No rear heat

[stilllearning6]

2003 Tahoe:
front heat AOK, coolant AOK.
Rear vents blow only cold.

Took apart the rear passenger side covering to reveal the HVAC system:

Hot coolant gets to heater core and return coolant line gets hot: heater core AOK.

Removed the Temperature Blend Door Actuator (HVAC transmission gear) box.
Inside gears look perfect, no cracked gears, nor cracked axles.
Inside transmission motor has continuity- AOK gear box box.
I feel that the TBDA gear box is AOK.

With the Actuator gear box removed, I could rotate the Blend Door axle to deflect hot air out of rear vents.
I conclude that the Blend Door is not being told to rotate.

With Actuator gear box unbolted and uncoupled from the Temperature Blend Door axle, I tested the Actuator gear box rotation via changing head unit temperature dial. The gear box Actuator never turned.
Conclude: either bad input signal to Actuator gear box or Actuator gear box is faulty. Since the Actuator gear box looked AOK and the gear box motor had continuity, I suspect that the Actuator gear box is not getting the correct signal.

I used multimeter to obtain the electrical signal from the HVAC Temperature Control Module ( possibly AC Delco PN 15832319 and /or PN 3734-07705535; Dorman PN 601-036) that sends signal to the TBDA gear box.

There are 5 wires from Temperature Control Module to the Actuator Gear box.

Results of multimeter with engine running at operating temperature and rear heater core is hot:
Brown wire: supply 12v-14v . Conclude AOK .
Yellow wire: ground . Yes, grounded to chassis , zero resistance. Conclude AOK.
Blue wire: Signal wire: 2.49 volts while engine running. Voltage holds at 2.49v even when I rotate the front dash area head unit temperature dial to change temperature from 65 degrees to 90 degrees. Conclude : ???
White wire jumped to Grey wire: 0.01 volts even when I rotate the front dash temperature dial from 65 degrees to 90 degrees. Conclude: ???

I suspect that the signal from the HVAC temperature Module is not giving the correct instructions to the gear box Actuator to change the position of the Blend Door. I believe that the white wire jumped to the Grey wire should have experienced a voltage change as I turned the temperature dial at the head unit.
That is my theory.
Can anyone add to this? Confirm or refute?

I plan to swap the $100 HVAC Control Module , as I believe it is the culprit.
Any feedback is appreciated.

 

[stilllearning6]

Update:
Before spending $100 for a Rear HVAC Control Module , I swapped the rear seat area (auxiliary) Vent Actuator ( which works) for the rear Temperature Blend Door Actuator.

To my surprise, the Temperature Blend Door moved.
This contradicted my theory that the rear HVAC Control Module ( 2 sheet metal screws attach it to the rear HVAC ductwork) was the culprit.

I now get heat in the rear.

The original Temperature Blend Door Actuator became the prime suspect., though it appeared to be in great shape. No visible gear damage, no teeth missing, no cracks seen, splines appear to be snug on the blend door axle.
I am curious to learn how this apparently good looking Temperature Blend Door Actuator behaves faulty.

I dissected the actuator and saw that the drive gear was over extended in its arc pathway.
I repositioned the drive gear, reinstalled the actuator.
Then, did an initiation index for over 30 seconds.
Reinstalled HVAC 10 amp fuse and started vehicle.

I rotated the head unit dial knob to force the rear vent between 50% rear ceiling and 50% rear waist area side vents.
Result:
The air blew from both ceiling / side vent at 50% each way to 100% from ceiling. Good.
I rotated vent dial to direct air to 100% side , 0% ceiling.
Result: FAIL. No change in vent air .
Actuator would not reverse direction.
Actuator appears to be turning in one direction only and will not reverse.

Ordering $40 actuator Dorman PN # 604-100 from Advance Auto, arriving within a week.

The one way rotation of the actuator is why I could not get heat when the actuator was residing in its original OEM position at the Temperature Blend Door control position.

Why did that one way Actuator rotation occur?
This was an unexpected finding. I was so close to blaming the HVAC Control Module as not providing signal to the Temperature Blend Door.

 

[RobbysDad]

Check out post I put up Heat AC Problems

I wanted to put this here too.... Check out the post I put up called that... It will fix your AC and Heat in the front and the rear.....They go bad and simple fix...

I will copy and paste it here too.....

I keep reading through posts and keep seeing problems with the heat and the AC. Here is the problems.... They can blow warm or cold air on the driver and passenger side, get stuck, turn the key on and off and it will work.. Weird I know.... That is until they go out 100%..... Make sure you get the p/n off old one.. They all look identical, but do different functions by the way the wires on the plug go in... Super Important to get the correct one.

Do not turn them to match how yours is.. They are already calibrated brand new in the box.... Their is a way to reset the module, but we will stick with this.

First one is located under the console and go at it from passenger side door and it a little pain to get at but use a flash light and you will need either I think it is a 6 or 8mm socket and an extension... This one controls if you are getting hot air out one side and cold from the other. Usually you hear a clicking when they start to go bad. I know its weird but turning off the truck and turning it back on will get it working temporarily, but then they fail 100%.

Second one is located behind glove box.. Get everything out and push on the sides so it can drop down... This one controls where the air is going to blow being defrost, vents, feet.... Super easy to change...

Third one I never changed and it a pain I hear due to you have to pull the drivers dash out to get at it... This one controls your fan speed...

Fourth is located in the rear that controls back heat/AC and is located under the passenger side huge panel in the cargo area... Pull up trim that is on bottom and it clips in, so take it slow and they pop out... Then you need to pull off the trim going up, which is same way and get both of them out of your way.... Then you can get the big panel off and you do not need to take it off all the way to get at this one... Just enough to get access at it.... Put new in and reverse steps putting it all back together....

Super important you pull the p/n off old one due to one may have 4 wires and one may have 5, etc.. They are not interchangeable.

Once again do not turn the new one with a screw driver or anything thing they are calibrated all ready to go and need to work it on so it will close the damper door. Just go slow.....

I have not changed one in a while, but I know I will be soon and their is a way to reset your heating and air system, but cannot think of the name... That is super easy and no scan tool or anything required...

Any questions send me a message and I will walk you through these.. Super easy and they are not the cheapest... AC Delco will cost around 100.00 and you can get generics a lot cheaper... I am not a big fan of generic parts that are electronic, but with how these can fail and go by the age of your truck you can always by another generic...

Warning though.. Some generics do not work out of the box and make sure once again you get the right one by p/n or it will not work, do not turn it being calibrated already...

Super easy to do and once you do it and the console one expect a couple little cuts on the hand and be patient... They are in pretty deep and hard to get at, but you do not need to pull the console... Some say yes you do, but I have changed a bunch and never pulled the console one time, just took my time and gave my hand a break from cramps... Just do not rush it and you will be fine... Two bolts hold them on and a plug... How easy is that to fix it yourself and save a fortune.....

 

[stilllearning6]

Perhaps my post was misleading; the issue is only in the rear . The front HVAC is AOK.

I dissected the Temperature Blend Door Actuator one more time because I could not understand what was wrong. Took many iPhotos.
When I edited the photos , enhanced the contrast, made them larger, made higher exposure in my Photo program , I saw for the first time that the largest drive gear in the actuator had a barely visible hairline crack .

I am speculating that the largest white drive gear within the actuator would not bite onto its central axle. This would occur in one direction only.

That is my conclusion after 2 days of inspecting / dissecting / ohming out the rear HVAC system .

Several years ago, I encountered a 1997 Suburban k1500 with two faulty HVAC actuators in the front dash (both had cracked gears), plus the vent door fell out of its perch in the dash. I had to remove the dash to access the components. This time , I was aware of the cracked gear issue and my eyes still could not detect the cracked gear. Without my camera and enhancing techniques, I would not see the crack. See photo attached.
I expect that a replacement actuator will resolve my issue.

 

[stilllearning6]

Installed the new temperature blend door actuator and followed the recalibration process.
FAILED.
Because ... I received incorrect $40 Temperature Blend Door Actuator from a national auto parts chain (although the Auto Parts spec sheet said " 2003 Tahoe 5.3 exact fit " ).
I see an incorrect electrical pin connector pattern.
I received Motormite 604-100. Wrong pin pattern.

Will end this post.

Good news:
When I swap the rear auxiliary Mode Vent door actuator into the position of the rear auxiliary Temperature Blend Door Actuator, I can see the temperature blend door axle turning as I adjust the head unit temperature dial. That tells me it is merely the rear auxiliary Temperature Blend Door Actuator that caused my lack of rear heat.
According to my research, the two rear passenger side actuators in my vehicle are supposed to be the same parts and they are swappable.