Trailblazer EXT Rear HVAC Issues

Kurb

Well-Known Member
First, I would like to say thanks to Roadie for all the great info on Trailblazers that have helped me a lot over the last several years. I came to this site since I just learned that he came over here.

Second, I have never had a need to request help from a forum before, but I am at the end of my rope with this problem, so this is one last desperate attempt to figure this out before I break down and take the Trailblazer to the dealership.

The problem: I have a 2006 Trailblazer EXT with manual HVAC controls. The rear HVAC temperature, vent mode, and blower controls stopped working completely. The rear HVAC can be operated using the front controls, but sometimes the rear vents will not change positions when I move the vent switch, and the rear blower acts really strange. If I turn the auxiliary blower control switch on the front console to the R position to activate the rear controls, the blower shuts off and will not restart when I move the switch out of the R position. If I shut the blower off for ~20 seconds or so, then it will start back up. Also, if the rear blower is running and I move the front vent switch to the lowest position (so that the vents blow at the floor position), then the rear blower will almost always shut off off and will not restart unless I again leave it off for ~20 seconds. Sometimes, the blower will also randomly shut off without switching any of the controls and will not restart unless I shut it down and leave it off for ~20 seconds. With the blower not working and the blower speed switch on the highest setting, I checked the voltage at the blower connector and found it to be 14.4 VDC. Normally, I would replace a blower for this condition, but I read on a forum that the blower speed controller should provide a pulse-width modulated signal to the blower instead of a constant 14.4 V DC signal. Between that and the other weird symptoms, I am guessing this is probably the Auxiliary HVAC Control Module, but I want to verify this is the problem before I throw money at it.

Also, the rear temperature actuator was not working with either the front or rear controls until I disconnected the battery for a few minutes and restarted the car to allow the system to reset itself. Now, the rear temperature actuator will work with the front controls only. However, when the control switch is set to the hot position, it only blows warm and not hot air, even after I let the car run for 30 minutes.

Another thing worth mentioning is that I recently replaced the rear evaporator when I had a rear line leaking that was seized to the evaporator. The aftermarket evaporator did not have mounting holes for the torx screws that secured the blower motor speed controller to the original evaporator, but the speed controller housing was pressed into the HVAC plastic housing and stayed there without the screws, so I do think this is an issue. Also worth mentioning is that I broke the blend door shaft when I unnecessarily removed the mode actuator during the evaporator replacement. I do not think this is a problem since only the tip of the star-shaped end of the shaft broke, so the end of the broken shaft still fit inside the star-shaped insert in the mode actuator, and I super-glued the end of the shaft to make sure that it stayed there.

Has anyone had similar issues with the rear HVAC controls and blower? Is there any good way to troubleshoot this with a digital multimeter? I do not own or want to spend the money on an oscilloscope.
 

Mounce

Well-Known Member
I'm in the same boat as you, although mine works less than yours.
 

The_Roadie

Founding Member
Administrator
Hiya! Glad you got the memo that I (and 100% of the volunteer mods and admins of Trailvoy) began this place and fled trailvoy when new corporate owners were abusing the members to make a buck.

The PWM signal is correct, but many hand held meters average the pulses and display odd readings, depending on the bandwidth of the meter. Reading 14V on the blower SHOULD run it at full speed, PWM or not. Suspect you have a loose or corroded ground on the other side of the blower. Or a bad blower. Did you confirm the ground was zero volts when compared to a known good frame ground?

You might indeed have a bad control module, but they're cheap on Ebay. Certainly a lot cheaper than a dealer's diagnostic charge, which is never a guaranteed diagnostic. They just charge it to make extra profit.

On this site, we have manuals available for download by Mooseman. Or I can dig into my bag o' schematics and talk to you through troubleshooting the wiring at the rear control module. Up to you how you'd like to proceed.
 
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Kurb

Kurb

Well-Known Member
Thanks for the reply. I did not check the ground voltage, but I will do that next. I'll also take a look at the manuals when I get a chance.
 
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Kurb

Kurb

Well-Known Member
I ruled out the blower. As Roadie suggested, I checked the ground voltage by back-probing the blower connector (with the other DMM probe on a good ground), and it was 2.7 V when the fan was running on high, but it jumped up to 14.2 V when the fan dropped out. The voltage on the power side of the blower was 13.43 V with the fan running and 14.2 V when the fan shut off, so the ground is opening up. The voltage across the fan was 10.73 V with it running and 0 when it dropped out. I was checking the voltage across the connector with it unplugged before, which was not a very good test.

The fan is also now consistently dropping out after running for a minute or two without changing any settings. I hooked the ground side of the fan directly to ground, and the fan ran without shutting off. I let it run for over 10 minutes.

It is either the blower speed controller or the HVAC control module. From reading the forums, I know the blower speed controller is problematic on the Trailblazers, but since I am having problems with the vent and temperature controls, I suspect the module is bad. I downloaded the manuals, so I've got some research to do.

Any advice on testing testing the blower speed controller or HVAC control module is welcome.
 
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Kurb

Kurb

Well-Known Member
I've been very busy, so I have ignored this for a while, but I decided to look at it today since I had a leaking water pump that could not be ignored. I think I figured out the cause of the blower shutting down. I think it is the blower speed controller.

The connector for the speed controller has an orange wire that is the power supply (fused battery voltage), a gray wire that is the PWM control signal from the auxiliary HVAC module, and two black ground wires: one input that runs from the blower, and one output that supplies the ground to the blower. I back-probed the connector and found that the when the blower cuts out, the control signal voltage increases by only 0.1 to 0.2 V for all blower speeds, so I do not think the control signal is causing the speed controller to shut off. I used a DMM on the VDC setting, which is not really the best way to check this, but I don't have an oscilloscope. The controller is getting very hot and when it does, it shuts off, which opens the ground to the blower motor. The ground from the blower goes to 14.4 V (alternator voltage), and the controller ground goes to 0.7 mV. When the blower switch is shut off, the controller cools off in about 30 seconds, and the ground voltage drops from 0.7 mV to 0.

I figure I probably caused this when I chose to not return the aftermarket evaporator that lacked the mounting holes for the blower speed controller. There is a metal plate on the back of the controller that mates with the side of the evaporator. I am guessing that the controller is cooled by the evaporator fins and the air that flows by the evaporator. Without the screws to hold the controller flush with the evaporator, the controller is not getting cooled enough. I pushed hard on the controller to hold it in contact, but the blower still shuts off, so I figure the controller is fried. I guess I will be looking for another evaporator with the mounting holes and a new speed controller. I do not want to drill new holes as I am not sure how far I can drill before I cause an evaporator leak. Hopefully, Autozone will take it back.
 

Einst-Hawk

Well-Known Member
Kurbinator said:
Without the screws to hold the controller flush with the evaporator, the controller is not getting cooled enough.
Funny you mention this. I had just replaced an actuator and the control module on my rear HVAC. I researched how to do this not only on here but other forums and youtube.

One post I read, someone had mentioned the same heat related issues with the module as he had not pulled the entire unit from the base and had a limited amount of space to work with. He had left the module hanging as he could not get the mounting screws back in. He related that it would overheat quickly and shut down the system. He posted that once he was aware of the two bottom mounting screws for the HVAC unit, he was able to pull it and mount the module properly.

So I decided to hedge my bet when I changed this control module. I coated the back of it with thermal cpu paste. I still had some from my last computer build decided that it couldn't hurt.

So your concern about heat is valid. I did not have to change the evap so I can't comment on drilling holes. Sorry
 
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Kurb

Kurb

Well-Known Member
Since I have to buy more parts and disassemble the case again, I figured I would replace the rear temperature blend door that I broke, but I cannot find it. The dealer said that I need to buy half the case (P/N 89023340) to get the door at $103, which I found online for $72, but I can't see buying the case when I do not need it. I could not find it at any local junk yards either. Can anyone recommend a potential supplier?
 
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Kurb

Kurb

Well-Known Member
While waiting on parts, I determined that the rear vent and temperature control issues are being caused by the auxiliary HVAC control module. When I went to troubleshoot this, the rear actuators stopped responding to the front controls, but the rear controls started working, except for the temperature controls.

This forum and the manuals link from Mooseman that the Roadie suggested above were very helpful for troubleshooting this. The Haynes manual also has a pretty good schematic of the rear HVAC system. There are two identical rear blend door actuators: one temperature actuator and one mode actuator for controlling the vents. They each have a 5 pin connector, and all wires from the connectors go to the auxiliary HVAC control module. Three of the pins are for a potentiometer: one is a 5 volt reference, one is a ground, and one taps off of a variable resistor that varies the voltage when the door moves. The voltage on these pins appeared to be fine.

The remaining two pins are a control circuit that use battery voltage to move the door. The polarity is reversed to reverse the door direction. When the switches were moved (except for the rear vent position switch that was working), no voltage was applied to the door control pins. Since both pins had a low steady voltage (~26mV), I figured this was not a wiring problem, but to be sure, I checked the voltage at the back of the aux control module, and it was the same and did not change with the switches. The aux control module was really easy to access; just pry gently on the top and pull it out, although there is not a lot of slack in the wire, so it was a little bit difficult to back-probe some of the wires on the connector. To rule out a bad data feed from the front HVAC control module, I removed the front HVAC control module (still easy but a little more difficult since a couple panels had to be removed), which had no impact on the voltages. I wasn't sure if the aux module needed the data signal from the front, but apparently it didn't since disconnecting the front module had no impact. There was a constantly varying voltage on the data wire before I disconnected the front module, so the data signal was present.

I ordered the Auxiliary HVAC Control Module (GM P/N 15250198) from Amazon for $138 plus $11 shipping. The cheapest one I could find was a used module from eBay for $125, but I figured I might as well pay a little more and get a new one.

I am still looking for the temperature blend door if anyone could suggest a supplier.
 
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Kurb

Kurb

Well-Known Member
Since I didn't close this out last year when I fixed this, I thought I would close this out. Replacing the blower speed controller fixed the fan issue, and replacing the auxiliary HVAC control module fixed the rear control problems. I never did find a replacement blend door. I ended up ordering a new case with both blend doors in it.
 

Tammy2002

New Member
First, I would like to say thanks to Roadie for all the great info on Trailblazers that have helped me a lot over the last several years. I came to this site since I just learned that he came over here.

Second, I have never had a need to request help from a forum before, but I am at the end of my rope with this problem, so this is one last desperate attempt to figure this out before I break down and take the Trailblazer to the dealership.

The problem: I have a 2006 Trailblazer EXT with manual HVAC controls. The rear HVAC temperature, vent mode, and blower controls stopped working completely. The rear HVAC can be operated using the front controls, but sometimes the rear vents will not change positions when I move the vent switch, and the rear blower acts really strange. If I turn the auxiliary blower control switch on the front console to the R position to activate the rear controls, the blower shuts off and will not restart when I move the switch out of the R position. If I shut the blower off for ~20 seconds or so, then it will start back up. Also, if the rear blower is running and I move the front vent switch to the lowest position (so that the vents blow at the floor position), then the rear blower will almost always shut off off and will not restart unless I again leave it off for ~20 seconds. Sometimes, the blower will also randomly shut off without switching any of the controls and will not restart unless I shut it down and leave it off for ~20 seconds. With the blower not working and the blower speed switch on the highest setting, I checked the voltage at the blower connector and found it to be 14.4 VDC. Normally, I would replace a blower for this condition, but I read on a forum that the blower speed controller should provide a pulse-width modulated signal to the blower instead of a constant 14.4 V DC signal. Between that and the other weird symptoms, I am guessing this is probably the Auxiliary HVAC Control Module, but I want to verify this is the problem before I throw money at it.

Also, the rear temperature actuator was not working with either the front or rear controls until I disconnected the battery for a few minutes and restarted the car to allow the system to reset itself. Now, the rear temperature actuator will work with the front controls only. However, when the control switch is set to the hot position, it only blows warm and not hot air, even after I let the car run for 30 minutes.

Another thing worth mentioning is that I recently replaced the rear evaporator when I had a rear line leaking that was seized to the evaporator. The aftermarket evaporator did not have mounting holes for the torx screws that secured the blower motor speed controller to the original evaporator, but the speed controller housing was pressed into the HVAC plastic housing and stayed there without the screws, so I do think this is an issue. Also worth mentioning is that I broke the blend door shaft when I unnecessarily removed the mode actuator during the evaporator replacement. I do not think this is a problem since only the tip of the star-shaped end of the shaft broke, so the end of the broken shaft still fit inside the star-shaped insert in the mode actuator, and I super-glued the end of the shaft to make sure that it stayed there.

Has anyone had similar issues with the rear HVAC controls and blower? Is there any good way to troubleshoot this with a digital multimeter? I do not own or want to spend the money on an oscilloscope.

Do you know where I can get a acuator blend door shaft? I've been searching for days.
 

Maverick6587

Silver Supporter
@Tammy2002 I've only seen the replacement gears (without the shaft). If you just need the shaft you could grab one from a local junkyard and just swap the internal parts. Otherwise, you'll have to buy a new actuator.
 

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