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From digest.v7.n1367 Tue Mar 24 14:14:00 1998
From: "David Leonard" <>
Date: Mon, 23 Mar 1998 11:03:15 -0500
Subject: Re Alternator woes

"The AAA guy "tested"
the alternator by disattaching one of the battery poles while the car was still running. He said "if the car keeps running, your alternator is fine." NO NO NO NO NO NO NO NO

NEVER disconnect a battery from a running charging system!

If you didn't clean out the alternator(and plenty of other expensive electronics) with this stunt, you're a lucky man!

The way a voltage regulator works is by sensing the system(including battery) voltage, and exciting the field to raise the voltage to 13.8 volts or so. If you disconnect the alternator, the regulator will sense 0 volts, and go to full charge, regulated only by the rpm's it is turning. You can test some of them by exciting the field with a 12 v jumper, and watching the output....It will vary with engine speed..completely unregulated.

Needless to say, your Bimmer's electronics won't respond well to the 20 or so volts generated
by your alternator at full tilt boogie output!

To test whether the alternator is charging, run the car at idle, and probe the cig. lighter with a volt meter. It should be about 13.8 volts at idle, and with the engine off, somewhere around 12.5 v if the battery is healthy. Charging voltage should be higher than battery voltage at rest.

On Marine applications, with a selector switch, and two batteries, there is sometimes an alternator field disconnect switch built in to disable the charging system before cutting the battery power during the momentary total battery disconnect. Most of these swithches are STILL labeled " do not switch with engine running".

Good Luck!

Dave Leonard ( Learned this one the hard way with my airplane, popped litebulbs, blew radio fuses, cleaned out a LORAN, etc, by flipping the wrong switch in flight!)

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