Alternator Voltage Spikes

Alternator Voltage Spikes

>A little while ago, a List member wrote of dramatically-fluctuating
>voltmeter readings, with lights going bright and dim and wipers speeding up
>and slowing down. At the time, I had a similar problem which appeared to
>ease when I cleaned some earth (ground) connections. However, the symptoms
>returned along with a new problem: charging ceased altogether and the
>battery would become flat in a day or two, with the car standing with no
>lights left on. The discharge seemed to be occurring through the
>alternator. I suspected burnt-out diodes, and this was confirmed by the
>electrician who fitted a new diode pack. He would normally have fitted a
>new regulator as well, but couldn't source one in the UK.
>BUT, after one day's use, I am now back where I was, with no charging and
>the battery discharging itself - presumably the new diodes have met the
>same fate as the old ones.
>Before I remove the alternator again and get yet more diodes fitted
>together with a regulator from, say, Dave Roberts), can anyone reassure me
>that this will cure the problem? I'd hate to toast yet more diodes. Or
>should I invest in an exchange alternator? Or would that (rather expensive)
>unit be in danger of being cremated by some other problem?
>If you're the guy who had a similar-sounding problem, what was your

I had a similar experience (with a Mercedes, however) and the Bosch mechanic told me that the diodes
could have failed due to spikes caused by worn out rings/brushes to the alternator rotor. The brushes start to jump and this creates lots of spikes which can kill both the diodes and the regulator.
The only way to cure it would be to re-work the rings and use new brushes (normally fitted to the regulator in one case. In my case, he did it (wasn't too expensive) and everything worked fine afterwards.

90 S4 A/T Euro

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