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1973 Cadillac Coupe DeVille
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Discussion Starter #1
Hey Everyone! I have what I hope is a quick question. The other day the GEN light came on in my 1973 Coupe DeVille. Thinking it was the alternator (it was original) I replaced it with a reman from NAPA. The car fires up fine, but the GEN light is still on. SO... with the car running, I disconnected the negitve terminal on the battery to check and see if the alternator will run the car. It did. I know the voltage regulator is built into the alternator so now I'm lost.... and about to start drinking.. :alchi:

Any ideas?!?! I hate to bring it into a repair shop for something stupid. :thumbsup:

Thanks everyone!
 

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'85 Fleetwood FWD
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728 Posts
You may have a wire being grounded somewhere between your GEN light and the alternator. The light has battery voltage on one side, and the alternator supplies the ground when the ignition switch is on with the engine not running. When the engine is running, the alternator has battery voltage on the the light instead of grounding it, which means no current flow through the light, and it turns off. That is controlled by the voltage regulator diodes in the alternator.
It could be other things also, like maybe the ignition switch etc
There is also a diode in the light circuit to keep the alternator from supplying voltage to the ignition when the ignition switch is off.
 

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'85 Fleetwood FWD
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I'm quite sure it's internal Bruce. I had two '73 Chevies with the internal regulator. He could have a fresh alternator with a bad internal regulator diode etc.
 

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1926 Model T street rod, 2000 Jaguar XJ8, 1999 Corvette.
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My experience is a failed diode will cause a dim gen light whereas a defective ground or alternator will be a normal bright light.
 

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Eldorado
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A dim light can be caused by voltage drop in the wiring, but a more likely cause is a bad diode. So the issue is proving the new alternator is bad, without voiding the warranty or having to fix it yourself. Swapping an alt would prove its bad. A scope on the bulb lead would show a drop in voltage on one phase (of 3) if a small trio diode is open. A higher spike on a phase means a main rectifier diode is open; without the battery load, the voltage shoots up and thru the trio to the bulb. Bruce Roe
 

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'85 Fleetwood FWD
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I guess he could unplug the alternator, and the ground for the light would not be there (??) The # 1 terminal on the plug is the ground path for the light.
If the light stays off with the plug out, it narrows it down the the VR I would think.
There also is usually a diode in the wiring behind the panel, but if that one were shorted, the engine would likely run-on or diesel a bit.
 

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1973 Cadillac Coupe DeVille
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Discussion Starter #9
Thanks the for all the help. Steely is correct, 1973 was the first year for the voltage regulator to be located inside the alternator. I can tell you that the light is not dim. It's as bright as when it first came on. I'm going to have to check the grounds and possibly the new alt itself.

:thepan:

If need be, do you think it is okay to drive this way, or will it leave me stranded someplace??
 

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1973 Cadillac Coupe DeVille
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Discussion Starter #11
I just wanted to post a quick update and let you guys know that I found and fixed the problem.....

Long story short, I replaced the alternator with ANOTHER unit from NAPA and the light stayed on. So I started digging around and noticed the the 10 amp fuse marked TRANS was blown in the fuse box. After looking up in the 73' Cadillac shop manual, I started looking at the wires for the trans kick down switch. Turns out, I had a cracked wire coming of the trans kick down off the carb. For now all I did was wrap the wire in electrical tape and replaced the fuse. The GEN light is off and the fuse did not blow.... All is well. :thepan:

Again, thanks for the quick response. Hopefully, I can return the favor sometime... :highfive:
 

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OK, forgot "check fuses". The bad wire touched ground & blew the fuse. Then the short or another circuit provided a path to light the light. Thanks for the update. Bruce Roe
 
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