Took the battery in for testing. Still @ 100%, but needs a charge. Duh.

Took the car to the alternator supplier. Alternator tests fine, but it is not turning on as it is supposed to.

Something is amiss between the PCM and alternator. But what? Where do I look? Ground is fine. It's the signal that's off.

I was told that I can jumper the signal 'red' to the output 'red' to force a charge cycle. True?

How to I find the WTF?

 

How can they tell the alternator is fine but is not, "turning on"? The only way is to put 12v on the red wire on the alternator plug. If that was done, then the fusible link in that red wire is blown and should be replaced. You should be able to measure 12v on the red wire because it gets power from the battery all the time through a fusible link. You can unplug from the alternator and use a voltmeter to determine this. Best put a load on the red wire when you measure its voltage, like a 196 side marker light because the fusible link could be blown but have resistance lower than the high resistance of most voltmeters.
Charge your battery up overnight with a battery charger!

 

ok, how GM wires the alternators up ................
There is a alternator plug. it's plastic and has a red wire and a black wire. the black wire goes to a resistor of some sort (light bulb maybe)then to the ignition key. (hence the gen or alt light that stays on [red bar?])
THe red wire goes to the battery lug where the big red battery wire goes to.
The red wire is the battery reference, and the black wire is the wire that "turns on" the alternator by biasing the field.
Ok........so ya wanna make it work..........
Here's what you can do.
go to the plug, and measure the volts coming out of the black wire with the key on. If you don't have any volts, you might have a bad ignition switch, or the fusible link that feeds the ignition switchcould be bad.

But you want to test the alt. in the car.
go to an auto parts store and buy a replacement alternator plastic plug with the red and black wires already installed. buy a small dash light bulb, say an 1819, and two wire alligator clips.
solder the black wire to the brass case of the bulb. Get another length of wire like speaker wire with the insulation peeled back about half inch on both ends. Solder that wire to the end of the bulb.
Test the bulb after soldering to make sure you didn't ruin it with extra heat. If it lights up it's good.
Crimp the alligator clips on the wires coming out of the new plug, including the black wire (speaker wire)with the bulb in the middle, clip both leads the red and black to the battery post on the alternator. Be sure to wrap the bulb with some electrical tape to keep it from shorting to anything, and then plug in the alternator plug.
The light bulb should be on.
Now, leave the hood open, start the engine. The light should go out if you rev it. don't go over 1000 RPM.
The alternator should be charging the battery and running the car.
Get a volt meter, and put the red lead to the battery lead on the alternator, and the black to a ground.
You should get 13.8-14.2 volts. That checks your alternator, and makes sure the nut on the front pulley is tight. Otherwise, the pulley spins, but the alternator stops turning with the load.
turn on the air conditioning and then whatever, lights, radio, etc.
At idle, you should get around 12.5 volts, and 13.8 or so at maybe 700-800 RPM.
This is only an emergency repair to get you home in the event the car can't be fixed in the middle of the desert, or in a ice storm, or a white out.
Don't leave it this way, the bulb will run the battery down slowly but it tests the alternator.
Chris

 

Update:

With the neg terminal disconnected:
Alt output to batt pos term - very low resistance
Alt red connector wire to batt pos term - very low resistance
This should mean that the fusibles are OK and that the ECM is not open, right?
All fuses OK. Will check fusibles to make sure.

Ign 'on' but not running:
Red alt connector @ batt voltage.
Blk alt connector @ batt voltage.

I put a charger on it at lunchtime and it is charging as we speak. Hopefully it was a bad alt that dumped the battery low enough to fail to turn on the new alt. We'll see. Currently @ 13v hold charge.

If all is still bad, what next? Bench test the new alt?

If I get it all charged up, how will I know if system is working before batt drains?

 

You know your alternator is defective when the red battery symbol light on the dash is lit with the engine running. Also, engine running and cold, your battery voltage should be 14.7 volts and the alternator voltage drops to 13.7 volts when it gets to its operating temperature. This is normal. Your dash voltmeter will read 0.3 volts lower than these numbers because it measures the voltage after the ignition switch contacts which have about 0.3 volts across them. Car batteries cannot have a voltage higher than 13.2 volts and normally they are rarely higher than about 12.9 volts fully charged.
The alternator could have been good at the supplier, but now, it is NOT!
Also, yes, if you connect 12v to the red connector wire, connector plugged in, or apply 12v to the socket pin on the alternator the red wire connects to, and engine running, you should have 14.7 volts on the output wire on the alternator. If you don't, then the alternator is defective.
Oh yeah, one of the ways new alternators fail is if the output terminal which is a bolt, gets turned by tightening the bolt and this breaks the wire on the inside of the alternator. Auto store tightens bolt to test and the wire touches inside and tester tells you your alternator is good.

 

Today I learned to have the alternator bench tested before I take it from the shop. They gave me a bad one. Then we tested and found another bad one before a good one.

Hmmm...

I am grateful for the learning experience of what should ohm and volt where. Only cost me a couple trips to my favorite store.

 

Congratulations. It is always good to learn new things.