HVAC system stuck on heater

fl0w3n

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Jan 8, 2015
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I've got two problems going on, I think I know the solution but want to confirm.

First is, truck randomly goes to full heater mode regardless of input from dash controls. It does it on it's own too, not like I go to heat and then it won't come back. Just now sitting idling it all the sudden just started blowing hot after 45m of running on cool. I believe this is the blender door(s)? Something about them getting stuck or the computer not knowing their position so defaulting to a preset location?

Question is, how do I fix it? Is this going to require full disassembly of the dash all the way to the firewall?

Second issue, #5 fan setting only intermittently works - more often it does not work. This is fixed by changing out the blower resistor, correct?

This is on my 04 with the basic dual climate system
 

2004LB7

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Pull the hvac fuse to try and reset the blend door position. I believe the resistor is correct for the fan speeds
 

fl0w3n

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Pull the hvac fuse to try and reset the blend door position. I believe the resistor is correct for the fan speeds
Just did a commanded relearn with my Tech 2 on the blend door position, seemed to have worked for now.
 

fl0w3n

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Jan 8, 2015
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That worked for about 30 minutes. On my way home it decided to melt me. Great timing because it was 100 out today.

Which part do I need? It looks like I’ll have to completely tear the dash apart to get to it. While I’m there I definitely want to only do it once and do anything else that might go wrong at the same time.
56AD43D9-51E2-4925-8E23-72E469DCB82F.png
 

2004LB7

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I remember a post from theferminator mentioning the grease in the blend door gear box dries out and they start to act like that. Supposedly you pop the cover off and clean out the old grease and repack it and be good for another 10 years
 

fl0w3n

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Jan 8, 2015
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I remember a post from theferminator mentioning the grease in the blend door gear box dries out and they start to act like that. Supposedly you pop the cover off and clean out the old grease and repack it and be good for another 10 years
@THEFERMANATOR care to shed any light??

Okay but which one? It looks like there's a passenger, driver, mode control, and I'm seeing on All Data DIY a Temp Door Actuator? 1592533901118.png
 

1FastBrick

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Dec 1, 2016
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This bad boy will be your best friend!!! It's a tedious task for sure. I have only had to do one so far about 4 years ago. I think the airbag comes out to do it.

There is not alot of room even if you remove the upper dash pad. Its right next to a vent and there is a small brace in the way.
 

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1FastBrick

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I recall it saying something about the bottom one being a blend door too. That one is easy to get to just remove the cover. How ever my case was similar it was hot and even though you turned the AC on it was blowing heat I believe from both sides. And of course it was a hot day like your case...

I also recall there being different parts for digital Vs manual controls. The first motor i ordered was wrong and I had to get a replacement from a local autoparts store.

Fun times. The upper cap of the dash off will give you more room. The stamped metal bracket is sharp so very careful.
 

1FastBrick

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Oh and the resistor also has to be correct for your style of ac system. This part is usually the cause of the loss of 1 of the speeds for the motor.

Mine shorted on a hot day in Stop and go traffic with the AC on full blast... I almost crashed when it filled the cabin with smoke blowing out the AC vents... I smelled it just before it came out the vents. Melted the pig tail to the resistor. Had to replace both.
 

2004LB7

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Found this on another site:

Blend Door Actuator:
The blend door actuator is controlled by the vehicle's climate controls. The blend door can open and close at various positions to block or allow air passing through the heater core in order to achieve the desired temperature inside of the vehicle. The blend door actuator is typically located closer to the heater core and evaporator than the mode door.

Mode Door Actuator:
The mode door actuator controls which vent(s) air blows through. The door on this actuator has two positions: closed and open. Today, most actuators are controlled electronically. As actuators wear over time, the plastic gears inside may be stripped or broken, causing them to fail

It almost seems that you may have both going out, but more then likely the blend door would be the issue for your heat problem.
 

fl0w3n

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Jan 8, 2015
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Just an update, the driver side is the one on the bottom (which wasn't shown on the diagram). Luckily it's relatively easy to reach.

I took it out and took it apart, and sure enough there was grease over the contact for the doors "position sensing"

Pulled the whole thing apart, used some MAF cleaner and a Q-Tip on the pad, put it back together and did a reset with my tech 2. It's working for now, I'll monitor over the next few days. I'm hoping it's fixed.

I'm hoping the top hard to reach one (passenger side) isn't suffering from the same thing, but it's likely installed with the motor on top so any melting grease would drip away from the contact pad instead of onto it.

Blend Door.jpg
 

dndj

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Apr 13, 2019
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Diagram in post #6 is correct, at least it matches the official GM FSM. Blend doors control cold/hot temp, there is one for the driver and one for the passenger side on trucks with independent L/R air temp controls whether manual or automatic option.

Driver blend door is not visible in the diagram as stated, but it's down on the bottom of the HVAC unit, access from the passenger side footwell.

Passenger blend door is the one up on top which is very difficult to get to - special tools as mentioned above. I've also seen Youtube videos where a guy sectioned the plastic air duct to get to it, then taped it back up with HVAC ducting tape when he was done to save time. Not a fan of this approach.

There are 2 other actuators - mode door, which selects which vents the air comes out of. You can see this one if you lay down in the driver footwell and look up above the throttle pedal. Lastly, the recirc door actuator which is up high on the right side as shown in the diagram.


Glad you got it fixed, and thanks for the high-res pictures of the internals. Very helpful!!!
 

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