06 Yukon REAR HVAC won't shut off -update, WAS resistor - SOLVED

[Edwards1337]

Gonna start out by saying I've been chasing this issue for a few weeks now, dozens of hours on forums and online, and checking stuff. Found similar issues, but not this one

I've already thrown every part at it, just to do it at this point
New upper control module
Rear hvac module
3x blower motor resistors
Blower motor it's self

The problem is the rear HVAC fan won't shut off, unless I unplug it or pull 30a fuse under the hood to kill power to the orange wire going into the blower motor resistor.

It will stay on if the rear hvac module, the upper controls, both relays under the hood are unhooked even all at once

I have verified the purple signal wire IS adjusting voltage when the fan speed is adjusted on the upper controls.

I have also verified the temp settings are changing as they should, along with the upper/lower blower positions.

What I've found on my testing is say I have 12.8v @ power/ground at 3pin resistor input plug, whenever a resistor is plugged in I'm getting direct 12.8v at the 2 pin blower plug (even if the motor is unplugged)

Whatever voltage I have going into the resistor is coming directly out of it whenever it is plugged in, even though the purple signal wire IS changing with the control knob.

I've put in 3x blower motor resistors from 3x different manufacturers, I just can't fathom every single one is bad, 4 including the oem one

 

[mikez71]

All I know is, when I unplugged my headliner wires to remove the headliner, the rear hvac ran full blast.

Have you looked at wiring diagrams yet?

 

[Edwards1337]

All I know is, when I unplugged my headliner wires to remove the headliner, the rear hvac ran full blast.

Have you looked at wiring diagrams yet?

Even with the truck off?

The diagram I found I traced essentially everything where the orange 12v lead runs up the d pillar, across the rear of the roof, down the driver d pillar, down the sill on the driver side and meets the white connector in the driver kick panel, this is the plug that commonly melts causing poor connection on the white plug in the kick


Then there is a line of probably 6-8 wires that travels from the hvac module up the top vent outlet that does travel the passenger side of the headliner, I'm assuming where it crosses the front of the headliner it's going into the rear hvac controls in the upper console.

I have already confirmed everything on those controls is working however as temp changes, direction all change via knob, and have verified the fan signal wire voltage changes based on knob setting


Do you recall if anything specifically was unplugged or a plug that fixed yours from being pegged when you plugged it in? There's 2 plugs that run down the a pillar and one go behind the cluster, and the other into the power distribution center in the footwell, those are both plugged in

 

[TJ Baker]

What I've found on my testing is say I have 12.8v @ power/ground at 3pin resistor input plug, whenever a resistor is plugged in I'm getting direct 12.8v at the 2 pin blower plug (even if the motor is unplugged)

Whatever voltage I have going into the resistor is coming directly out of it whenever it is plugged in, even though the purple signal wire IS changing with the control knob.

I've put in 3x blower motor resistors from 3x different manufacturers, I just can't fathom every single one is bad, 4 including the oem one

This sounds like you have the automatic control system that uses a solid state blower speed control.module. This is not truly a resistor at all even though it is frequently called that.

This is key in understanding what to look for in testing. As I understand it, the speed control works like this: 12 volt positive is supplied at the orange wire at all times. Ground is applied at the black wire. The interesting one is the purple wire. This is a Pulse Width Modulated ground signal from the HVAC control. If this wire gets inadvertently grounded the blower will run full speed all the time.

Since you already tried several new blower speed modules perhaps it is time to check the purple wire between the HVAC control and the speed control for a short to ground.

On my own 2005 Yukon it was easy to see the issue as the blower speed module had failed and a new unit fixed the issue.

 

[TJ Baker]

Mine was quite clearly toasted...

 

[Edwards1337]

This sounds like you have the automatic control system that uses a solid state blower speed control.module. This is not truly a resistor at all even though it is frequently called that.

This is key in understanding what to look for in testing. As I understand it, the speed control works like this: 12 volt positive is supplied at the orange wire at all times. Ground is applied at the black wire. The interesting one is the purple wire. This is a Pulse Width Modulated ground signal from the HVAC control. If this wire gets inadvertently grounded the blower will run full speed all the time.

Since you already tried several new blower speed modules perhaps it is time to check the purple wire between the HVAC control and the speed control for a short to ground.

On my own 2005 Yukon it was easy to see the issue as the blower speed module had failed and a new unit fixed the issue.

I've checked for short to ground on the purple wire in the rear, it's not resting at ground. I've I take my meter and put positive lead on the 12v orange and ground probe on the purple wire with the vehicle on or off, no "resistor" plugged it in reads 0v and will gradually increase as I change the fan speed, all the way to 12v when on max

With the "resistor" plugged in and motor unplugged, this wire acts the same way. However I place another meter on the blower output side of "resistor" I'm getting full 12v out regardless of what the purple wire is doing, with the blower plugged in and hooked up the purple wire is still acting normal, according to its voltage fluctuation with the fan speed knob. It's still working as should going into these resistors but everytime a resistor is plugged in the voltage appears to just flow directly through it to the motor.


Does that purple wire going into the resistor come direct from the upper hvac controller at the front of the vehicle? If so I can try bypassing oe wiring and running a lead across the vehicle to test if it that's the case

 

[mikez71]

Even with the truck off?

Nope! Sorry I missed that part!

With the "resistor" plugged in and motor unplugged, this wire acts the same way. However I place another meter on the blower output side of "resistor" I'm getting full 12v out regardless of what the purple wire is doing, with the blower plugged in and hooked up the purple wire is still acting normal

Weird! Here's a diagram for the rear blower from charm.li
It looks like that purple wire goes to the HVAC control.. And it looks like that's the primary method for controlling rear blower...
Is the blower running full speed?

 

[Edwards1337]

Nope! Sorry I missed that part!



Weird! Here's a diagram for the rear blower from charm.li
It looks like that purple wire goes to the HVAC control.. And it looks like that's the primary method for controlling rear blower...
Is the blower running full speed?
View attachment 489180

Blower runs full speed.

The circuit is complete with almost no resistance from the hvac module to the resistor as far as the purple signal wire goes

However I did test that the purple wire will go right to 9v when the resistor is plugged in, however not plugged in the voltage fluctuates fine

I guess at this point I have to find if there's an issue with the fan speed control wire coming from upfront overhead controls to the hvac module

Do you have access to a diagram for that?

 

[TJ Baker]

I've checked for short to ground on the purple wire in the rear, it's not resting at ground. I've I take my meter and put positive lead on the 12v orange and ground probe on the purple wire with the vehicle on or off, no "resistor" plugged it in reads 0v and will gradually increase as I change the fan speed, all the way to 12v when on max

With the "resistor" plugged in and motor unplugged, this wire acts the same way. However I place another meter on the blower output side of "resistor" I'm getting full 12v out regardless of what the purple wire is doing, with the blower plugged in and hooked up the purple wire is still acting normal, according to its voltage fluctuation with the fan speed knob. It's still working as should going into these resistors but everytime a resistor is plugged in the voltage appears to just flow directly through it to the motor.


Does that purple wire going into the resistor come direct from the upper hvac controller at the front of the vehicle? If so I can try bypassing oe wiring and running a lead across the vehicle to test if it that's the case

This sure sounds like a bad blower speed control module. But you replaced that 3 times, right? Same part number each time? Does it match the original part number? For my 2005 I used one from Amazon, labeled Aitken. Shows currently unavailable though.

Amazon.com: AITKEN Blower Motor Resistor for 03-06 GMC Yukon Tahoe/ 2006 Cadilac Escade Replace# 89023386, 89040361, 93803634. : Automotive

Buy AITKEN Blower Motor Resistor for 03-06 GMC Yukon Tahoe/ 2006 Cadilac Escade Replace# 89023386, 89040361, 93803634.: Blower Motors - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

 

[Fless]

Blower runs full speed.

The circuit is complete with almost no resistance from the hvac module to the resistor as far as the purple signal wire goes

However I did test that the purple wire will go right to 9v when the resistor is plugged in, however not plugged in the voltage fluctuates fine

I guess at this point I have to find if there's an issue with the fan speed control wire coming from upfront overhead controls to the hvac module

Do you have access to a diagram for that?

Wiring diagrams can be found at www.charm.li.

Confirm that it has the AUTO (with digital temp setting on front control) or Manual HVAC system?

Manual diagrams:

HVAC Systems - Manual — 2006 GMC Yukon 2WD V8-5.3L VIN T Service Manual | Operation CHARM

Detailed repair manual for the 2006 GMC Yukon 2WD V8-5.3L VIN T.

charm.li

Did this happen all at once, and were any other symptoms present before that, or start at the same time?

Does it have HVAC controls in the back panel of the center console?

 

[TJ Baker]

Have you ever depinned a harness terminal?

Depin the purple wire at the three terminal connector to the blower speed control module ('resistor').

Reconnect the harness connector without the purple wire in it. If the blower still runs then the fault MUST be the blower speed control.

 

[TJ Baker]

Looking through my stash of parts and such it occurred to me..... I 'think' I once got a replacement blower speed control module where the pin assignments on the replacement were not the same as the original and I had to depin the harness connector and rearrange the order of the connections.

Have you verified the "A", "B", and "C" connections on your replacements match the original part with regard to Battery, Ground, and Input designations?

 

[mikez71]

However I did test that the purple wire will go right to 9v when the resistor is plugged in, however not plugged in the voltage fluctuates fine

That's weird... Makes me think it's the main hvac controls..
Or the wrong rear module as TJBaker mentioned.. (or repinned!!! Although you would think you'd see 12V instead of 9V if that was the case..maybe not if A and C are swapped?)

...upfront overhead controls to the hvac..

Do you have access to a diagram for that?

It's in that same diagram.

 

[Joseph Garcia]

Just a question....... Is there a ceiling temperature sensor for the auxiliary HVAC unit on your truck?

 

[Edwards1337]

Have you ever depinned a harness terminal?

Depin the purple wire at the three terminal connector to the blower speed control module ('resistor').

Reconnect the harness connector without the purple wire in it. If the blower still runs then the fault MUST be the blower speed control.

So I did depin the center pin during testing

I just went out and depinned it and plugged it in, so purple was not connected and it still runs

I totally agree it has to be the resistor, but I've tried 3 different ones all with the same result, they just free-flow the input voltage to the motor output and I've tested that before even plugging the blower in so I wouldn't think it's a blower issue

I do have a 4th resistor on the way that was more expensive maybe it'll work but I'm gonna wire up the hot and grounds externally before putting in the vehicle to see if I get the same result with the output voltage for the blower side being the same as the input side from the car

 

[Edwards1337]

That's weird... Makes me think it's the main hvac controls..
Or the wrong rear module as TJBaker mentioned.. (or repinned!!! Although you would think you'd see 12V instead of 9V if that was the case..maybe not if A and C are swapped?)


It's in that same diagram.

I thought so too but does it even with the hvac controls unhooked up top and below the radio

The 9v I think only appeared when it was plugged into the resistor on the purple wire, not plugged in it goes from 0-12v as the controls are increased

 

[mikez71]

Mine runs full blast with headliner & roof controls disconnected. Maybe that's just how the rear blower controller works if not getting a proper signal?

TJBaker said purple was supposed to be a PWM ground signal.. so I wonder if it's supposed to measure voltage at all? (when unplugged from the blower module)

 

[Edwards1337]

Mine runs full blast with headliner & roof controls disconnected. Maybe that's just how the rear blower controller works if not getting a proper signal?

TJBaker said purple was supposed to be a PWM ground signal.. so I wonder if it's supposed to measure voltage at all? (when unplugged from the blower module)

The purple wire references ground so with a probe on 12v and the other on the purple wire you'll see the voltage change based on the knob up front, which mine does

Does yours run full blast even when the truck is off tho?


The only thing I havnt physically tried a new one of is a upper roof control, so guess I'll do that... however it still does it wth those unplugged and the blendor actuators work properly when adjusted as well as the temp. The purple wire voltage works fine when the resistor wire unpinned, but the fan still blows constantly

 

[Edwards1337]

Just a question....... Is there a ceiling temperature sensor for the auxiliary HVAC unit on your truck?

Yes. Already replaced it tho

 

[mikez71]

The purple wire references ground so with a probe on 12v and the other on the purple wire you'll see the voltage change based on the knob up front, which mine does

Ah, Ok.

Does yours run full blast even when the truck is off...

Doh, I forgot already!