechanic was removing the pulley off the previous alternator, he discovered that it wasn’t assembled properly.    They had used the wrong pulley, and to make it fit, omitted the little key that prevents it from spinning on the shaft.   It had been tightened up, so wasn’t obviously slipping, but who knows what was happening under load?   My mechanic had another dead alternator with a good pulley he was able to use.

In any case, there was a huge difference in performance with the new W111 alternator.   Even with headlights on high beam, AC on max, I was still getting just under 13 volts at idle.   It would drop down into the high 11s before at times.

The one thing that it did not improve which I was hoping was the radio performance.   I’ve been having all sorts of strange behaviour with my radio.  I event sent it back to the seller to test and it worked fine on the bench.   I was hoping that a better and more consistent power supply might fix these issues, but it hasn’t.   More investigation to do there.

As well as the W111 alternator, the car also had a regular service and a tune.   Previously the idle was very low and it would be almost stalling in gear.   Now its driving really nicely – better than it has in a couple of years.   Its a really nice car to drive again.

While it was in the workshop, I also had the subframe mounts checked.   The front end feel is a little vague.   The subframe mounts made a big difference on my 450SLC.   The verdict was they were fine.


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