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Re: alternator not charging my 91 735i (archive)

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Posted by Max Fretter (NZ) on January 03, 2001 at 12:12:34:

In Reply to: alternator not charging my 91 735i posted by Barney on January 03, 2001 at 11:22:41:

Sounds stupid, but it sounds like you've got bad wiring and/or bulb in the idiot light curcuit.

Apparently (as I learnt on one of the BMW 2-wheeler FAQs from a guy who [again apparently] works at AC-Delco - see the URL below) most alternators use the idiot light to derive the reference voltage between their output and the battery voltage - hence at maximum difference of no alternator o/p the light is brightest. However, the light also in doing this provides an intial current path to the regulator to energise the stator core (can't be energised off the output terminal as this is isolated from drawing current by the diode pack; there is no o/p from the alternator itself yet, so the chicken & egg standoff is to use the idiot light to get the very first output, then it's self-sufficient as long as the alternator is turning...

Again, main upshot:
- check the idiot light, correct type & wattage. Maybe swap it with another original bulb?
- Get an Ohmeter (digital probably best to safeguard any electronics it might come into contact with) and check continuity to the alternator take off.
- Check the condition of the battery, yes
- Check the battery voltage to the idiot light when the engine is running; one side should go direct to the laternator, one will go to the battery via some ignition-based switch (probably not a relay, but who knows?).

Looking back at all this, I might be advisable to get an auto electrician involved...

Hope this helps as a starting point.


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