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Battery light came on

Tiny
JMORAN09
  • MEMBER
  • 2004 PONTIAC SUNFIRE
  • 2.2L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 300,000 MILES
Hi, I don't know if it would make a difference but it's from Canada. So the battery light came on a couple weeks ago and the car shut off on me. I bought a brand new alternator (the one I took off I put on a week before from a salvage yard) still nothing later in the day I noticed the wire from the alternator to the starter was off. I put a new end on it and fixed it and still nothing. I even tried putting an additional wire on the stud on the starter and going straight to my battery and still wouldn't charge it. The plug on the alternator has a red wire and a grey one the red wire lights up the test light and the grey one won't. The schematic shows that the grey wire goes to my computer. My buddy thinks the computer is bad but I have no clue at this point. Any help with this would be amazing. Thank you
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Saturday, July 13th, 2019 AT 9:59 PM

9 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Hi JMoran09,

Take a look at the attached documents.

The PCM is responsible for turning the generator on and off. This is done through the smaller red wire on the generator. The grey wire is the duty cycle that the PCM monitors. It is only grounded when there is an issue. So I would not think this is your issue.

A battery light normally comes on when the running voltage drops below a threshold. Normally we think this is caused by the alternator but it can also be caused by a weak battery.

I assume you are measuring the voltage on the battery when the engine is running and expect to see close to 14volts. If you are not seeing this then first thought is the alternator. However, if the battery is discharged enough then the alternator will not be able to reach this voltage.

Think of it this way, the battery is your foundation and if it is not around 12.6 volts on its own then the alternator cannot reach 14 volts of the overall system.

I would stop by a parts store and have the battery load tested. If it passes then let me know and we can dig deeper.

https://www.2carpros.com/articles/car-battery-load-test
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Sunday, July 14th, 2019 AT 7:54 AM
Tiny
JMORAN09
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The battery is brand new. I've slow charged it overnight and the battery was fully charged and after a day if driving it shuts off again. We tested all the wires the only one not giving out anything with the engine running is the grey wire on the plug for the alternator.
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Sunday, July 14th, 2019 AT 10:18 AM
Tiny
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Okay. Per the service information (highlighted in attached) the grey wire is only grounded when the voltage regulator detects a problem. Otherwise the circuit is not grounded so there should be no voltage coming from it to the PCM.

If the battery has 12.4 volts or more and the alternator is new, here are the next steps.

1. Inspect the serpentine belt (replace if suspect) and the tensioner to make sure your alternator is not slipping due to insufficient tension.

2. What is the voltage on the B+ wire with the engine running? Large red one that probably has a bolt holding it on. Not the red wire going to the PCM. I know you said you have voltage on it but what is the number?

3. Check you battery terminals and wires for looseness and corrosion, including the ground for the battery negative. The I would suggest doing a resistance measurement on them. Basically, this is just unhooking the cable from both ends and measuring resistance from end to end. This is where you set your meter to ohms and it should be around.5 ohms.

4. Just run through your fuses with the engine running and make sure you don't have any blown fuses. This is unlikely but is easy enough to check and could drive us nuts if not checked.
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Sunday, July 14th, 2019 AT 7:16 PM
Tiny
JMORAN09
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Update. I replaced the alternator a couple more times with the warranty my local parts store doesn't test them. We put power to the red wire on the two wire plug going to the alternator and it is now maintaing the battery but still isn't charging it. I work at a GM dealership but don't wanna pay to have them look at it for days but I got the schematics from the GM computer
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Friday, July 26th, 2019 AT 6:00 PM
Tiny
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Ok. I am not sure I follow. What do you mean it is maintaining the battery but not charging it? The alternator is only there to maintain a sufficient voltage. It is a misleading idea that the alternator is supposed to charge the battery. At its peak it is only producing 2 more volts than what a fully charged battery has. This will charge a battery that is slightly drained but it will not make up for a depleted battery.

What is your voltage across the battery with the engine off and the engine running?

Also, if you work at a GM dealer, I am sure if you talked with the service manager he would have one of his guys just check it out real quick.
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Friday, July 26th, 2019 AT 6:34 PM
Tiny
JMORAN09
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I already talked to the lead tech and he has no idea I went out with the volt meter last week and the alternator wasn't putting anything out. The B+ wire on the stud read 12.4 with the car on and off. This was all before we made the red plug wire hot. As of right now the battery will stay at 12.7 volts until my daytime running lights come on and it drops to 11.7 but it stays steady but as soon as I put the radio fuse in it starts to slowly drop like the alternator can't handle those few things on.
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Friday, July 26th, 2019 AT 6:52 PM
Tiny
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Ok. As stated in the first response, have you monitored the PCM? The lead tech should be able to do this. The PCM is responsible for sending the turn on signal to the regulator when the engine is running. If the PCM never sends this signal then the alternator will not have any output. Go back to the first post and read that circuit description.
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Friday, July 26th, 2019 AT 7:51 PM
Tiny
JMORAN09
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I'll see if he can do it Monday but I got a new computer for the car too. I checked the wires for continuity and everything's good. I don't know what else it could be if I basically replaced the whole charging system
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Friday, July 26th, 2019 AT 10:10 PM
Tiny
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Unfortunately, it looks like that is all that is left. I would like for you to find a "smoking gun" before replacing it but I agree, everything else has been replaced. Let me know what happens and I will help how I can. Thanks
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Saturday, July 27th, 2019 AT 5:05 PM

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