Climate Control Module Problem

[FlyingV92]

96’ 2d Tahoe.

I’m on my 5th climate control module in a week.

It seems like something is grounding out possibly but have no idea where to start. The AC and Recirc buttons do not light up at all once it goes out. Blower, temp control still work. No power going to compressor to kick it on after the short. Tried jumping high pressure and low pressure switches with no luck. The problem lies with the control module and it’s wiring my guess. It will last at best 1 day or so before it goes out.

Attached you should see the only indication of a problem on one of my control modules. Another one looked completely fine. The first had melted the transistor underneath the switch for the recirc.

I have seen other post on this but nothing certain other than the posts on the transistors.

I really don’t want to open the dash to look for exposed wires but I may have to at this point unless someone can give me some solid pointers or systemic troubleshooting.

I checked my connections in the pigtail with the ten wires on the right hand side of the control module but nothing was bare, crimped or mangled.

There are no blown fuses in the cabin or under the hood and the ac relay is fine.

Really at a loss here as it’s mid June in Texas.
Any help is welcomed.

Thanks,
Matthew

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[kennythewelder]

Im kind of having the same issues with mine, but I haven't narrowed it down yet. In my testing, I realized that when I test the relay with a test light the compressor would come on with the relay in place but lifted slightly to access the relay pins. Only one of the pins would do this. So then I figured out it was because it was getting a ground through the test light. I jumped this pin with a wire and toughed it to a ground, and the compressor stayed on as long as it had a ground. So I did a bunch more testing and got no where. As a temp fix, I added a toggle switch to a ground, and ran a wire to that pin on the AC relay. After doing some research and looking at wiring diagrams, I realized that the ground it what activates the AC relay. Only thing is, if you do this temp fix, when the toggle switch is on, the compressor will be on no matter what. It by passes all safety systems. Like you, I live where it is very hot ( south Louisiana). So like you, I need my AC. This is not a permeant fix. Just something to get my by. If there is a short, then this may not be a good idea. You could add the ground wire and let the system run like that in your drive way for a while and keep an eye on the wires, checking to see if any of them get hot. If not, then this will at least give you a temp fix.

 

[FlyingV92]

It’ll definitely give me a temporary fix albeit not the safest one like you mentioned. Worst comes to worse I may have to do that for now.

Could the blower motor resistor, although mine is working, be causing an overload in the circuitry on the climate control board? Same thing with the blower motor itself?

 

[kennythewelder]

It’ll definitely give me a temporary fix albeit not the safest one like you mentioned. Worst comes to worse I may have to do that for now.

Could the blower motor resistor, although mine is working, be causing an overload in the circuitry on the climate control board? Same thing with the blower motor itself?

It is hard to say. I wouldnt think so, but without testing everything, its imposable to rule anything out. I was looking for this grate video that explains how an automotive HVAC system works but I cant find it. Its some where on youtube. Basically the controls are logic, and the controls open the ground to the relay if all of the perimeters are meet. Like as long as everything else is ok the the logic side lets the relay get the ground to turn on the AC relay. I have not narrowed mine down yet, but I dont thing I have power at the HP and LP switches. Everything that feeds the logic side, is all +, I think. So if the control side sees all of the + from each section, them it lets the - close completing the circuit, and engaging the AC relay. But if any of the + systems sees an issue, then the control side, will not let the relay see the ground it needs to kick on the Ac clutch. After seeing this, thats when I jumped my relay and decided to leave it like that for a while. Also, by doing this, the compressor runs all of the time. As long as the toggle switch is on, the compressor is running. So if it gets cold in the truck, it will not disengage the compressor. Not that its going to get cold in the summer months in the deep south where we live. But just FYI.

 

[AK49BWL]

@FlyingV92 : Something is pulling far more current than it should be, and the traces on the controller are melting because of it. That trace also seems to be the one that burns out on a LOT of these control units... Which honestly leads me to suspect the replacement control units more than the truck. That trace doesn't really provide power to any high-current components, so I can't wrap my mind around why I see so many burned up in the same place, unless something on the control head itself is burning it out. It's not the blower or blower resistor either - that's a separate circuit.

@kennythewelder : The HVAC pressure switches on the 96+ trucks are GROUNDED on one side, NOT powered. The other side of each runs to the PCM. While both switches are closed, the compressor will (should) run. There's also a third switch on the 96+ models (maybe the 95s too, not sure) -- THIS one is where the positive (battery) signal wire from the control head goes, and then the other side of it goes to the "A/C Act" input on the PCM. On mine, the pressure switches High and Low are on the high line leaving the compressor and the dryer respectively, and the activation switch is in the back of the compressor. On the earlier models, both pressure switches run in series to the compressor clutch, which is activated directly by the HVAC control head unit. -- HVAC control head unit -> A/C Relay (Battery +) -> Low pressure switch -> High pressure switch -> compressor -> ground.

 

[kennythewelder]

@FlyingV92 : Something is pulling far more current than it should be, and the traces on the controller are melting because of it. That trace also seems to be the one that burns out on a LOT of these control units... Which honestly leads me to suspect the replacement control units more than the truck. That trace doesn't really provide power to any high-current components, so I can't wrap my mind around why I see so many burned up in the same place, unless something on the control head itself is burning it out. It's not the blower or blower resistor either - that's a separate circuit.

@kennythewelder : The HVAC pressure switches on the 96+ trucks are GROUNDED on one side, NOT powered. The other side of each runs to the PCM. While both switches are closed, the compressor will (should) run. There's also a third switch on the 96+ models (maybe the 95s too, not sure) -- THIS one is where the positive (battery) signal wire from the control head goes, and then the other side of it goes to the "A/C Act" input on the PCM. On mine, the pressure switches High and Low are on the high line leaving the compressor and the dryer respectively, and the activation switch is in the back of the compressor. On the earlier models, both pressure switches run in series to the compressor clutch, which is activated directly by the HVAC control head unit. -- HVAC control head unit -> A/C Relay (Battery +) -> Low pressure switch -> High pressure switch -> compressor -> ground.

Yes, you are rite. It all works on low voltage. As you mentioned, it very well could have a short.

 

[FlyingV92]

So I believe I figured out the problem.
I combed over the diagrams and schematics and came to one conclusion. It has to be a ground.

I didn’t believe there could be a wire exposed inside the cab since everything was factory and still had the matching manufacturing dates on it so I decided to leave that alone as the last option.

So I went through and found every single ground for the ac system as well as the frame to cab/engine grounds.

I found one mount that was loose/off. This mount was one of two that are directly under the firewall on the passenger side frame. Both are anchored on the frame at the same point on one side however on the other they have two different mounting points. The one going to the firewall under the evap canister was fine and connected. The other going to the bell housing which also holds a harness guide for the bank 2 sensor 1 o2 sensor was not connected. I had recently changed the o2 sensor and did not put this ground mount on.

I believe this was the cause of my problems because soon thereafter the frying ensued. Once I would hit a bump this would cause intermittent connection/lack thereof causing the overload on the board. The times it lasted and went out I was driving along a bumpy road.

Since relocating the ground I have had no issues with the ac and have been steady cold air.

I’ll update the post from time to time and say whether or not the air is still holding. I’m pretty confident it will, but again shall see.

Thank you for your help guys. And Kenny, maybe this could be your cause too. Check all your major frame to engine/cab grounds and see if any are loose/off. If they are replace them and see if anything changes.

Regards fellas,
Matthew

 

[AK49BWL]

Thanks for coming back with an update and potential fix! Too many don't do that lol.

 

[FlyingV92]

Update: haven’t had any problems since the fix. Things have been nice and cold.

 

[someotherguy]

Grounds are important. That power will find a path, and without proper grounds, it'll find the path you don't want it to.

Richard