Parasitic battery drain from HVAC-B circuit

Rwhite65

Original poster
Member
Jul 23, 2013
4
I've recently developed a 1.72 - 1.78 amp parasitic battery drain on my '03 GMC envoy. I've traced the draw to the HVAC-B fuse located in the under-seat fuse box. By pulling the HVAC-B fuse I can see on my multi-meter a gradual drop from somewhere in the 1.7 amp range to under 100 milliamps. With the fuse pulled the entire HVAC system is dead - not surprising. And with the fuse in the entire HVAC system functions just fine. I have no idea how many control modules are fed off this circuit but I'm assuming one or more of the control modules are not falling into sleep mode after the ignition is switched off. How do I isolate the parasitic drain any further? What modules might be suspect?
 

Denali n DOO

Member
May 22, 2012
5,596
Rwhite65 said:
I've recently developed a 1.72 - 1.78 amp parasitic battery drain on my '03 GMC envoy. I've traced the draw to the HVAC-B fuse located in the under-seat fuse box. By pulling the HVAC-B fuse I can see on my multi-meter a gradual drop from somewhere in the 1.7 amp range to under 100 milliamps. With the fuse pulled the entire HVAC system is dead - not surprising. And with the fuse in the entire HVAC system functions just fine. I have no idea how many control modules are fed off this circuit but I'm assuming one or more of the control modules are not falling into sleep mode after the ignition is switched off. How do I isolate the parasitic drain any further? What modules might be suspect?

Welcome to GMTNation :thumbsup:

I think the roadie might chime in on this :undecided:
 

AtlWrk

Member
Dec 6, 2011
674
HVAC-B feeds the logic of the main hvac control module and the auxilliary control module in the center console. If you can get to one of the modules and unplug it you could isolate which one is having the problem. Not sure which one would be easier.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Denali n DOO said:
I think the roadie might chime in on this :undecided:
Man, I HATE being so predictable. :raspberry:

But anybody with a meter who can accurately describe their parasitic drain problem without guessing and flailing about - THAT guy gets my respect. :thumbsup:

Your profile doesn't show you having an EXT/XL, but even a short wheelbase truck may have an intelligent rear seat control module. So it could be either the module in the dash, or one in the console facing the rear seat. As you said, they should both go to sleep with the ignition off. See if you can find a buddy with the same model TB or Envoy and swap. Or check Ebay or the junkyards.

Do you have the digital automatic system or the manual slide pot controls?

EDIT: Damn! Scooped by typing too slowly. That almost NEVER happens! [congrats!]
 

Rwhite65

Original poster
Member
Jul 23, 2013
4
AtlWrk said:
HVAC-B feeds the logic of the main hvac control module and the auxilliary control module in the center console. If you can get to one of the modules and unplug it you could isolate which one is having the problem. Not sure which one would be easier.

Thanks for the VERY QUICK reply. I'll attempt to locate both and disconnect each with multimeter in series with the battery to see what happens to the amp draw. I'm assuming if one of the control modules does not fall into sleep mode than the only option is to replace it. is that assumption correct?
 

Rwhite65

Original poster
Member
Jul 23, 2013
4
the roadie said:
Your profile doesn't show you having an EXT/XL, but even a short wheelbase truck may have an intelligent rear seat control module. So it could be either the module in the dash, or one in the console facing the rear seat. As you said, they should both go to sleep with the ignition off. See if you can find a buddy with the same model TB or Envoy and swap. Or check Ebay or the junkyards.

Do you have the digital automatic system or the manual slide pot controls?

Thanks also for the quick reply! My Envoy is the short wheelbase version. It does have the rear seat / rear passenger HVAC control (not sure if this is std. On all models). I also have the digital automatic system. I don't have access presently to a doner vehicle but I have a brother-in-law that own a TB that's maybe only a year newer ('04). I'll pursue swapping mods once I figure out where and how to.

I'm presently traveling so it will be the weekend before I can follow up. Thanks for the tips thus far.
 

Rwhite65

Original poster
Member
Jul 23, 2013
4
AtlWrk said:
HVAC-B feeds the logic of the main hvac control module and the auxilliary control module in the center console. If you can get to one of the modules and unplug it you could isolate which one is having the problem. Not sure which one would be easier.

Problem found!!! It turns out that I have a faulty auxilliary control module. (This is the HVAC module mounted into the center console that faces the rear seats and allows the backseat passengers to control their climate.) As AtlWrk suggested, I disconnected the rear climate control module - which after some experimenting turned out to be pretty easy since the control module simply pops out of the rear console with a little help from a small flat screw driver. Once the module was outside the console I could easily disconnect the wiring harness feeding it.

With the wiring harness disconnected I tested the draw with my multimeter and after a couple of minutes the draw dropped below 0.00 amps. Just to be sure I plugged the module back in and the amp draw went up to 1.75 - 1.80 and held constant, indefinitely. After 30 minutes I disconnected the rear HVAC module again and after a couple of minutes the draw, as expected, dropped below 0.00 amps.

I just looked at e-Bay and can find a used, warrantied rear HVAC control module for $39 plus shipping. Considering that I've only used the rear temperature control module about 3 times since I bought my '03 Envoy new it's no big deal if I ever replace it.

Now I can leave the HVAC-B fuse in while the ignition switch is off and not worry about waking up to a dead battery. Thanks everyone for all your help!! :wootwoot:
 

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