Alternator Question

Huw Powell human747 at
Thu Aug 29 18:34:47 EDT 2002

> Using good old deduction I believe I have narrowed down my problem, but a solution still eludes me.  I have had earlier postings about electrical problems and many, many incorrect assumtions on my part.  I have finally listened and worked on eliminating systems.  I found that I was getting a 1.5 amp draw on the battery with no fuseable equipment connected.  I disconnected the alternator and the draw went away.  I wanted to prove what the problem was so I performed the following experiment.
> Here is my test that in included 4 alternators and one known good battery.  I constructed the circuit with only the battery and the alternator on a bench.  I ran the ground to the alternator chassis and put my multimeter, set to measure current, between the postive battery post and the alternator post.  I tested with the exciter wire on and off of ground as I was not sure about the proper testing conditions.

hooking up the alt chassis to ground and the big wire to battery +
should result in no draw, unless there are bad diodes leaking current
the wrong way through the stator windings

the way the exciter wire works is it is hooked up to the battery (via
fusebox, etc.) when the car is running.  Depending on the voltage on it,
up to about 5 amps will be drawn via the brushes through the rotor
windings.  if the alt is spinning, this will result in charging current
flowing out through the big wire.

> I guess what I am looking for someone to tell me why this test isn't showing me the right thing

I think it may be showing you something, but it is hard to tell what.

I would reassemble it all on the car, and re-verify that current draw.
Undo the exciter wire, does the draw go away?  reconnect and undo the
big red wire (watch out for sparks!) and retest for draw.  (I would
disconnect battery ground while undoing and reconnecting these wires,
reconnect ground to test, or put your meter there).

I don't think your test has isolated any components usefully - you may
have bad diodes, or a bad voltage regulator, or voltage to the exciter
wire when the car is off (bad ign. sw?  bad X-relay?), or some sort of
random short in the alternator.

Huw Powell

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