Electrical gremlins

Blue924.9 · December 23, 2022

Hello new member here. Picked up a 03 gmc 2500hd 4x4 with the 8.1 and the Allison this summer with 140k no rust which is rare up north. Drove it a couple times this summer with no issues and then this winter I started having battery drain issues. Checked for drain with a meter but did not have anything only 33 milliamperes. Put a maintainer on the truck with a new battery as the old one had been drained enough to cause it to go bad. The battery drain was enough to trip a code on the maintainer which happens over 1 amp.

I’m currently having issues with the hvac module the truck has dual zone auto hvac. while the battery was going dead the hvac was stuck at 75 with no fan I later figured out that was a calibration procedure. Once the calibration would complete the hvac would work as normal at this point the new battery and maintainer are on I unplug and plug it in to charge up to 2 amps for 12 hours to at least keep the battery from going dead. Thinking the hvac is fixed I go to use the truck about a week later and the hvac is stuck on 71 none of the buttons on the hvac work or respond to inputs and the fan says it’s on but is not. Didn’t have time to mess with it and used a different vehicle. That night the error code is not on the maintainer like it was every night for the last two weeks. Left the maintainer for a week and it never tripped the excessive drain code. Went to try the truck this morning and hvac module still stuck on 71 with no response. The module does seem to be replaced by previous owners as the color doesn’t seem to match the other pieces of the interior and there is virtually no wear on the buttons. I pulled before the battery was replaced after driving the truck which I will attach to the post. All the other functions seem to work like radio and dash with the exception of the gear display which washes out after 15 or so minutes of driving. The other functions working make me think the bcm is not faulty

David89GMC · December 23, 2022

If the auto hvac unit didn't come with the truck, what did? Look at your option codes in the glovebox and verify it was originally manual or auto.

Keep in mind that cheap accessory units can fail very quickly, best to get a used OEM unit than a a new one made in China. And stay away from Dorman if you can. And get the exact replacement unit if you know the original number. There are many different units that will may not be compatible with the system controller.

Good luck.

Blue924.9 · December 23, 2022

9 hours ago, David89GMC said:

If the auto hvac unit didn't come with the truck, what did? Look at your option codes in the glovebox and verify it was originally manual or auto.

Keep in mind that cheap accessory units can fail very quickly, best to get a used OEM unit than a a new one made in China. And stay away from Dorman if you can. And get the exact replacement unit if you know the original number. There are many different units that will may not be compatible with the system controller.

Good luck.

is there a list somewhere of the option codes? It’s a well optioned truck so I would guess it was automatic, is it Possible to switch it to a manual or vice versa hvac?

I’m saying it appears to have had the oem hvac module replaced with an aftermarket one due to things not quite matching up

davester · December 24, 2022

16 hours ago, Blue924.9 said:

is there a list somewhere of the option codes? It’s a well optioned truck so I would guess it was automatic, is it Possible to switch it to a manual or vice versa hvac?

I’m saying it appears to have had the oem hvac module replaced with an aftermarket one due to things not quite matching up

In the glovebox, there's a sticker with a table of 3-character alphanumeric codes, which describe the options your truck came with. The best source for getting what most of the codes mean (some are internal gm codes that don't have a public description) is the full service manual for that year, but there are online sources that are pretty good. Google "RPO codes" for sites that have lists of what the various codes mean.

It's useful to keep those codes handy for ordering parts as well, as they will help select the correct parts for your vehicle, based on it's options.