Alternator not charging? The vehicle stops running?

A0302

MEMBER
1999 CHEVROLET TRUCK
5.7L
V8
2WD
AUTOMATIC
150,000 MILES

C2500 truck not charging. Truck will run for and drive for a bit and the voltage meter in the truck stays at 12.5 volts for 20 minutes.
Then slowly to 11 volts and then 9 volts and then the vehicle stops running.
Charge battery again and then starts and runs and repeats same thing.
Voltmeter test reading when truck running is same as battery voltage and not going up at all to 13 or 14v as it should.
The battery light in truck dash stays on permanently. The alternator lug on top has same reading as battery voltage, assuming the fuse able link is good then. As there is no difference in voltage. Would this conclude it’s a bad alternator/ voltage regulator? Could there be a peristatic draw in alternator?
Any suggestions appreciated.

7 Replies

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi,

It sounds like the alternator itself is bad. What I would recommend is to remove the alternator and have it bench tested at a local parts store.

If you have battery voltage to it, it should be working. You are checking the power to the red wire where it attaches to the alternator, correct?

Also, as a reference for you, I attached the wiring schematic below specific to your vehicle.

Here is a link you may find helpful:

https://www.2carpros.com/articles/how-to-replace-an-alternator

I attached the directions below for the removal and replacement specific to your vehicle.

Let me know what you find or if you have other questions.

Take care,

Joe

See pics below.

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Saturday, August 6th, 2022 AT 10:42 PM

A0302

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62 POSTS

Yes, the power wire with the protective boot at the back top of the alternator was checked. Picture provided showing the wire. This had the same voltage as the battery itself. I will remove it and take it to get it bench tested. I read that on certain vehicles if the battery is below a certain voltage, the regulator inside the alternator won’t trigger the alternator to produce any charge, is this correct?
Clearance

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Sunday, August 7th, 2022 AT 7:31 AM

A0302

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62 POSTS

I saw in the pictures you sent, the smaller wire from alternator goes to the light indicator etc. I read somewhere if this indicator lamp doesn’t light up when first turning on the ignition switch this will cause alternator not to work? Or is this vehicle specific? The only things that light up as seen in pictures. Last owner of vehicle taped up the battery on light as you can see, probably to stop the glare when driving at night?

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Sunday, August 7th, 2022 AT 7:51 AM

A0302

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62 POSTS

My wife put a light tester on battery positive post and touched bare metal on alternator in two places to show that the body ground is good to further rule out no ground issue to alternator.

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Sunday, August 7th, 2022 AT 7:26 PM

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

The battery would need to be nearly dead. What voltage reading are you getting when you test it?

As far as the light, that won't prevent it from working. Have you had a chance to have it bench tested?

Let me know.

Joe