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04 GMC Yukon no rear heat only cold air. The rear control…

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04 GMC Yukon no rear...
04 GMC Yukon no rear heat only cold air. The rear control module and the hot/cold actuator are working. Rear hot/cold door turns easily manually.
Submitted: 2 months ago.Category: GM
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Answered in 5 minutes by:
3/25/2018
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago
Jerry Newton
Jerry Newton, GMC Technician
Category: GM
Satisfied Customers: 5,662
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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Are the heater hoses to the rear of the vehicle both hot?

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Customer reply replied 2 months ago
I can manually turn the blend door and get hot air.
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

Ah, OK, you said the actuator was working. The electrical actuator that moves the temp door, is that not working? Did you remove it so you could move the door manually?

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Customer reply replied 2 months ago
I swapped the acuator and module from my othe Yukon and same thing. I pulled the hvac fuse and did a relearn and the acuator just goes to full cold and will not mover back
Customer reply replied 2 months ago
I removed the acuator and can move it manually
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

You should have power on the brown wire, ground at the black wire for that actuator, can you test those circuits?

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Customer reply replied 2 months ago
I will test a minute
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

OK

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Customer reply replied 2 months ago
Battery is dead in my volt meter. I will go get a new one.
I reset the acuator to center , do a relearn and the acuator moves to full cold so it must be getting power
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

How did you reset the actuator to center? You shouldn't attempt to move those by hand, or put power to them, it typically ruins them if you try to manipulate them like that. They shouldn't even be connected electrically unless they're installed and screwed in place.

If we have power and ground, the signal from the HVAC control would be the next circuit I'd test. I don't have the expected voltage in front of me, but I would measure it and just watch to see that it does something different when moving the dial from hot to cold.

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Customer reply replied 2 months ago
I opened the acuator pulled out the center gear out and moved the lever back to center.
Then I swapped the floor/face acuator with the hot cold acuator. Both acuator swirl in the floor/ face door but go directly to cold in the hot cold blend door
Customer reply replied 2 months ago
Both acuators work in the floor face blend door
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

OK, are you able to measure the signal circuit to the actuator with your multimeter?

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Customer reply replied 2 months ago
I am buying a new volt meter now
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

OK.

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Customer reply replied 2 months ago
What the best setting for signal current?
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

I'll have to do some research to verify this, but my recollection is that this actuator responds to a 0V-5V signal, so DC Volts is what you're looking to measure.

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Customer reply replied 2 months ago
I have a brown white yellow blue and gray what wire should I have signal voltage at
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

By my schematic you only have three wires there. Let me look again.

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GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

OK, I have the right schematic now. Power on the brown wire, ground on the yellow wire.

White/black should be the command. Gray should have 5v on it. Dk Blue is the signal wire back to the processor that tells the controller whether the door moved as commanded or not. Here is a pretty good description of the circuit and how you can measure it.

The auxiliary air temperature actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5-volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5-volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometers adjustable contact changes the door position signal between 0-5 volts.

The auxiliary HVAC control module uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count range. When the module sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module. Once the position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.

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Customer reply replied 2 months ago
Gray has 5 volts , white black has 2.5
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

OK, when you move from hot to cold on the controller, is that 2.5V reading changing at all?

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Customer reply replied 2 months ago
No change just 2.5 volts
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

Which control head are you using, front or rear?

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Customer reply replied 2 months ago
Front I did try the rear earlier but did not check the voltage, I will
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

OK, you know you have to set the front control to REAR so the rear controls will work, yes?

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Customer reply replied 2 months ago
it needs to be on AUX
Customer reply replied 2 months ago
Same thing, no movement from 2.5
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

That is interesting. Based on those results, it looks like the processor is not commanding this actuator to move. I think you need a new processor. Let me get you a part number and location on that, bear with me.

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Customer reply replied 2 months ago
I don’t see anything about a hvac Processor is this in the front or back of the car
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

Let me get the location and procedure for you. I'm sorry for the delay, I've taken a number of phone calls this afternoon.

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Customer reply replied 2 months ago
No problem, I have a identical Yukon so maybe I can swap the processor and try it
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

#9 in this image.

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Customer reply replied 2 months ago
Are you talking about the rear hvac control module? I already tried that it works fine in my other truck
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

The one that is pointed out in that image, attached to the rear HVAC assembly?

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Customer reply replied 2 months ago
Swapped #7 blower control processor,
Centered the acuator did relearn same thing back to full cold
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

OK, I am looking at #9, not #7. #7 controls the blower motor, not the actuators.

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Customer reply replied 2 months ago
I already swapped #9 , that seems to be the most common problem, videos all over YouTube about it but that is not my problem, the control modules work in one truck and don’t in the other
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

This device is what commands the actuator to move, based on input from the control heads. Does the other actuator work, the one that changes modes from ceiling to floor?

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Customer reply replied 2 months ago
the floor/face works, just not the heat/ac. I have swapped 4 acuators and 2 control boxes , I am pretty sure that is not it
Customer reply replied 2 months ago
all acuators and control modules work in my other Yukon ( modules moves the heat / air in one truck but not in the other)
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

I understand. Let me get back to the schematics and study this a bit.

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Customer reply replied 2 months ago
Any other ideas?
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

So much has been done here.... have you ever done this recalibration procedure?

  1. Clear all DTCs.
  2. Place the ignition switch to the OFF position.
  3. Install the HVAC actuator.
  4. Connect all previously disconnected components.
  5. Remove the HVAC/ECAS fuse for a minimum of 10 seconds.
  6. Install the HVAC/ECAS fuse.
  7. Start the vehicle.
  8. Wait 40 seconds for the HVAC control module to self-calibrate.
  9. Verify that no DTCs have set as current DTCs.
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Customer reply replied 2 months ago
I have done the recalibration several times. The control module must not be getting signal from the controls. The default setting is cold if no command is received.
Any commen problems with the wiring from the controls to the rear module?
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

I can't imagine a scenario where two separate HVAC control heads have failed to send the signal to the processor. It's just about impossible.

This has scan tool diagnostics, frankly, if this were in my shop for repair, my first step would have been to put a scan tool to it, retrieve trouble codes if any, command the actuators and watch them move, etc. This is really a ten minute diagnosis with the right tool.

There are also temperature sensors that play into this, if there is a sensor that is reading way too high, the processor assumes it's too hot in the vehicle and moves the door to cold. The processor is moving the door on purpose, it's being told to do so by something.... what that something is, only the scan tool can tell you.

You can continue swapping parts, you have a donor vehicle and that's something most people don't have. I would personally spend the $75 or so and have this diagnosed by someone with a good scan tool, simply because my time is worth something to me, and I wouldn't want to spend hours and day fussing around with it. I just need to get to the root cause, fix it, and move on.

Looking at the schematic, a temp sensor problem is the only thing left that makes sense, there are two in the rear, one in the front, and we don't have any idea which one of them is causing the problem. Signals from the control heads come in two different forms, one directly, one over class 2 data bus. Neither of them is affecting the operation, though, so there is definitely something telling the software to swing that door full closed.

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GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

Did you get this worked out?

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Customer reply replied 2 months ago
I did I little more research and it looks like gm updated the control module. The old module allows the acuator to over travel as the blend door wears. The new module has different programming to prevent this. I have a new module ordered
GM Mechanic: Jerry Newton, GMC Technician replied 2 months ago

Excellent, I look forward to hearing a success story!

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