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05 suburban keeps burning out the hvac control module

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    05 suburban keeps burning out the hvac control module

    I have a customer vehicle, an 05 suburban with the manual hvac controls for the rear. In January of 19 It came to my shop with the rear blower motor not operating. I diagnosed it to find out that the front hvac controls up in the headliner had a melted circuit. I replaced the hvac control unit with the 3 knobs and everything worked as it should. Now a few months later it came back to my shop with the same problem, same circuit melted in the upper front hvac control unit. My first thought was that the blower motor must have a high current draw. I tested it and found it to be at 8.5 amps running and over 10 amps starting. I haven't been able to find a spec to know if these numbers are ok or not. Since then I have reviewed wiring diagrams and it looks like the rear blower motor is fed through a relay so I dont think the motor amp draw could be burning out my hvac board.

    I have 2 questions:
    1. what is the allowable current draw for the rear blower motor so I can rule that out for sure?
    2. Do you have any other suggestions for what could be causing my problem.


    No hard data on what you are asking, and my area is heavy trucks, but I will throw this out. I have chased my tail on a problem only to find an internal short inside the relay case, it is rare and I burned some electronics before i found it. Also depending on the relay type, sometimes there are SPDT relays and DPST relays that look the same and will take the same plug but are most assuredly NOT interchangeable.
    Are the wires themselves what is melting or is the circuit board that is damaged?
    At 15 years old, I would be looking for cross shorts in the wiring that feeds the board. Possibly so sensor wire that is supposed to get +5 volts seeing full 14.2 volts somehow?.
    I know, not all that helpful but the best I can do.


      A problem with diagrams is still finding that relay I suspect highly. Inside those are real points (at least others) that do fail. Should cost you under $20 is worth tossing it over this crap. If it works again check the draw but also open the relay you'll probably see its junk inside.

      Plain harness plugs as well look for ones that show they've been hot. Also knob for fan speed in question look if that's separate behind that or the whole unit? I really can't be sure how rear is just the flipping electrical relays stink in general and wiring connectors. How does that run? Along rocker panels and up or exposed under it? I have one to go look at this exact year whole A/C is FUBAR just for cooling so a neighbor says but otherwise works. I have to wait and ask to go look only if need be it's not mine nor wanted to fix a thing about now if pricey it's a bit rusty too common for anything used and not garaged and it isn't.

      Check away the things mentioned are common to me but check that draw it way too high IMO will check if I can on another for comparison,
      MetroWest, Boston


        For comparison only, My IHC truck has two blowers, each fed individually with its own breaker and resistor pack. Each has a 30-35 amp breaker so 10-20 amps doesn't sound excessive to me.


          Thanks guys for the responses. Sorry for the delay in getting back to you guys. It is the circuit board itself that is melting.
          I bought a new blower motor to test the amp draw of a new one. It is the same as my old one at 8.5 amps.

          Upon further review of the wiring diagram I see that each speed is fed by a separate relay. Those relays are internal in the rear blower motor resistor assembly.
          I have decided to order a new resistor assembly for $95. This will replace all 3 of the relays. I suspect that as mentioned above I have a relay that is internally shorting out and back feeding into the control side of the circuit. I will post back in a few days once I have my new resistor assembly installed. I'm open to any other ideas too.

          Thanks again.


            We LOVE to hear how it turns out, we ALL learn in the process.
            Sometimes you have to really scratch your head at "progress" A switch, a resistor board and may be one relay worked for years, now it is a "package" that cost $95, or worse yet a "stepper motor and digital driver"!


              This would end up a snag like this? Is there the original owner's/operator's manual still with this thing? Look up without fail the plain fuse that covers this could be either inside or under the hood? See what's in there now and the rating that, that matches.

              I just trying not to blow another assembly a fuse hopefully just for this should blow first, common sense may not apply you said it's a 10 AMP circuit blower uses too close to 10 but they didn't ask me - this isn't funny of course.

              Maybe lower it if you can find a 7.5 or so, such that this doesn't just happen again. By rights a little lower a good name brand fuse should blow if exceeding AMPS at a blower or any other reason not yet found??

              You've found the weak link if it doesn't unfortunately at a price that's unacceptable of course for this again. Show owner of this thing if that works again right away if at all possible I know it's a customer's I would hope reasonable people would have a clue if you showed that just that much.

              Other thought may not help is ask if there's prior work for the rear and just what was done. A clue has to be somewhere for a wire that's chaffed could be either way or just a wire enabled by ground or adding power to make this fool thing work somehow I doubt they would use just body metal for ground housings are plastic so look how it's grounded if diagram is good enough it should show that. The arrow pointing at decreasing sizes of lines you know that.

              Does this have a sunroof that gets used at all? Aftermarket or OE the first more of a horror if so.

              Where do wires go along the way to get there? Up the rocker behind the latch for rear door? That's a problem with the best of diagrams doesn't really tell you how it was routed, mounted in a protective cover like that split plastic (forgot the word for that stuff) follow that as much as you can.

              Sorry for the novel but some stuff just has to be ruled out. Said back a few that a neighbor as this one, this model year but can't go digging this much as a favor so when all hell does or did (mostly done taking on much work - did this type dirty hunt for area shops if I wasn't busy) and would go to junkyards that allow "pull your own parts" some do some never for you locally? That was great if torn up exposing everything if you find one already apart. IDK this thing is on the cusp of 15 years old around me will let you cherry pick junk not whole assemblies by this age rust ruins everything usually before this or such that any accident or expensive something just give up.

              Another thought just while here. What type ducts are there that move by actuator it must - I'm asking as I don't know? If this blower is fighting full fan with ducts that aren't working it might stress the motor out?

              Been a while with rear A/C I wouldn't fix any on stuff this old for A/C isn't the whole point just out costs having just front only now plugs so you can NOT have rear A/C family owned Suburbans from 1974 every decade on up almost sh*t when my own MOTHER paid for rear A/C on the last one (1996) the ones from 70s + 80s didn't have it so outlasted the vehicles. Another (an in law) just fixed a 1999 in Louisiana to just front only does Summers in New Hampshire and has for ages now it went fine with two passengers loaded to the gills for routine run can't count on LA to be cold or what for May - Sept. runs takes that one of two to beat miles on.

              In short and just can't be this thing is about a fuse or chaffing wire where the heck did GM run them in their infinite wisdom and yes I own one and would again!

              MetroWest, Boston


                Sorry for two post if you already read "fuse" stuff above. Why lower it with known new? There was a recall now some years ago fuses were random by a Chinese maker despite the markings/colors causing some others (another site) wild headaches or worse totally wiped out wiring up to back of fuse panels where wires are live either right there or how that's fed all power,
                MetroWest, Boston


                  Well I installed the new control assembly and resistor. All is well and vehicle has been returned to customer.
                  I talked with the customer and he decided he wanted a new blower motor installed too in case that could have been causing the problem.
                  I did find that the old low speed resistor was not working (blower didnt work on Low). Hopefully this is why the control unit was shorting out.

                  So in conclusion I believe that the resistor was the cause for blowing the control unit circuit board.
                  Thanks for the help.


                    Sounds like about all you could do. Best wishes it just stays working now I'm not sure it couldn't happen again?

                    Just off the wall other with sealed units wires or the printed circuit board in some plastic if like that there with the assembly take it apart if time if even bust it to see the flaw if anything like that.

                    My own not A/C but GM Pick-up the (1997) was using sealed things. OMG what a chase it was blowing one bulb and clicking from the flasher would quit this game for days or weeks?? Now what?

                    Found in a pretty common looking (it is) whole lens wires go into a plastic area then on to silly bulbs sockets CAN'T be replaced. Saw what I thought was the flaw - heated plastic spot. Turn bulb around was just brake and directional contacted both sides. It wasn't touching hard enough? Bent tiny prongs out and like a sewing needle bent pulled the sealed socket contacts out a bit nothing to lose can get the whole thing.

                    That's been months now made to be real tight with toothpicks, bent and broken not exposed the whole thing seals up back on is so hack of course but hasn't failed!

                    Go figure that crap out?

                    If that was a customer the time spent would be insane - no way could charge much it was a vendetta for fix the stupid thing!
                    MetroWest, Boston