I came across something that all GM owners should know about. One of my neighbors came over to seek my advice on a baffling problem he was having with his GM (I think it was a Chev) and its charging system. He was having dead battery problems (sort of like Don was in another story) and had taken his alternator off to replace it. Fortunately he had taken it to "Wheels" the former National Auto store, and they had tested it on an alternator testing machine. There was nothing wrong with it. This neighbor, I'll call him Artie, had the foresight to do some diagnostics ahead of time and determined that there was no output (14.5 volts) at battery or at the alternator stud, so a logical conclusion was that the problem was in the alternator - it has a built in voltage regulator so logic prevailed. Replace the alternator and solve the dead battery problem, right? Nope.
I walked over to see what I could see. We had the service manual open to the circuit describing the charging system. I traced the wires and determined which one was the field and which was the bat and which one was ground. Then I noticed something strange. I thought it was a typo - the wire from the 12 volt source to the field winding went through the idiot light filament, that's right, the bulb is in series with the field supply. Alternators don't have a true field supply like the old generators had with external voltage regulators which regulated the output by switching the field winding on and off. But they do need something to start the system working. Once the system is putting out its current the system becomes sort of self sustaining.
I took a short length of wire and connected it to the field terminal while the engine was running. Suddenly, the voltmeter across the battery went to 14.5 volts! I took the wire off the terminal and sure enough the system continued to charge at 14.5 volts. Fixed, right?? Nope.
Artie shut the engine down and restarted it again - nothing - jump the field, 14.5 volts.
I asked Artie if the idiot light was on while it was running and not putting anything out. Nope. He shut it down again and we tried it all over again, only this time I sat in the driver's seat and Artie did the jumper thing. Same results, only I did notice something strange. "Just where is the "alt" idiot light on the dash panel Artie? I didn't see one - just an engine light and an oil pressure light."
There was no idiot light for the charging system! I took a flashlight and held it at an angle to the dash panel. There, not visible under normal lighting conditions, was a place for the "bat" light. Yet it did NOT light when the switch was turned to the "ON" position. Then it hit me like a ton of poop! Remember I thought that the circuit diagram was in error - that the idiot light was shown to be in series with the field terminal on the alternator?? Well, that was not a typo! The bulb in the dash panel is part of the charging circuit. It is REQUIRED to be functional to supply the initial field voltage to get the alternator started doing its thing. IF THE BULB IS BURNED OUT THE SYSTEM WILL NOT CHARGE!!
It took us another half an hour to get to where the bulb should be but it became obvious that it was going to take a LOT more than a half hour to replace the bulb. It looked like part of the instrument cluster was going to come out before we got to the bulb. Plan "B" went into effect. I found the wire under the hood that supplies the idiot light and wired in a small socket and a bulb under the hood. I connected the other end of the wire to the field terminal and started it up. Voila! Charged just like new. I taped the bulb and socket to a handy vacuum hose and closed the hood. To this day that GM product is still riding around with a bulb taped to the vacuum line under the hood. I would love to see the face on the mechanic who discovers that bulb and wonders what the heck it is doing there!!
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