Low battery light stays on with key removed

Tiny
BILL TAYLOR2
  • MEMBER
  • 2001 GMC SONOMA
  • 4.3L
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 126,000 MILES
After replacing the electrical ignition switch I now have a low battery light staying on when key is removed from the ignition. Light goes off while driving and idling. I've tested the alternator output and it was reading 14.6 volts. Not sure where to start diagnosing this problem.
Tuesday, December 3rd, 2019 AT 4:25 PM

16 Replies

Tiny
ASEMASTER6371
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Good evening,

Do you mean the battery light is on?

Check the 2 red wires in the diagram for power with the key off.

Roy
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Tuesday, December 3rd, 2019 AT 4:53 PM
Tiny
BILL TAYLOR2
  • MEMBER
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Yes, the battery light is on with the vehicle off and key removed from the ignition.
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Tuesday, December 3rd, 2019 AT 4:58 PM
Tiny
ASEMASTER6371
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Okay, did you test the 2 wires I highlighted in the diagram?

Roy
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Tuesday, December 3rd, 2019 AT 4:59 PM
Tiny
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I'll do that in the morning. I am at home now and away from the shop.
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Tuesday, December 3rd, 2019 AT 5:04 PM
Tiny
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Sounds good.

Keep me updated.

Roy
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Tuesday, December 3rd, 2019 AT 5:21 PM
Tiny
BILL TAYLOR2
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Tested the wires highlighted showing 12v on b+ and nothing on the L wire.
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Wednesday, December 4th, 2019 AT 5:53 AM
Tiny
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Okay, start the truck and see if it is hot. Turn on the headlights when testing.

Roy
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Wednesday, December 4th, 2019 AT 5:58 AM
Tiny
BILL TAYLOR2
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I started the truck and it is hit when running. However, I noticed that the alternator is making a faint high pitched beeping noise with the truck off. Could this be a clue?
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Wednesday, December 4th, 2019 AT 7:45 AM
Tiny
BILL TAYLOR2
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I need to add removing the connector will stop the noise but not turn the light off. I also started pulling fuses and checking to see if the light went out. I found that the light goes off when I pull the radio battery fuse. And of course the cluster fuse. Neither fuse stopped the alternator noise though. I am leaning towards a short or bad ground in the radio circuit. Am I on the right track?
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Wednesday, December 4th, 2019 AT 9:23 AM
Tiny
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Okay, most likely the alternator has a bad diode inside turning on the light with it off. If you remove the cable that attaches from the battery, the light should go out.

Roy
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Wednesday, December 4th, 2019 AT 10:50 AM
Tiny
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I disconnected the main cable going to the alternator the noise stopped and the light went out. This told me the alternator was back feeding the light after the engine was off. So I replaced the alternator wired every thing up connected the battery and viola the same noise from. The new alternator and the low charge light is back on as well as the odometer stays on and the shift indicator. I'm still leaning towards something in the aftermarket radio might be to blame or in the ignition switch connector as changing that and the lock cylinder housing is what started this in the first place.
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Thursday, December 5th, 2019 AT 11:30 AM
Tiny
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Okay, the alternator is being commanded full field by the PCM. That is what the whining noise is. The L terminal is a pulse width signal from the PCM. Looks like you need a PCM.

Sorry for the delay. I have been sick the past day.

Roy
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Friday, December 6th, 2019 AT 12:49 AM
Tiny
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That makes sense I'll look at a new PCM. I am sure the price for that is going to be a tad expensive. Sorry to hear you've been sick hope your feeling better.
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Friday, December 6th, 2019 AT 6:03 AM
Tiny
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Thanks. Today is better.

Keep me updated.

Roy
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Saturday, December 7th, 2019 AT 8:48 AM
Tiny
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Just finished replacing the PCM with one I found online I just had to send the VIN in to get it programmed right. Plugged it in attached the battery cables did the pass-lock relearn procedure and she fired right up. No more lights on the dash and it actually seems to run better. Thanks for all of your help.
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Saturday, December 7th, 2019 AT 9:56 AM
Tiny
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You are welcome.

Always glad to help.

Roy
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Saturday, December 7th, 2019 AT 9:58 AM

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