Rear HVAC Fan

BuffettTruck

Well-Known Member
#1
I have read through other posts on this matter and done some troubleshooting (TS). During my first round of TS, I noted that both leads to the fan had 14 vdc. This struck me as odd and so I cut the black and attached an extension to a ground point and, sure enough, it spun up. So I replaced the resistor pack those leads ran to figuring I had a short; but with the new resistor pack I get the same results and still no fan and 14 vdc on both leads. To my mind the black lead should be open or different resistance values to ground, not holding 14 vdc, but maybe I have the concept of the resistor pack wrong. Another thing I had noted during my initial TS is that the blend doors did not respond to changes on the console controls, but figured I would tackle those after I got the fan going. Now I am wondering if maybe I am looking at the wrong area and my issue might be in the wiring between the controls and the Rear.

Any sagely wisdom or advice on where I might look next?
 
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freddyboy61

Well-Known Member
#2
First thing to check are fuses #13 & 36 in rear fuse box.

What happens when you use the rear blower selector switch located at the shifter console?

If you are up for some voltage measurements, look for the following at the blower motor module (resistor)

Pin A (Blk) - 0VDC
Pin B (Blk) - varies with blower speed selection
Pin C (Grey) - varies with blower speed selection
Pin D (Org) - 12 - 14VDC constant
 

budwich

Well-Known Member
#3
Hopefully, you have a typo on your "TS" about the "1V" on both leads after the change of the pack... especially since you first indicated the "14v" previously. Your problem is obviously not a voltage issue but a "lack of ground" / broken connection thereof. You proved that by connecting to a ground source and the motor ran.
 
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BuffettTruck

BuffettTruck

Well-Known Member
#4
What happens when you use the rear blower selector switch located at the shifter console?
No controls make any changes to the rear. During my initial TS I had checked the harness in the rear for any changes that would suggest the controls were doing something. I wasn't sure what I would see, but I was expecting to see something like what the Shifter switch does where I saw a change in voltage from 3 to 5 Volts at the switch (or something like that, it was a while ago).

I will check those voltages at the pack tomorrow.

Hopefully, you have a typo on your "TS" about the "1V" on both leads after the change of the pack...
Indeed it was a typo. I fixed it as well as a few others.

Edit: If all else fails, I will just put a potentiometer on a long lead with the black wire and attach it to ground. At least then the kids can turn the fan speed up and down.
 
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freddyboy61

Well-Known Member
#5
To control the blower, you will need a high wattage potentiometer. The blower draws quite a bit of power. You will still have to deal with the blend and direction doors not working.
 

Kurb

Well-Known Member
#6
I am a little confused since I did not think there was a resistor for the rear blower. There is a blower speed controller, which I think is a power transistor, that mounts between the blower and ground. If you have 14 volts on both sides of the blower, then it is not getting a ground. The blower speed controller is a common problem that eliminates the ground.

Also, the rear HVAC control module failed and caused my blend doors not to respond. I bought a replacement from Amazon.

This sounds very similar to problems I had years ago: https://gmtnation.com/forums/threads/trailblazer-ext-rear-hvac-issues.10662/
 
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BuffettTruck

BuffettTruck

Well-Known Member
#7
Okay, it took me way too long to get back to this issue because... well... life. I pulled off the panels today and checked the above listed pins.

Pin A: 0 VDC and a continuity check shows that it goes to ground.
Pin B: 14 VDC constant. Switches have no affect on this.
Pin C: 8.25 VDC constant. Switches have no affect on this and upon testing, the fan spins up to full speed.
Pin D: 14 VDC constant.

After doing these checks and with the vehicle/AC running, I ran a jumper from Pin C to Ground figuring the kids could at least have full blast air and close/redirect the vents as needed. Upon shutting the vehicle off, I learned this is a no-go as the fan was still running. I may need to hit a hardware store to pick up a 12V toggle switch and rerun a longer jumper.

Is it possible the new controller module is bad as well? Or should I be looking further up the line? Perhaps the armrest controls?
 
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BuffettTruck

BuffettTruck

Well-Known Member
#10
Okay, a little confused this morning. I picked up a 12v switch to run the temporary jumper to turn the fan on and off. Instead of just tapping the gray wire that put out the 8.25 vdc, I figured I would tap it where it connected to the armrest module. But I couldn't find 8.25 vdc on any of the pins at the module. I found a wiring schematic (posted below) that says it should run to Pin B12, but I get nothing there. So I go to the rear harness and check the gray wire there and instead of finding 8.25 vdc and the fan spinning up as it did the other day, I find 13.8 vdc and the fan doesn't do anything. Nothing has changed in the past two days aside from a beach trip and me pulling out the armrest module to test the pins. Which I am thinking could point to an issue with that module if things have changed after manipulating it to test pins. What I don't quite get is why I am getting 13.8 vdc at the rear but nothing at b12. The obvious answer is "there's an open in the line, pull the carpet and trace it", but wouldn't that mean that by testing that line and completing the loop the fan would spin up? I guess it's been too long since my Electronics Principles class, because I am feeling frustratingly stumped.

(edit: forgot the diagram)
 

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budwich

Well-Known Member
#11
what the "heck" are you doing????? I think you might want "retake" some of those "electronics courses" again... or maybe go to another "school". Just because you measure a voltage somewhere doesn't mean you can "tap into it" to drive another circuit.

The more likely answer to your "why"... is the other day, by putting a ground on "C", you likely took out the aux controller speed circuit.... :-(

And lastly... you are likely looking in the wrong place that started all this... :-( The likely cause in the first place is the front control switch is either broken or disconnected. Of course, you might now have other problems because of your "work around wiring".
 
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BuffettTruck

BuffettTruck

Well-Known Member
#12
I didn't tap into any circuit to run another. I tested pins for voltage and one of those tests spun up the fan. When I created a path to ground on Pin C it caused the fan to cut on. That is it.

On another note:
After popping the center console unit back into place, Pin C now shows 7.46 VDC and the fan spins up again. I am hoping the open between Pin C and Pin B12 is a blown fuse, but I have checked fuses multiple times and not found a blown one, so I am dreading having to pull out my carpet to trace the line.

On a happier note:
I bumped into someone today who has access to a full scanner and said he'll run a test when he has the free time.
 

budwich

Well-Known Member
#13
but was that with pin C disconnected?... ie. no other wire was on pin C.... the gray wire was no longer connected back to the aux controller.

lastly... what ground were using for your metering?
 
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BuffettTruck

BuffettTruck

Well-Known Member
#14
First thing I did was test for continuity of ground (Pin A) with the grounding point next to the tail light assembly. When I got a closed circuit there, I used that point as my ground for the other pins. If I did anything, circuit-wise, by testing Pin C with ground, I bypassed the speed control of the center console. Pin B and Pin C voltages (according to Freddy's info above) should change with the center console controls, but both are fairly static, excepting the odd voltage jump on Pin C from manipulating the control module itself.

And the voltage isn't coming from pin b12 on console, it is coming from the 14v constant. The more I mess with it, the more I believe the center console controls how easily the voltage flows to ground. That is the only live connection in the molex connector. Pin B and Pin C only have voltage when Pin D does. Which makes sense circuit-wise. As Pin B is just the other leg of the blower. So the power appears to be flowing from D to the blower then to Pin B and then to Pin C. And with an open between C and B12, the circuit isn't being completed.
 
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BuffettTruck

BuffettTruck

Well-Known Member
#15
Except that might not be correct, I just remembered originally getting 14v on both legs of the blower, even with the fuse removed, but I think that was before swapping the module.
 

budwich

Well-Known Member
#16
I think you might be making some "assumptions" on how the controllers work. They aren't necessarily sending DC / GRD to do things.

NOTE: in the diagram pins D and C are NOT connected to each other.

Anyways, I would check the selection switch (output) on the main unit to see if indeed is working as you indicated earlier that even the vent selection wasn't working.
 
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BuffettTruck

BuffettTruck

Well-Known Member
#17
I am making assumptions, no doubt. Haynes manuals no longer have wiring diagrams and Chiltons, last I checked, were still stupid expensive for the shadetree who only does the odd repair here and there. I am stuck with looking online and trying to find schematics, which tend to be a bit incomplete as they are missing legends and such. The above schematic seems to show the gray wire running through something at points G and K, perhaps just a cable run, but I don't have the accompanying sheet to see what C309 and C401 are. Connectors? The symbology is different from what I am used to. It's not like the AF when I had to repair a Comms drawer and grab the TM with the correct flow charts and circuit diagrams Short of gutting my interior or buying a Chiltons and hoping they still have wiring diagrams, I can only check pins and connections and try to figure out the flow. As far as the flow goes inside the blower module, the diagram doesn't show anything and the new module is sealed so i can't check the runs inside (I wonder if I still have the old one).

There is definitely no power on the car-side of the molex for the gray wire, I checked for that before checking continuity to pin b12. There is also no voltage on pin b12 when the module is plugged in.

I rechecked the fuses, even pulling them and checking continuity from leg to leg to make sure I wasn't missing a hairline break. All are good.
 

budwich

Well-Known Member
#18
I make NO assumptions either... just follow the circuit diagram that you posted. You can readily see what is connected / interconnected and what is not in the circuits shown. You just have to look at the other portions to understand that there is no direct connection between C and D or A and B for that matter as they have made it quite clear that there is some form of "logic" inside.

You could disassemble the motor blower control module to see what's inside... but I am not sure that would tell you much.

Anyways, GM is notorious for using PWM control signals to vary things as a result, you may not be seeing "straight voltages" at points beyond what the circuit diagram displays.

Hence, your checks of "known" conditions is needed to at least ensure those are working / set as required. Then you check switches and such to see that they operate electrically correct as shown.

Lastly, I should have pointed this out earlier... your statement about the "blower resistor pack"... IF the diagram is as you show, that ISN'T a blower resister pack, it is an "active power circuit" which controls the "average DC voltage value" by turning on and off at a some rate corresponding to a "voltage drop" that effective controls the speed of a DC motor.
 
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BuffettTruck

BuffettTruck

Well-Known Member
#19
The resistor pack comment was based on what I had learned of how the front blow speed control worked. Voltage came in nd depending on the switch setting the power would go down a different path with a resistor, except for 5 which was a straight shot through.

Regardless, I think I am done crawling around the back of my TB and torturing my pinched nerve. At least until I take it to the guy I was talking to today (a mechanic at a used car lot) so he can run a diagnostic and see what that says.

And thanks for the help thus far.
 

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