Cadillac Deville alternator troubleshooting

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Hello everyone, I am trying to help a friend with his 1998 Cadillac Deville after the alternator completely stopped charging and left him stranded. I thought it was going to need an alternator, but I just checked it out and found that neither of the two wires on the alternator's plug have power with the key on or with the engine running. I was expecting one of the wires to be the signal to turn the alternator on when the ignition is on and light a test light like on the other cars I have worked on, but neither wire has any power. Unfortunately I have not been able to find much information on the wiring, the only diagram I found is this one. From what I can tell, the grey wire (F terminal) is just to tell the PCM what the alternator is doing and the red wire (L terminal) is for the battery light. So which wire tells the alternator to turn on? Is it just activated through the battery light? I have never been so confused by an alternator wiring diagram.

Thanks in advance for any help, it is greatly appreciated! :)

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That grey wire going off to the "generator feedback module" holds all your gremlins, but you'll want the Cadillac FSM for diagnostics. F= "Field."

The battery light wire could be an issue if you had it directly wired to an incandescent bulb (which burns out) but since it's another solid state module that make that hard to diagnose as well. Sometimes they want a "pure" +14V power source to sample to see what the alt is actually putting out, one not spliced into other fuse boxes and current drawing circuits. That said, I think you should find +12V there.

Car might do monkey business like "soft starting" the alt a few seconds after the engine catches which is why they put the PCM in control.
 
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Avery4

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That grey wire going off to the "generator feedback module" holds all your gremlins, but you'll want the Cadillac FSM for diagnostics. F= "Field."

The battery light wire could be an issue if you had it directly wired to an incandescent bulb (which burns out) but since it's another solid state module that make that hard to diagnose as well. Sometimes they want a "pure" +14V power source to sample to see what the alt is actually putting out, one not spliced into other fuse boxes and current drawing circuits. That said, I think you should find +12V there.

Car might do monkey business like "soft starting" the alt a few seconds after the engine catches which is why they put the PCM in control.
Thanks for the information! I tried to get the alternator to charge by powering the alternator's L terminal through a test light, but it didn't work. It just lit the test light. From what I can find, any GM alternator from that era should charge with just the L terminal being energized through a test light or other small incandescent light. From this test, it seems that the alternator is bad.

Also, it appears that using the test light to check for power at the connector may not have been a good test. From what I have read, it takes only a very small amount of current to excite the alternator and make it charge, so perhaps by design the L circuit just can't supply enough current to light a test light.
 
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That grey wire going off to the "generator feedback module" holds all your gremlins, but you'll want the Cadillac FSM for diagnostics. F= "Field."

The battery light wire could be an issue if you had it directly wired to an incandescent bulb (which burns out) but since it's another solid state module that make that hard to diagnose as well. Sometimes they want a "pure" +14V power source to sample to see what the alt is actually putting out, one not spliced into other fuse boxes and current drawing circuits. That said, I think you should find +12V there.

Car might do monkey business like "soft starting" the alt a few seconds after the engine catches which is why they put the PCM in control.
Good advice. That resistor wired parallel to the bulb is there to provide current if that bulb burns out. Bypass it
 

Avery4

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Update: After doing a lot of research and still not finding much information, I took the alternator off and we had it tested at an auto parts store and found that it was indeed bad. Installed a new alternator and it's charging perfectly again and he's back on the road.

Although it sucks to spend so much time trying to properly diagnose a problem without adequate information, I did learn several valuable lessons such as to not necessarily expect any of the pins on the alternator's control plug to supply enough current to light a test light because some clearly can't. Never saw that before.

Thanks for the help, it was greatly appreciated! (y)
 
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I'm completely guessing here, but usually a PCM pulls an output wire to ground in order to make something happen on the device connected to that wire. This is because electronics to do that are a bit simpler and cheaper than driving it to battery voltage. So the F wire would not show a voltage when unplugged from the alternator.

A set of small diodes inside the alternator provide positive voltage to the field as it turns. This only works while it is turning. A small amount of power to bootstrap the process comes through the L wire via the warning bulb. If the alternator is working, the voltage from the diodes takes over and raises the voltage on the L terminal to approximately battery voltage. No more current is needed from the ignition and dash circuit on the L wire, so the bulb goes out. If the bootstrapping fails, the L wire continues to draw power which makes the bulb remain lit.
 
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I'm completely guessing here, but usually a PCM pulls an output wire to ground in order to make something happen on the device connected to that wire. This is because electronics to do that are a bit simpler and cheaper than driving it to battery voltage. So the F wire would not show a voltage when unplugged from the alternator.

A set of small diodes inside the alternator provide positive voltage to the field as it turns. This only works while it is turning. A small amount of power to bootstrap the process comes through the L wire via the warning bulb. If the alternator is working, the voltage from the diodes takes over and raises the voltage on the L terminal to approximately battery voltage. No more current is needed from the ignition and dash circuit on the L wire, so the bulb goes out. If the bootstrapping fails, the L wire continues to draw power which makes the bulb remain lit.
Did that era GM alternator still have the “full field“ test tab?

Regarding bulb turning off when field voltage equals batt voltage- wouldn’t it turn on when field voltage is greater than batt as it flows the other way now?
 
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So the F wire would not show a voltage when unplugged from the alternator.

I think the F wire is just a duty cycle or pulse width modulated output from the alternator which tells the PCM how much load the alternator is under. That was common for GM vehicles in that era.

The PCM can't control anything on the alternator through the F wire.
 

Avery4

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Glad you got it figured out. Unfortunately getting bad parts right of of the new box is more common today then ever before.
Sure is. Whenever I buy a starter or alternator locally, I have it tested before I leave the store for that reason. The first alternator he tried to buy from Autozone was completely locked up right out of the box and it was brand new too, not remanufactured. Ridiculous, but not too surprising.
 

Avery4

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I think the F wire is just a duty cycle or pulse width modulated output from the alternator which tells the PCM how much load the alternator is under. That was common for GM vehicles in that era.

The PCM can't control anything on the alternator through the F wire.
That was my understanding too, thanks for clarifying.
 
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