Re: I'm Kinda Dense: Alternators Again

Posted by Bob on March 09, 2005 at 10:13:05 from (66.163.134.177):

In Reply to: I'm Kinda Dense: Alternators Again posted by Allan in NE on March 09, 2005 at 09:37:56:

On a gas tractor with a 2-terminal ignition switch, enough current can "backfeed" from the alternator's #1 terminal to the "switched side" of the ignition switch to power the ignition system, and keep the engine running with the ignition switch "OFF". This could apply to a diesel engine with electric fuel shutoff. This "backfeed" of current also causes stress on the diode trio and internal voltage regulator, as these parts are not designed to send charging current OUT of the alternator through the #1 terminal.

The diode prevents this backfeed of current. It is NOT needed to prevent draining of the battery with the tractor shut down. Using an "idiot lamp" does not PREVENT this backfeed of current, however, it limits the current to a much lower "safe" level.

I always use either a diode or a #194 lamp, as this is how the alternator was designed to be wired.

With a direct connection from the switch to the alternator #1 terminal, it will work just fine, however, if there is ever a poor connection in the high-current circuit from the alternator's large output post to the battery, the small wire, the voltage regulator, and the diode trio inside the alternator will provide an alternate current path for the alternator's output, and the smoke will be let out of the system, requiring alternator repairs.

I repair alternators for a living, and have seen this a few times. People who don't fully understand what goes on in the regulator cicuit will tell you the "idiot light" or diode is not necessary, however, there are times having one, as the designers of the alternator intended, will save on $$ repairs.

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