Alternator problem on 2021 GMC Terrain

[Doug Domm]

My daughters Terrain had an issue of absolute stone cold dead battery when parking overnight in -40 temps due to a parasitic draw that the techs couldn’t track down. It turned out to be the module for the heated seats was sensing the temp and turning them on. Our daughter in laws Acadia was having a voltage regulating issue which turned out to be the ECU. Our neighbor is a tech and has had several issues as you’ve described and each time it was the ECU.

[Danny Byrd]

Thanks guys for the input, I read in an automotive journal that the alternators are controled by computer chips, in other words cycling up or down as the load demands in order to save fuel. When the tech that checked my car & showed me the printout claiming the alt was within GM specs I offered to bet him $100. dollars that the printout was wrong, he would not go there. Then he said GM would not allow the replacement of the alt if the printout showed it was good.
I went to well respected alternator shop & asked if he could build an alt for my car, he checked & said he couldn't get the parts to do it.

[Kirk Morgan]

Anybody can put a volt meter on the battery or alternator to check voltage. But when you are having problems like you have the system needs to be loaded at different RPMs to be properly tested. Iam thinking the tech may not have tested the charging system under the correct conditions. Did you have the alternator shop test the charging system. The alternator can have diode problems causing these symptoms.