Hi everyone,

This is my first forum post on this website and before I ask my question, I just want to thank everyone here for their time and effort in trying to solve people's problems.

I have a 1991 Mercedes 300E, it is very lightly used and currently has around 55,000 miles on it. Recently, I have been experiencing a problem with my vehicle. Hopefully, somebody on this forum has experienced the same issue with their car.

So here goes: while in traffic for longer periods of time - with the gear in drive - the tach will start to oscillate up and down very fractionally around the idle RPM. The tach will reach 650RPM then drop to 550RPM and then start bouncing back and forth. A small amount of time later, the speakers begin to cut out (this has no effect on the actual stereo display) and then the air conditioning blower motor begins to cut out. Just to note, I refilled the AC system and had a thermometer in the vent. While experiencing these issues the vent temperature rose significantly (obviously meaning the components of the AC system were not receiving proper power.) After this, the SRS warning light comes on, and then so does the ABS antilock break light.

However, once the car begins to drive again, all the symptoms instantly disappear. My first thought after reading extensively on this forum was that the alternator brushes were to blame. (According to some people, 50,000 miles is where the brushes tend to wear out. My car being at 55,000 miles this made perfect sense to me.) Obviously it's worth trying new brushes before replacing the entire alternator, as it brings the cost of repair down significantly.

Unfortunately, after taking it to a garage, they said the only thing they could do to help me was replace the alternator. "They needed to warranty their work." At this point, I let them replace the alternator with a Bosch premium rebuilt 70A alternator.

A few days later, I experienced the exact same issue with the car. I brought it back to the garage, and they told me the alternator was failing when hot. They replaced the alternator once again.

Now, I am still experiencing the exact same issue - with one stranger quirk. While the tach is oscillating up and down, it also spikes for moments to 1000 RPM. I did not notice the engine revving or running roughly whatsoever.

This problem does not happen constantly and cannot always be reproduced. I took the car to a Mercedes dealer, which managed to replicate the problem. The battery voltage was normal when there was no load on the system. However, when the AC system and radio was turned on, the voltage actually reached a low of 11.81V. When I took it back to the garage I was originally at, they have been unable to reproduce the problem and have told me they were unable to reach a voltage of below 13V.

A couple more things to note in case they matter:
- This is a Canadian car and therefore has daytime running lights
- I have recently replaced the NA headlights with a pair of used Bosch European headlights - this is the only modification that has been done to the car.
- The stereo and all components of the stereo system are completely original.

I am leaning towards thinking my solution at this point is to install a larger alternator, as many people have done on this forum. (Like the 143A and 150A alternators found on some S Class models from 1996 to 1999.)

However, I'm not completely sure obviously. Again, I really appreciate everyone's time and effort in helping me solve this situation. It means a lot to know there are people who care about these cars and how to fix them when things go wrong!

Thank you everyone for your help,

Winston