Alternator problem on 2021 GMC Terrain

Danny Byrd · 01-08-2024, 11:13 PM

Hope some GM Techs on here may chime in to give info on alternator problem on my 2021 GMC Terrain.
Problem is the alternator will not keep the battery charged up to specs.
I took it to 2 different GM Dealers while it was in warranty & both of them ran tests on it & said everything checked ok.
I have a radar detector that reads the cars battery voltage & most of the time the volts reading is between 11.3 & 12.2 Battery aways checks good.
When winter time hits & I start using the heated seat & steering wheel option it won't start, so now I have to use them just a short time. Thanks to anyone that can shed some light on this problem.

340Cuda · 01-09-2024, 02:43 PM

I am no GM tech but my street car always shows high 13 or even low 14 volt readings even with seat heaters on. You voltage reading seem way too low for a working alternator.

Kind of a long shot but have you had the battery checked?

Maybe a wiring harness problem passed were they were checking the alternator?

Hope you figure it out!

Danny Byrd · 01-10-2024, 09:43 PM

Yes the battery checks good.

Kirk Morgan · 01-11-2024, 12:26 AM

Do a voltage drop test. Start the truck with nothing on and check the voltage at the alt. and the battery. It should be same. If the voltage is low you will need a inductive amp meter to see how many amps the alt. is putting out. If it putting out lets say 50 amps or more with nothing on you have a battery problem. If the voltage is low and there are no amps coming out of the alt. the alt. is bad. On some newer cars and trucks the computer is the voltage regulator. I am not familiar with you GMC how the voltage is regulated.

Doug Domm · 01-11-2024, 06:00 PM

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 · 01-13-2024, 12:01 AM

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 · 01-15-2024, 11:18 AM

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.

01-08-2024, 11:13 PM	#1
Danny Byrd Junior Member Join Date: Jul 2023 Posts: 19 Likes: 0 Liked 9 Times in 3 Posts	Alternator problem on 2021 GMC Terrain Hope some GM Techs on here may chime in to give info on alternator problem on my 2021 GMC Terrain. Problem is the alternator will not keep the battery charged up to specs. I took it to 2 different GM Dealers while it was in warranty & both of them ran tests on it & said everything checked ok. I have a radar detector that reads the cars battery voltage & most of the time the volts reading is between 11.3 & 12.2 Battery aways checks good. When winter time hits & I start using the heated seat & steering wheel option it won't start, so now I have to use them just a short time. Thanks to anyone that can shed some light on this problem.

01-09-2024, 02:43 PM	#2
340Cuda VIP Member Join Date: Apr 2003 Location: Tulsa, Oklahoma Posts: 1,621 Likes: 3,560 Liked 720 Times in 310 Posts	Re: Alternator problem on 2021 GMC Terrain I am no GM tech but my street car always shows high 13 or even low 14 volt readings even with seat heaters on. You voltage reading seem way too low for a working alternator. Kind of a long shot but have you had the battery checked? Maybe a wiring harness problem passed were they were checking the alternator? Hope you figure it out!

01-10-2024, 09:43 PM	#3
Danny Byrd Junior Member Join Date: Jul 2023 Posts: 19 Likes: 0 Liked 9 Times in 3 Posts	Re: Alternator problem on 2021 GMC Terrain Yes the battery checks good.

01-11-2024, 12:26 AM	#4
Kirk Morgan Member Join Date: Jan 2009 Posts: 212 Likes: 0 Liked 62 Times in 36 Posts	Re: Alternator problem on 2021 GMC Terrain Do a voltage drop test. Start the truck with nothing on and check the voltage at the alt. and the battery. It should be same. If the voltage is low you will need a inductive amp meter to see how many amps the alt. is putting out. If it putting out lets say 50 amps or more with nothing on you have a battery problem. If the voltage is low and there are no amps coming out of the alt. the alt. is bad. On some newer cars and trucks the computer is the voltage regulator. I am not familiar with you GMC how the voltage is regulated.

01-11-2024, 06:00 PM	#5
Doug Domm Member Join Date: Apr 2006 Location: canada Posts: 248 Likes: 27 Liked 84 Times in 21 Posts	Re: Alternator problem on 2021 GMC Terrain 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.

01-13-2024, 12:01 AM	#6
Danny Byrd Junior Member Join Date: Jul 2023 Posts: 19 Likes: 0 Liked 9 Times in 3 Posts	Re: Alternator problem on 2021 GMC Terrain 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.

01-15-2024, 11:18 AM	#7
Kirk Morgan Member Join Date: Jan 2009 Posts: 212 Likes: 0 Liked 62 Times in 36 Posts	Re: Alternator problem on 2021 GMC Terrain 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.