Alternator dont Charge - red warning lights are on

[Huw Powell]

> Symptom: alternator is not charging
> 
> Things tested:
> 
> 1. Started engine and tested voltage at the alternator.
> Big post -> 12.0 volts
> small post - blue wire -> 1.9 volts

1.9 volts bad.  Was gonna say that earlier today...

> Swapped out the Voltage Regulator with a known good one -- same symptoms.
> 
> 
> New test just completed:
> 
> Disconnected blue wire from alternator to see if wire was ok and was not
> 'High Resistance' - open or bad contacts.
> Started engine
> Voltage on disconnected wire is battery voltage - 12 volts.
> Put a load on the wire(small light bulb), voltage on wire is  3 volts,  and
> bulb glows. Wire is ok.

Actually, you just proved it isn't.  It should hold 12 volts with a 
several amp load (as I recall, the VR draws up to about 5 amps).

Make a little jumper with a 10 A fuse in it (and maybe a switch) and set 
it up so you can connect the big post 12 volt battery power to the small 
post, with the blue wire disconnected.  Do this with engine running. 
See if anything changes at the voltmeter...

> Diagnosis:
> I suspect an internal short or low resistance between the alternator blue
> wire post and ground.

Yes, the VR can be as low as two ohms or so.

> An open circuit would read 12 volts at the post with blue wire connected.
> A short would read 0 volts at the post with blue wire connected.
> A reading of  1.9 volts is telling me that a low resistance exists between
> the blue wire post and ground.

Low resistance *and* resistance in the blue wire.

> Questions:
> 1. The normal voltage reading on the alternator blue wire post should be 12
> volts, right?

Right.  It is basically a LRR switched battery wire.

> The same voltage that exists in and around the ignition switch
> feed?

Yes, post LRR though (alternator does not run while cranking).

> 2. The blue wire feeds the voltage regulator input?

Correct.

> Does it connect to anything else? 

Yes, it serves a bizarre grounding function for the dash lights, that's 
why they all go on when the belt snaps.  But you can ignore that part 
for practical purposes.

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/