The battery light upon start up doesn't illuminate, then at random times it illuminates solid for a few minutes then goes off, for a few minutes. I hooked up the gauge and the voltage fluctuates from 11.4 volts to 13.5 volts...the light comes on around 12 volts from what I can tell.

I am going to take the batteries in tomorrow to be tested, I just purchased this truck from a GMC dealership 2 months ago, and they changed out one of the batteries because it was bad.. I'm sure I will need to replace both batteries, should I take the alternator off and have that tested too? Can the auto parts place test the batteries on the vehicle or do I need to take those out too? Thanks.

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sounds like a Bad Alternator

The OEM alt is underpowered so best to get a High Output

 

As you verified with the voltage readings, the battery light is pretty much right. As always verify battery connections are clean and tight, including cables from the engine block to the frame. There's one on each side on my '06.

The connector plug on the back of the alternator is prone to overheating. Look for the plastic part to be brittle and corrosion in the connector.

Batteries can be load tested in the truck but one of cables (doesn't matter which one) should be disconnected. Batteries are wired in parallel and a good battery will "cover up" for a bad one if everything is connected during testing.

Alternator issues are often intermittent and more frequent as it heats up. A bench test should include a good load and it may take a few minutes to show up. The alternator is likely your problem and hopefully it hasn't taken a battery out with it. Also, low voltage is hard on the power side of the FICM. Try not to drive it much until repairs can be done.

The factory charging system (single alternator) is borderline as far as amperage output. If the budget allows there are several options for a higher amp alternator, from higher rated factory units to aftermarket. Many threads here at FTE with details on the options.

 

If all grounds and batterys are good, you have a failing alternator. Batterys have to be tested disconnected from one another.

 

I had the same issue. I questioned it foolishly and ended up needing batteries, an alternator and had to re-solder my FICM because it ran too long with low voltage. So don't delay just because the light isn't on constantly.

 

The HO alternators are around $170 on Ebay, that's 250 amp right?
http://www.ebay.com/itm/251140362113...84.m1438.l2649

 

Seems sketchy.

A good deal is step-up to the 140 amp 6G commonly used in the dual alternator and snow plow equipped 6.0 trucks, along with a 58mm pulley. Or a 130 amp 3G alternator from a 94-97 7.3 truck. That option needs a conversion pigtail or hardwired.

Josh

 

You do NOT want this alternator. I had one by the same manufacturer that didn't last a year. You're money ahead getting a DC from Ed @ Ficmrepair.com.

 

Thanks for the info everyone.

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