Alt/Gen Light on with engine off

[Jimb1231]

I suspect I have a bad alternator and m going to replace it.  Problem started out as a whining noise in the radio's audio as a function of engine RPMs.  Yesterday, the whine or noise got very loud - much louder than just the annoying whine - but now almost at the level of the audio itself from the radio.  When I arrived at home following a 1 hour drive on the Interstate, the GEN/ALT light came on and stayed on, even after turning the engine off.  I'm thinking the light is remaining on - possibly from a shorted diode that is allowing the battery voltage to get to the light itself.  Also planning to install an alternator filter between the battery and the replacement alternator.  Until I solve this, to avoid running the battery down, I have disconnected the battery cable while the car is not being used.  I have a re-manufactured Alternator en-route.

[ctaylor738]

As I understand this, the light gets positive from Run on the ignition switch, and ground from the alternator until it starts turning.  So it's unclear to me how the light would stay on with the ignition off even if there was positive from the alternator.  I agree that replacing the alternator will probably fix it, but I'm at a loss as to understand why.

Hope it works out,

[Shvegel]

You are correct that a bad diode can cause the light to stay on.  The whine you are hearing is an AC voltage whine caused by the shorted diode allowing current to flow through the rectifier bridge when it shouldn't so rather than a close approximation of DC voltage what you are getting is sort of DC with a little spike in it.

[Jimb1231]

Appreciate much the opinions.  I have an alternator on order and once installed, I'll follow-up on the symptom.  Thanks,  Jim

[Shvegel]

If the old one looks original to the car you might want to hang on to it.  The way things are going with values someday our cars will all be restored to a high level and people will be searching for a correct alternator with date coded carbon brushes that match the production date of the car (only slightly tongue in cheek).

[ctaylor738]

So how does power get to the light with the ignition switch off?

[Shvegel]

From the B+ battery post on the alternator, across the bad diode in the rectifier bridge(the wrong way), across the diode trio in the proper direction and up to the bulb.  Ground is usually across some other load on the same circuit so the light isn't as bright as normal.

[ctaylor738]

Thanks.  That's the first real explanation I've seen for this situation.

[Shvegel]

Thanks for the kind words.