04 chevy Silverado hvac issue

So I haven't actually worked on the vehicle yet, in doing research on the hvac system for this 04 silverado I found a wiring schematic I dont understand and looking for guidance. So the customer complaint is his HVAC wont change from heat to a/c. Assuming it's a blend door activating issue I wanted to know what I was looking at for testing the hvac module wiring and actuators. Looking at the schematic below, using the left air temperature actuator. Here's what I know,

Pin 5 (brown wire) -12v
Pin 6 (dk blue) - control
Pin 9 (Lt blue) - control
Pin 10 (Lt blue/blk) - 5v reference
Pin 7 (yellow) - low volt reference (I'm assuming ground?)

Can anyone clarify how this operates? I looked at the description in identifix but it wasnt much help. Trying to be proactive before working on this truck. Thanks

Awesome man that really helps. Btw where do you get your working diagrams. Identifix as all that information but in separate sections. I like how yours is laid out with it all together. Did you make it that way or does it come that way?

3 wire potentiometer
2 wire reverse polarity DC motor
Low ref is the sensor ground circuit

Should be temperature door commanded and actual counts in hvac module data. There’s going to be a dtc in there to do worse comes to worse follow the flowchart for the dtc.

good info thanks yal

is there any difference between a ground and low referance? or is low referance just another name for a ground?

Little update as i worked on the truck the last 2 days and pulled my hair out the whole time. The only issues i found were the left and right blend doors were stuck in Heat and wouldnt move with hvac control module controls. Also found there to be a leak in the A/C freon system and zero freon. I was able to read codes and view data as well bi-directional control the HVAC using my Verus. I then Back probed at the left side blend door actuator on the control wire and saw 2.5v, there was no change with HVAC operation. I then jumped the control wire at the actuator to low referance and watched the actuator change position, then i jumped control to 5v ref and it went the other way. I did the same for the right side except I gained access at the right side dash junction block. There was a noticiable difference in the air coming out of the vents from a/c to heat and I confirmed actuator operation. I then back probed both control wire for the right and left blend door actuators at the back of the HVAC control module. Both read 2.5v and had no change with operation of the control module. I condemed the control module and replaced with a new one. I followed the procedure to "program" the control module by pulling the HVAC fuse and waiting 60 secs and then installing, key to start and run for 60 secs without adjusting the controls. I wasnt able to communicate with the new module and see data or bi-directional controls. I was however able to see the codes in the HVAC module, they were coming and going sometimes clearing themselves and then coming back, very odd. The new module was adjusting the right side blender actuator but the left was still stuck in heat, as a matter of fact the right side was blowing warm air on cold setting and ambient air on the heat setting. The mode door was constantly moving from vents to defrost and back. I back probed both right and left blend door actuators on the control wires several different times and found different resting voltages from 1.9v to 2.8v, never the norm of 2.5v. I forgot to mention earlier but the other wires at the actuators always had 12v on the brn wire, 5v ref and low referance. With all this i finally felt comfortable to condem the new HVAC control module as defective and im awaiting a new one.

Few questions here for the guys with more knowledge than I.
1. Did I diagnose this right the first time and the second time, and is there anything that im missing?

2. Focusing on the actuators, how does the control circuit operate? what does the control module send down that control wire to the actuator? Is it a signal and the actuator motor uses the 12v, 5v ref and ground the move or does the control wire actually carry a load?

3. The potentiometer is used as a feedback loop so the HVAC control module knows the actual position of the actuator. However does this circuit have control to keep the module from moving the actuator down the control wire or is it just a simple feedback loop?

Sorry for any confustion in my post I tried to explain my thoughts and process. Im in the process of reading pauls book and following along with the videos but im not very far in it and had this issue come into the shop so i dove head first into it.

Could you bi-directional control the actuators? If you can move the actuators with the scan tool, then the HVAC is not at fault. Remember, the computer can disable an output if there's a code for it. Check if inputs are being received by the module and check the feedback circuit as well.

The "feedback" circuit, aka this potentiometer circuit, is ENTIRELY the basis for code setting. When the HVAC module first gets powered up it will check this "feedback" circuit at each actuator for proper range of motion (self calibration) and can disable the actuator from moving again until next key cycle if something's not quite right. This means if the HVAC can't monitor where the door is at, it might not move it. Don't condemn the HVAC without checking this signal wire.

The HVAC monitors the voltage on the signal wire and converts it into "counts" as displayed on the scan tool. (see image). Scope the signal wire, turn the key on and off, or run the self calibration test in scanner, and see if the signal voltage is nice and smooth like in a TPS sweep test. No dead spots!

These three wires (constant 5v, constant 0v ground, and changing signal voltage from motor movement) are identical in operation to a TPS test:

Thrashnasty13 wrote: I back probed both right and left blend door actuators on the control wires several different times and found different resting voltages from 1.9v to 2.8v, never the norm of 2.5v.

That's odd, I thought these voltages were discrete. Check your meter ground.

In the end what matters most is do these actuators move or not when commanded? Do they have codes? Maybe the actuator is fine with that voltage for resting voltage. If it's a problem then I would cut the wire and see if this voltage returns to expected 2.5V and might be affected by a faulty actuator.

Thrashnasty13 wrote: I forgot to mention earlier but the other wires at the actuators always had 12v on the brn wire, 5v ref and low referance.

Lol, I hope you're not saying you have 12v on all those 3 wires, because if you do then you have a bad ground. Also put the first HVAC back in, the replacement sound like it's more trouble than it's worth.

I could access the bi directional controls but it wasnt actuating them just the ones that were working from the knobs on the HVAC module

I used the same ground for the new and old modules and changed them out several different times, the new one had a variable control wire resting voltage and the old had a constant 2.5v on all control wires. The actuators did move when jumped at the actuator or the junction block but wouldnt move with the old module. The codes came and went, B0414, B0424, B0418, and B3770. No i didnt have 12v on all those wires lol. I wrote it a little confusing, 12v on BRN, 5v on the blue/blk and ground on the yellow.

So a signal wire from the actuator not matching the commands by the modules will keep the module from controling the actuator. Also I read useing a jump wire to use battery negative was acceptable, would that be the same with using the ground side on a power probe hook connected to the battery?

How can i verify the integrity of the signal wire? i know i need to load the cricuit and do a voltage drop just from the back of the HVAC module and the back of the actuator. A good reading would be less that 100mA?

I messed with it again today, back probed both the right and left blend door actuators signal wires at the control module. I then jumped them one at a time by using tee pins on the control wires and connecting them to ground and then to 5v ref. Both actuators showed a smooth transition on the signal wire, by this I felt is was safe to assume they both have good actuators and wiring.. I then proceeded to change the control module with another new one. Low and behold every function on this one works. However my left side is still blowing heat, I can literally see the actuator moving and I'm still getting heat. I pulled it off and verified the door was moving and I could control heat to ac. Also when back probing the left side control and signal wire. The signal wire and control look normal when on ac but when I go all the way to heat, once the actuator makes its transition the control wire will go to 2.45v and then starting jumping around anywhere from 1v to 3v. The signal wire is still a steady transition with no dips. When I first started looking at this truck I took the left actuator off and forced it to move. I've read that will mess them up. It still moves but could I have damaged it?
I checked the ground on the left one and it had 40-60 mV on a volt drop test with the circuit loaded.
Also I can still not communicate with the new hvac controller with data I can see the codes (their are none). With the old module I can go into every setting on my scanner. Even now if I plug the old one in I have total control. Both new ones haven't let me communicate with the data.