Alternator Failing?

[email protected] · October 3, 2011

I have a 2007 Suburban -- 5.3L AFM with 55k mi. that has failed to start a dozen or more times since I bought it a year ago.

I had the battery tested at multiple places (autozone, local mechanic) and it always tests out fine. I've checked it w/ multimeter myself as well -- even after sitting over night or longer and it always has over 12.5V.

I have noticed that it tends to fail if it is driven for a while with the alternator is producing in the lower range (~12V), I make a stop and then try to start it again and it just clicks. Interior lights, radio, etc all work fine and it jumps quite easily.

I've read about the potential drains on the battery (windshield washer fluid heater, heated seats, etc) but it doesn't seem to be related to my issue since I can start it up w/o issue, drive it for 20 minutes, stop it for 3 minutes and then it just clicks when I go to start it. Has to be the alternator, right?

What controls the variable output of the alternator?

Thanks in advance,

Dan

nickvrebel · October 3, 2011

doesn't sound like the alternator. sounds more like a bad connection or bad wire. I've seen a couple people on this board comment about similar issues and fixed it by replacing the ground wires from the battery to the block and chassis

impalabelt · October 3, 2011

I agree with NickV's assessment. Definitely sounds like a connection or wire issue. Additionally, it's been my experience that the alternator puts out to match system load demand (i.e. the alternator is capable of 13v-14v output or higher, but the load on the systems are only demanding around 10v-12v depending on battery state thus the alternator only outputs 12v [edit: this is just an example and over-simplied of course.]) with respect to its rated base output. It sounds like nothing in your system is making a large demand of the charging circuit, thus the alternator isn't working very hard. I could be wrong. But I don't suspect the alternator in this case.