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Date:         Sat, 9 Dec 1995 06:32:28 -0600 (CST)
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Dan Beatty <danb@sound.net>
Subject:      Re: Alternator & O2 lights on

At 07:57 AM 12/8/95 -0600, Drew Jenkins wrote: >Yesterday, my wife was driving the Vanagon when the O2 light and >the Alternator light came on. She stopped and restarted the Vanagon >and the lights stayed on. > >So she called and I go pick her up. Quick check of the engine >shows no broken belts, nothing apparently wrong that I can see. > >I started it and the two lights stay on. I let it idle for about >a minute and I felt I could drive it the three miles home. Drive >out of the parking lot and the lights go out. Hmm, that's interesting. > >I drove the Vanagon around the block after dinner. It had the 2 lights >on when I started but they went out after a minute. This morning, I >started it and the lights were not on. > >Has anyone seen or experienced these symptoms? Answers and insights >are certainly appreciated. > >Oh, I last reset the OXS milage counter at 94K miles, there are now >117K on the van. > >

Many (most if not all) automotive alternators rely on an external power source for the initial excitation current. The reason that this is necessary is that would tend to be difficult to control because of the very low field current that is needed. Bosch alternators typically contain three exciter diodes that provide field current during operation. But because of the very small amount of residual magnetism these need a boost to get started.

If you want to try this out here is how. Disconnect the alternator leads from the vehicle. Now connect them to a COMPLETELY discharged battery. Start the vehicle and see when the alternator begins producing power. It probably will when you hit about 5,000 rpm <g>, there is a tiny bit of residual just not much.

In vehicles with alternator warning lamps (not diodes) the current flow through the bulb provides the current necessary. Otherwise a small amount of current is feed through a resistor or in some other fashion. Bottom line.. It's cheap, reliable, and easier to control. Will it always be this way.. NOT.

Dan


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