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34 replies

Alternator Light always on

M mkehMember
I noticed a bearing was making noise in my alternator. So I took it to a reputable shop that rebuilt it. Problem is that now my alternator light is always on, even if the ignition is off and key removed. This drains the battery quickly. I call the shop and they said to bring back the alternator WITH the voltage regulator. I did so and they told me the voltage regulator was bad. They had a replacement that I purchased, got home, hooked it all back up and immediately the alternator light came on. It remains on even if the voltage regulator is unplugged.

If I disconnect the wire marked "D+" the light goes off. According to the attached wiring diagram "D+" goes from the alternator to the Regulator, to the Amp Light and then to the ignition switch. The Ignition switch "seems" to be functioning fine.

I find it hard to believe that the voltage regulator AND the ignition switch BOTH went bad as soon as I had the alternator rebuilt. The shop says they tested everything and it's working fine.

Anyone have any thoughts on this?

MH

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jag13catjag13catMember
Mike H.; From your wiring schematic, you have 12v going to B+ on the alternator from the battery. To have voltage coming from D+ on the alternator to D+ on the regulator, then to the amp light, you must have voltage crossing from B+ to D+ within the alternator (short/open circuit). With a voltmeter have you tested if voltage exists between B+ & D+ with the various conditions you described? You may also check for continuity between B+ & D+ with no voltage from battery. If continuity exists, then B+ & D+ are connected someway within the alternator.
M mkehMember
I found that diagram in here somewhere... so I'm not 100% sure it's accurate, but it's the only thing I had to go on... I just got the alternator back at lunch today, stopped by the house and plugged it all in... saw the light on and knew the problem was still there.

I'll check for continuity between B+ & D+... that makes perfect sense. Of course we're assuming it's not supposed to have continuity... I profess my ignorance on that!!!

Looking at the diagram DF switches the regulator right? If that's the case then it appears when engaged D+ goes to D- (Ground) which illuminates the dash light.

It would also make sense for me to check if D+ is getting voltage when the key is off. If it is then that would likely be the problem.

I think...

Does the alternator start charging when D+ goes to ground?

MH
goodrocgoodrocMember
quote:
Originally posted by M!ke H.:
I found that diagram in here somewhere... so I'm not 100% sure it's accurate, but it's the only thing I had to go on... I just got the alternator back at lunch today, stopped by the house and plugged it all in... saw the light on and knew the problem was still there.

I'll check for continuity between B+ & D+... that makes perfect sense. Of course we're assuming it's not supposed to have continuity... I profess my ignorance on that!!!

Looking at the diagram DF switches the regulator right? If that's the case then it appears when engaged D+ goes to D- (Ground) which illuminates the dash light.

It would also make sense for me to check if D+ is getting voltage when the key is off. If it is then that would likely be the problem.

I think...

Does the alternator start charging when D+ goes to ground?

MH



You are on the right track. I would suspect your ignition switch is failed. Confirm the lamp is extinguished once engine running and also that you can measure about 13,5-14,5 volts on the battery with engine running?
M mkehMember
The switch appears to be good... if the D+ wire is grounded then the light goes on an off with the switch as expected... I'll check it again at lunch, but it appears there is continuity between the B+ and the D+... since D+ goes to ground this would explain draining the battery. The part I can't figure out is how the light comes on with the switch off if D+ & B+ are both hooked up. That would seem to indicate that when the switch is off that the wire going to the light is grounded and the B+ is backfeeding through D+.

The most annoying part is that it was working fine, I just took the alternator in for a noisy bearing...
jfb05177jfb05177Member
quote:
Originally posted by M!ke H.:
The switch appears to be good...
if the D+ wire is grounded then the light goes on an off with the switch as expected...


The part I can't figure out is how the light comes on with the switch off if D+ & B+ are both hooked up.
That would seem to indicate that when the switch is off that the wire going to the light is grounded and the B+ is backfeeding through D+.



If that is the case, that is one of the rectifing diodes has failed and allowing current to leak through, the back feed to make the lamp come on could be through a normal "running" device.

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rockyrockyMember
If JFB is correct (failed diode in one of the legs), you should be able to find this with a voltmeter (actually an ohmmeter) with the alternator in the car.

If all the diodes are good - you should be able to measure resistance between B+ & D+ in one direction, and basically, an open circuit by just switching your meter leads.

If one of the diodes is failed, and is conducting, you should see a similar resistance in both directions.

Or you could use the "diode-checker" setting of your Harbor Freight meter to evaluate it.

Rocky
larrywlarrywMember
I just went through hoops chasing a similar issue on my excavator. Wouldn't crank, battery relay wouldn't release and charge light stayed lit when the key was shut off. Batteries stayed charged though.
I thought critters had nibbled at the wires. I was unwrapping sheaths, chasing wires, testing connections. Finally took alternator to my auto electric guy for a test-N/G. He rebuilt it, I reinstalled, gremlins gone. All were backfeeds through the alternator.
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