GEN lamp Q: need to check assumption

Apr 17, 2004 6 Replies
M

I need to check an assumption.



If the following is true:



The alternator is a working one, and the big fat wire from the alternator main terminal to the battery + terminal is good, and the wire from the alternator D+ terminal to the GEN idiot lamp is good, and the connection from the (take deep breath) GEN lamp through the ignition switch and then back to the battery + terminal is good, and the ground connection from the alternator case to the battery is good (battery and transmission ground straps both good).



And: The engine is running at over 2000 rpm, so that the alternator is charging the battery just fine.



The GEN lamp should be dark. This is because the voltage at the alternator D+ terminal and the battery + terminal is the same.



Then you cut one of the ground straps. The alternator can no longer charge the battery and it starts to discharge. But (is this correct?) the voltage at the D+ terminal and the main output terminal of the alternator are still at the same voltage . . . and since the GEN lamp is connected between these two terminals, the lamp should stay dark -- yes? no?



In other words, will a failed battery ground strap on the battery, or the transmission, light the GEN lamp?


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S

All your assumptions are correct.

You'll know if a ground strap fails the next time you go to crank the engine.

Speedy Jim

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M

So you're saying that a broken ground strap will keep the engine from cranking (no current through starter motor), but a broken ground strap will /not/ light the GEN lamp when the engine is running?

Or even when not running -- in other words, the potential across main and D+ terminal on the alternator is zero as long as the alternator is good.

"I'm trying to think, but nuttin' happens!"

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S

Correct

Ummmm. When *not* running/turning, there may be +12V on B+ (from battery) but zero volts on D+. That could light the lamp.

Well, if the battery ground strap is broken, nothing will work. If the tranny strap is broken, starter current *could* try to flow thru things like the accel/clutch/heater cables. Interesting... Jim

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J

Alternators, ach! :) My lesson was learned when I STUPIDLY painted my alternator stand black wrinkle. See, it didn't ground. No ground. No charge.

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M

Oh right. So, I think the proper sequence of events before attempting to jump start the vehicle would be to first inspect for a good ground return path from the starter to the battery. Check. Wilco. Roger that.

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P

Internal/external volt regulater shot/shorting out??

J.

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