Idiot light.

This is for Bob the alternator rebuilder.

What protection does an idiot light give the alternator? You made this comment earlier in your reply and others are asking this question. Maybe you didn't see it.

The idiot light serves a couple purposes:

1 - Provides an instant warning should the alternator stop charging. (And incidentally in most applications warns when the fan belt breaks...)

2 - When the ignition switch is opened with the engine running, the lamp limits the backflow current from the alternator back thru the coil to a level less than required to fire the coil. In other words without the lamp - or an equivalent series resistance or diode - the coil will continue to fire with the ignition switch open. And hence the engine will continue to run after the ignition switch is turned off.

The first and main reason..........
The idiot light allows you to disconnect power generated from the alternator from the ignition system.
The coil wire and the wire (with the idiot light installed inline) to the alternator both need keyed power. Power only with the key on. But if you just hook up 2 wires and do not use a idiot light or a one way diode the alternator will supply power to the coil with the key off and you will not be able to shut the engine down.

The second reason
This only comes into play if you have a problem in your wiring. If for some reason the main charge wire on the back of the alternator gets pulled off; broken; ect to where it can no longer carry the charging power to the battery the alternator will try to charge the battery threw the small wire that has the idiot light installed in it. If this were to happen the alternator would fry because it is not suppose to work this way. What causes the alternator to fry is it stops charging threw the heavy duty diodes and starts feeding the charge threw the diode trio.

Bob has a few pictures he can show you that may explain it better.

BobM, I assume you are also aware of the following...

<img src = "http://ndtc3500.stellarnet.com/~blweltin/Bob/DelcoSIA1A.gif">

<img src = "http://ndtc3500.stellarnet.com/~blweltin/Bob/DelcoSIB1A.gif">

Hey, John! Can we be FRIENDS? (GRIN)

I took a few hours off from bugging you guys to make a parts run, and take my 9-Y-O out for a little shopping and games with a friend at Micky D's.

Sorry for going a little overboard with you, but on the other hand, "don't shoot the messenger". The information I posted was TRUE and RELEVANT, and IMPORTANT for the long service life of an alternator.

I have worked with these alternators in OEM and conversion settings since they came out in '72 or '73 and have seen MANY examples of alternators being damaged by the scenario I posted about that would otherwise have not been harmed by a loose wire or bit of resistance in the charging circuit, had they had a current-limiting device in the lead to the #1 terminal.

I usually use a 1N5408 diode which costs less than $0.20 in quantity, a SMALL price to pay for increased alternator life/reliability. (A resistor or lamp serve the same function, but require a mount and get HOT as compared to the diode.)

Over the years, I have seen MANY systems with an "idiot light" that the customer notices is glowing slightly when the machine is operated in the dark. Often, this is a sign the charging system needs to have some connections tightened up and/or cleaned and WOULD have lead to alternator damage WITHOUT the light to limit current flow.

Of course, a dimly-glowing "idiot light" CAN indicate a leaky/shorted diode or a shorted stator, even while the alternator is STILL putting out some current.

Incidentally, "john in la" is "up to speed" on this, and "gets it".

HAPPY NEW YEAR... I'm out of here for a while!