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Discussion Starter · #1 ·
Ok, i have searched this thread for some answers but have not found any problems that are exactly like mine. So here is my problem: When I turn on the rear blower motor it turns on for a second and then turns off. I have the digital controls and the EXT model. I actually got it to work last night by turning on the ignition with the fan switch on high and about 3 seconds later starting the engine. It would stay on until I touched the speed switch and then turn off. So I believe that the rear HVAC module is turning off the blower motor for some reason, but why? I have already tried pulling all the fuses under the drivers side rear seat and letting the module recalibrate but that was no help. I do not have a Tech 2 so I haven't scanned it, but I am sure there are codes for something. I have friends that can do that for me but I am just wondering if anyone else has had this prblem and how they fixed it. The front controls work fine.
Thanks for your time and assistance.
 

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I don't know, just asking...Does the rear system incorporate a relay that could possibly be getting hot and shutting itself off?
No. There's a fan speed module that's under control of the rear control module.

Here's an experiment I'm not sure you've done. So far, you've been using the FRONT fan speed switch to control the rear fan, right? Put the front fan speed switch into the REAR position, then go into the back seat and try the rear fan speed control and see if it does the same thing.

I don't know what your skill level is for poking about in electrical systems, but I could post the schematic if you want. The rear blower motor has 12V on an orange wire and the speed control module grounds the black wire to turn it on. You could jump the blower motor black wire to ground to confirm the fan works. You would need an oscilloscope to look at the gray wire of the speed control module to look at a PWM pulse train to see if the HVAC control module was sending the proper signal.

What typically happens is the wiring gets oxidized, heats up, and ends up damaging the connector pin on the 12V input to the speed control module (actually a power FET based amplifier). You might just find the problem by tracing the wiring back from the blower motor to the speed control module.
 

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Discussion Starter · #6 ·
Thanks, I am actually an electrical technician for Freightliner Trucks so i am very good with electrical. I have not removed the rear panel to get at the blower motor to check connections at the motor. I am thinking that it may be a bad connection there because the problem has seemed to get worse over the last few months. I'm thinking that the control module is seeing too high a current when the motor turns on and is shutting it off. But it doesn't blow the fuse. Not sure how the circuit works but that schematic would be very helpfull. However you have to take the entire side panel off the rear passenger side. Tonight I will be doing that.

I have tried the rear seat controls and it still does the same thing.

I don't have an oscilloscope but i will try jumping the wires tonight to see if the fan stays on, then check all the connections at the blower motor and module.

Is the rear hvac module under the seat with the rear fuse panel?

Can you post the schematic for me?

I will post what I find. Thanks.
 

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The control module is the one with the knobs on the back of the console that the rear seat passengers can fiddle with. It's so nice to share a schematic with somebody who won't go "Huh. Send me somethin' in English, willya." :D
 

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Discussion Starter · #8 ·
Update
I removed the panel to access the aux blower motor and ac unit. checked all the connections and they all appear to be good. The fan speed control logic that is shown in the schematic is either hidden or incorporated into the aux control unit. There is a group of wires that disappear behind the rear ac unit that i couldn't trace without moving it out of the way. I also rebooted the rear control unit and that didn't help. The fan did run for a while with key on and the engine off, but as soon as I started the engine it would turn off as before.
I have a friend with a Snap on Solus scanner that he thinks should communicate with the rear hvac module so I can see if it has any fault codes that I can use for diagnostic purposes. If that doesn't work I guess I'll have to go to the dealership and beg an old co worker to scan it for free...or a 12 pack....
 

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I don't see any possible connection between the DVD player and the rear HVAC. Do you have a meter and want to check power at the DVD player? Please post a question about this in the Audio subforum.

Anyway, the HVAC blower speed control module is item #5 in this exploded drawing. ACDelco #15-72813

 

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Discussion Starter · #13 ·
I have scanned the Tblazer with a Snap on Solus scanner and it will not communicate with the rear hvac module. It just doesn't have the option. I have a friend that works at a chevy dealer that can scan it for me, I am trying to coordinate schedules to get over there.
I really hope it is not the blower speed module because it is on the back side of the rear hvac module. You have to remove the whole hvac box to access it which means the ac needs evacuated and the coolant will have to be drained as well. As soon as I can get a scanner on the thing I will know for sure. Beleive me I will post a solution.
 

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Solution?

I took my TB to a local garage, and the guy replaced the blower motor, which he said didn't fix the issue, and the control module. He said he talked to a GM dealer who told him that when the motor goes, it may short out the module. I'm a little skeptical, but am probably stuck at this point. Also, I realized the tech wasn't aware of the delay that occurs (3-4 minutes on my vehicle - typical?) before the rear a/c kicks on. System is now working properly.
 

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Exact same problem

I am facing the same problem with my trailblazer. Rear blower will run with the key in the ignition, but shuts down immediately when the engine is started. I have run the blower at length with the engine off and with the front selector in R or 3. As soon as I turn it to 2, that also shuts the blower down. I don't believe it is a switch issue. I believe it is a voltage issue.

Any resolution to your problem???
 

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Welcome! What's the voltage and where are you measuring it that makes you conclude that? I don't see any way to cause that set of symptoms without the rear control module being flaky.

Are your front controls digital or manual?
 

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I had tested the voltage to the front speed selector switch (black to gray wire). I had 4.75v with the engine off and 5.1v with the engine running. Maybe that's normal, but I would have thought a 5v supply to be more tightly regulated than that.

Here is some more information that might help.

Engine on or off, the rear blower only work with rear blower's front selector switch in "3" or front in "R" and rear selector in "3". (that is, the rear blower must be at it's fastest speed)

If the rear blower stops, the ignition must be off 3-5 seconds to reset it.

If I turn the ignition very quickly (0.5 sec), the engine will start and the rear blower will continue running if left at high speed. If I turn the ignition slowly (i.e. long enough to hear it turn over, about 1 sec), the rear blower shuts down immediately.

I've read on other forums that a faulty ignition switch could affect voltage supply to the rear fuse panel, but the directions for diagnosing this problem are a little unclear. Any help would be appreciated.
 

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OK, now I can start to trace things. The 5V reference isn't that tight, since automotive systems have to be incredibly noise tolerant.

Even though you have the manual controls, and the front blower speed control is a discrete resistor pack, both manual and automatic systems on an EXT use a solid state module to run the rear blower. It's a FET amplifier that converts a 5V PWM signal to a high current output to run the blower motor. The PWM signal comes from the rear HVAC control module in the console, and that's where I think the trouble is concentrated.

The rear blower motor has a fuse (rear fuse #13) for its power, and the control module is fused by rear fuse #36. Both of these are fed by constant 12V bus, not from anything that's switched by the ignition switch. The ignition switch feeds a few items in the rear fuse block, but this system isn't one of them. Most interior trailvoy systems are like this - fed by constant 12V, and the control modules are sent to sleep on ignition off by the BCM. Even the radio is like this.

So I don't think it's the ignition switch EXCEPT that the front HVAC control module has a power feed that's only on in the RUN mode of the ignition switch, and the front HVAC module is capable of sending any sort of malformed data link message to the rear module to suppress the rear blower.

Since the ignition switch is cheap, fails often, and is available in the aftermarket, I'd recommend you do that just to avoid a failure down the road that could leave you stranded.

http://forums.trailvoy.com/showthread.php?t=50141

Meanwhile, find a buddy with the same rear control module and swap for a while, or haunt Ebay until you find one cheap, or call a few junkyards.

I'm leaving Wed morning for a 2 1/2 week road trip, so I might not be able to help you through schematics more than once more. If I scan a couple of pages are you able to poke at wiring at the control module and see if the module is sending a proper PWM signal to the blower power amp?
 
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