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mikeha ... 3/14/2019

2007 Chevrolet Colorado WT 4 Cyl 2.90L


Why would my truck have issues starting / maintain high voltage

Hi all,
I've recently gotten a 2007 Chevy Colorado with 37,000 miles on it. The truck runs great, however I noticed on two occasions it was rough to start. I correlated those two instances, which were the next time I drove if I had sat in the truck or had the interior lights on after turning the ignition off.
I went to Advanced Auto Parts to have the battery and everything checked, and the battery came up good, however it said there was a charging system - high voltage. With no load, it's running 15.46, with a load it's running 15.31. I changed the alternator, hoping that would fix it, and I needed to change the belt as well so I figured knock it out. I got an OEM alternator.
I replaced it, and got it checked again - the exact same issue came up. All of the wiring, from what I can see, look clean. Any input would be greatly appreciated.

1 Answer


Jimm 3/17/2019

The alternator is NOT on during starting, so immediately after the heavy load of starting, the voltage will read in the 11+ volt range. After the engine is up to speed for a 2 to 5 seconds, the computer turns on the alternator and the voltage goes to 14.1 or 14.2 volts. After that the alternator maintains 13.8 to 14.2 volts. Running in the 13.8 range as the alternator and battery gets warmer. This exactly meets specs for charging a lead acid battery. The spec is 13.8 to 14.2 charging voltage. Charging at voltages higher than 14.3 will cause the battery to gas (boil).

High charge rate indicates either a full field condition in the regulator, a shorted field wire or a bad battery.


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