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Author Topic: Help please in testing Bosch alternator on the vehichle  (Read 2979 times)


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Help please in testing Bosch alternator on the vehichle
« on: July 21, 2009, 11:20:49 AM »
First, let me say thank you for your effort in having this forum and your hepful information on your website!

I have a friend with a 1999 Pontiac Montana, 3.4L with a Bosch alternator. She has had recent work done on the engine and now the alternator has stopped charging and ask me to look at it. What I found is the pigtail going to the 4 pole plug on the alternator had been replaced (spliced in) with what appears to be a used one. Don't know why that was done, might have been damaged when it was worked on.

The battery indicator on the dash works during prove out with the key on and then goes off like normal when the engine is started. There is no volt meter on the dash. The battery indicator light performs the same way if the wire is unplugged from the alternator. As if the system was charging but it is not. Battery voltage only at the alternator output terminal when running.

My question is this: What does the alternator want to see through the single small wire on the 4 pole pigtail to make it start to work? It measures approx. 4.7 volts coming down the loom to it. Does it see some kind of a frequency like some body control modules send? or can I just apply 12V to see if the alternator will go to work? I have not been able to find that information (yet) on the internet and don't want to damage anything by guessing.

I'm a bit mystified why the battery light seems to function correctly on the dash as if all is well. I would just take off the alternator and test but it's a very difficult vehicle to remove it on and I'd like to test first.

The sinlge wire on the 4 pole connector appears to go to the L terminal as near as I can tell working with a mirror.

Any light you can shed on the operating strategy of this vehicle would be greatly appreciated so I can proceed to test it on car and determine if it's a problem up stream or I need to take off her alternator.

Thank you very much,
Gus (retired old mechanic)

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Help please in testing Bosch alternator on the vehichle
« on: July 21, 2009, 11:20:49 AM »


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Re: Help please in testing Bosch alternator on the vehichle
« Reply #1 on: August 19, 2009, 10:14:46 PM »
Like all late model GM vehicles the alternator must be activated with a resisted ignition wire. If you put a full 12 volts to the "L" terminal it can damage the voltage regulator and the alternator will not work.
4.7 volts seems a little low but it should activate the alternator, I'm used to seeing about 9 volts.
Could be a fuse not making proper connection, broken wire or something like that. You don't want to run a full 12 volts tp the "L" terminal. What we use in field testing is a test lead with a small light bulb in it. I have activated alternators with LED lights but some offer to much resistance and the alternator will not activate. You can use a test light with alligator clip on one end and a sharp point on the other. Clip the alligator clip on the battery plus terminal and the other end on the "L" terminal. When you start the vehicle the test light should go out and you should get `14 to 14.5 volts at the battery if the alternator is working.