Here is what Ive gathered so far:

Stock 2000 LS1 f-body alternator setup, PCM fed exciter wire.

1. Alternator quit charging one day, completely out of the blue, had been working fine for months and 3k miles since install.

2. Took the Alt to the parts store, tested good, but changed it anyways. New Alt also tested good.

3. Install new alt, still no charge. Verfied no charge with volt meter at battery.

4. Battery cable coming off the back of the alt reads 12V. Battery reads 12V at posts.

5. Pulled exciter wire off and tested continuity to PCM connection, full connection. Tested resistance, 0 ohms resistance. Wire seems to be perfect.

6. Tested exciter wire at the plug that connects to the alt, ignition on, 10V.

7. Hook wire back up to alt, start car, still no charge. Verify there is no charge with volt meter at battery, shows about 11.5 V with engine running.


What wrong with this picture?

Ive read about running new exciter wires from switched 12v source etc, but this is a stock setup and something is wrong that just need fixed. Dont want to alter the wiring setup to get around whats really wrong.

 

here's what i know from 2002 model year via gm service manual:
DTC P0620: pcm uses the generator turn on signal circuit to control the generator (i.e. alternator). A high side driver within the pcm allows the pcm to turn the generator on and off. when generator operation is desired, the pcm sends a 5 volt signal to the voltage regulator via the generator turn on signal circuit. this causes the voltage reg. to begin controlling generator field circuit. once generator is enabled by the pcm, the internal voltage regulator within the alternator controls voltage output independently of the pcm. under certain conditions, the pcm can turn off the generator by turning off the 5v signal on the generator turn on signal circuit. the pcm has fault detection circuitry which monitors the state of the generator turn on signal circuit. if the fault detection senses a voltage other than what's expected this DTC will set. the voltage regulator also has fault detection circuitry, if a problem is detected the regulator will ground the turn on signal circuit pulling the voltage low. this also causes the pcm to set the DTC. when this dtc is set, the pcm sends class 2 serial data message to the instrument panel cluster (IPC) to tell it to illuminate the charge indicator lamp.


if 2000 model year is the same as 2002, i would not rig the "exciter" wire going to the alternator to 12v. you most likely won't get it to work, if you don't break something.
you said "stock 2002 alternator setup". does this mean only the alternator is stock and the rest of the motor is modded? or is the motor and electrical system and pcm all stock? if there are other problems, the pcm might not be sending the turn on signal purposely and you should have a dtc.

and i know on my 2002, there is only one wire going to the alternator on the 4-pin plug. sorry don't know which terminal of the plug. upon engine start, the pcm should send a 5v signal on this wire. if it doesn't you may be looking at a new pcm, the driver for that output from the pcm might have crapped out.

also, you asked what's wrong with this picture. hopefully you have DTC's set. if you don't then that's what's really wrong with this picture.

 

By stock 2000 alternator setup, I mean its a stock replacement alternator from the local oreillys with the factory red wire coming from pin 15 on the red connector coming from the PCM. The charging system as a whole has never been modified, and has never given me any problems until now.

The engine however is modded, but the battery, starter, alternator system has never been altered.

So if the PCM is supposed to send 5V to the alternator to engage, why/how could mine be getting 10V? Also, me seeing 10V at the plug that plugs into the alternator (single red wire of the 4 pin connector) is with engine off, key on, connector unplugged from alternator. I have not probed the wire with it plugged into the alternator, nor have I checked it with engine running.

 

check the fuse.

 

Every fuse in the car checks good.

 

Have you checked the battery cables for continuity?

 

with engine off key on, the pcm i don't think is commanding the alternator to turn on. the pcm knows the engine isn't running, and i believe from from stuff i've read over the years the pcm waits for the engine to be actually running before it sends the turn on signal.
and as for seeing 10v with the connector unplugged, that also may be part of the problem. I would plug the connector in then use a wire tap and a good high-ohm meter to measure voltage. the info given in my service manual is a little vague, it mentions 5v is sent to turn the voltage reg. on, and if the regulator grounds then the pcm can sense that, and i think the pcm also monitors this circuit to sense alternator output or duty cycle, and the pcm can "turn off" the 5v signal. "turn off" in the world of computers and electronics doesn't specifically mean 0v, or ground, or open circuit. it can mean almost anything, and i haven't come across a specific definition, in this case it could mean a high voltage.

 

what you could do is get a usb phone charger or something equivalent that plugs into the accessory plug. that is a dc step down converter and provides a good 5v output, which you could hack up and use to manually send a 5v signal to the alternator voltage regulator as the turn on signal.
if you decide to do that, have the alternator wire coming from the pcm unplugged.
and you should get a dtc for that being unplugged.
you haven't mention if you have any dtc's either, have you had your pcm reprogrammed?
but having the alternator disconnected from the pcm and the usb charger 5v output connected to the L terminal on the alternator, you should be able to start the engine and get an alternator dtc and service engine light and have your volt gauge reading below 12v. then plug in the usb charger into the accessory plug and the alternator should start charging and your volt gauge should jump to 13-14v.
if it works, then i would venture a guess that something is either wrong with the pcm not being able to send the alternator turn on signal or some other condition is preventing the pcm from commanding the alternator on. and you would need a gm tech2 or something that allows you to command the alternator on/off function in the pcm and troubleshoot from there. if the usb charger doesn't work, then that pretty much points to a problem with the alternator.

 

Probably a PCM problem, you covered everything else.

 

Here is what it ended up being: Intermittent connection with the exciter wire and exciter wire pin itself inside the connector that plugs into the alternator.

What I did was go and pull the exciter wire on a buddys car that works correctly. Engine off key on, it checked 10.9 volts to the pin. I was puzzled because thats what my car was reading. Went back to my car and again mine read 10.9 volts. So at this point I was certain it wasn't PCM related, or exciter wire integrity related, UNTIL, I noticed that in certain positions I lost all voltage to the pin. Took the entire connector apart and the pin had became a little loose on the wire. Re-arranged that around some and everything works.

Simple answer, loose crimp on the pin for the exciter wire inside the connector.

 

I've had the same issues with my 1998 SS. After replacing my head gaskets I had a new charging issue. That red wire hanging with nothing attached. Repair manual told me what it was and went. I have a 4000 repair manual on my phone I'm trying to find one spot and one part with other pages and it's most beautiful sucks but you got to do. All the information is in there but you sure have to take your time to find it. A 98's exiter wire is 12 volt out of the little red wire that goes to the L (second pin from the outside of the plug. 99 and newer are suppose to use 5 volt. I've done it both ways and it worked. I used a small light bulb for marker lights or interior stuff. After your alternate starts changing this light goes out. I could have put this wire up to that fuse box you don't come on issues on but I just hooked it up to resource that was always on. When I used this light it stayed on for days or even a week and didn't kill the battery. Shocker huh? I'm having no choice you should write down don't you speak I can get you time for a new alternator. But before you do that I'm going to check to see if I'm getting continuity from the battery cable that comes out of the back of the alternator. That cable goes over to one of the fuse boxes so that should be easy disconnect and check.

 

I've had the same issues with my 1998 SS. After replacing my head gaskets I had a new charging issue. That red wire hanging with nothing attached. Repair manual told me what it was and went. I have a 4000 repair manual on my phone. Trying to find one spot and one part with all those pages sucks but you got to do. All the information is in there but you sure have to take your time to find it. A 98's exiter wire is 12 volt out of the little red wire that goes to the L (second pin from the outside of the plug. This wire comes from a huge in the dash, temperature I think. 99 and newer are suppose to use 5 volt and are fed by the PCM. I've done it both ways and it worked. I used a small light bulb for marker lights or interior stuff. I just spiced it into the wire.. After your alternate starts changing this light goes out. I could have ran this wire up to the fuse box that was ignition control but I just hooked it up to resource that was always on. When I used this light it stayed on for days or even a week and didn't kill the battery. Shocker huh? I'm having charging issues now. Time for a new alternator. But before you do that I'm going to check to see if I'm getting continuity from the battery cable that comes out of the back of the alternator. That cable goes over to one of the fuse boxes so that should be easy disconnect and check. Wait, that's not true. Without going under the car nothing is easy to on this alternator from up top. Good luck. Peace out

 

11 years later........