3900 Alternator function?

[Guest]

So far I have confirmed that the alternator is either "ON" as default or something unusual is going on.

On start up the voltmeter reads 14 volts with only the battery cable connected to the stud on the back of the alternator.

Connect 5 volts to either the F or L terminal at the plug and the voltmeter doesn't budge.

One of the terminals L I recall puts out about 2.3 volts with the engine running, connect terminals F and L together the alternator shuts off as made evident by the voltmeter dropping down to about 10 volts. Disconnect the short and you can hear it turn back on and see the voltmeter climb back up above 13 volts.

Does anyone have a wiring diagram for the alternator in an 06-07 Chevy Uplander to help figure this thing out?

My guess is the PCM grounds the terminals together to control output by apparently shutting it off intermittently via PWM which makes sense to me considering a failure of the PCM circuit controlling it would render it functional at 100% due to the default setting rather than turning it off altogether.

[Joseph Upson]

It appears I am correct, I had the alternator tested along with an alternator from the 3500 and the operate differently. The 3500 alternator required an input and would not work un plugged. The 125 amp 3900 alternator required nothing more than a ground and battery wire connection to put out 15 volts with nothing connected to the plug.

It's either PWM or variable resistance controlled to reduce output and is otherwise on at startup.

[americasfuture2k]

would the 3900 alterrnator be better to use than the 3500's? or are the not direct swapable?

[Joseph Upson]

would the 3900 alterrnator be better to use than the 3500's? or are the not direct swapable?

Not without knowing exactly how it works. I'm still not sure or confident about its proper function needs despite the test by the parts store. My gauge reads about 13.5v with instances of a steady 12v. The last test drive datalog showed a mid to high 12 v output although it appeared to be working and that's to close for comfort to the low end.

It should be interchangeable going to the 3500, but it doesn't work the other way around because of the battery wire stud on the 3500 alternator which sticks out the side and is either two close to or contacts the upper intake on the 3900. The stud on the 3900 alternator sticks out the back end. I want to use it because it has 125 amp output capacity.

[geoffinbc]

Pretty much all the New GM alternators are controlled by the PCM. Hell every damn bulb and switch runs through some sort of computer interface whether it be the PCM or BCM

[MidnightriderZ24]

i can get u a wiring diagram no problem.. let me know if u need it. but yes theyre a variable rate alternator, the pcm controls it. kinda stupid for us, but its smart.

[Joseph Upson]

i can get u a wiring diagram no problem.. let me know if u need it. but yes theyre a variable rate alternator, the pcm controls it. kinda stupid for us, but its smart.

I have the diagrams, it's the method of operation that I need, to make sure it gets the proper voltage necessary to turn it on. The 3500 alternator does not work the same as the 3500 alternator from what I've discovered so far.