Rear Blower Speed Control Module (BCMP) OR Rear HVAC Controller??

[joee]

Making my way from the other site. Sorry that my first post on this site is a question, rather than a solution. Within the last week or so, I've replaced the low beam headlight relay and the oil pressure switch. Meant to post what I've done but, Hurricane Isaac's had my attention lately.
Today was the first day I could get back to working on the TB. My problem is that there is no air flow from the rear upper and lower vents. It doesn't matter if I use the rotary dial by the shifter or the rotary dial on the back of the console. The front vents blow great on all 5 speeds. This system is manual, not the automatic with digital readout.
There are many posts around dealing with rear blower issues but, I found this post was the most detailed concerning troubleshooting these two modules: Rear AC issues...and glove box - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum. Hope it's okay to post this?
Post #7 talks about the putting a meter on the BCMP to check for different voltages. I recorded the voltages while turning the dial by the shifter. Then I put that dial in "R" (for rear) and recorded the voltages while turning the dial on the rear of the console.

I disconnected the wiring harness from the BCMP and measured the voltages between Pin 3 (Gray) and Pin 2 (Black). When I measured between Pin 3 and Pin 1 (black), there was a constant 0.022 volts no matter the position of the dial. Between Pin 4 (orange) and Pin 1 (black), the meter read 12.29 VDC. I used a Fluke 117 DMM.

DIAL BY SHIFTER
Position VAC VDC
0 0.037 VAC 0.111 VDC
1 0.364 VAC 11.68 VDC
2 0.105 VAC 11.81 VDC
3 0.035 VAC 11.85 VDC

DIAL ON REAR OF MIDDLE CONSOLE
Position VAC VDC
0 0.037 VAC 0.107 VDC
1 0.345 VAC 11.69 VDC
2 0.171 VAC 11.82 VDC
3 0.038 VAC 11.89 VDC

I don't know why Position 0 and Position 3, on both dials, have the same voltages. It seems as though they should be different but, I really don't know. There is a difference between the other positions, though. So, does this mean that the Rear HVAC Controller is doing it's job properly and, maybe, the BCMP isn't responding to adjust the blower speed? I'm trying to determine if the problem is most likely with the HVAC Controller or the BCMP.
Thanks

 

[Wooluf1952]

Others will be able to help. I just wanted to say, we have no problem linking to the OS (especially since most of the info there, was written by the people that are here now).

Also:
Welcome

In case you haven't heard:
http://gmtnation.com/f5/welcome-gmtnation-com-heres-story-226/

 

[The_Roadie]

Excellent first post, and welcome! (From one of the ringleaders of the jailbreak from the other oppressive site)

Sometimes, PWM signals like those need a load to reveal their true values (if there's a pull-up resistor on the destination), so if you can, measure them again while the harness is plugged into the module.

But it smells a lot like a problem with the rear HVAC module in the center console. Got a buddy to swap with for troubleshooting?

 

[joee]

Thanks for the reply. Unfortunately, I don't know anyone with an EXT.

While taking the trim out, in order to access the BCMP, I left several doors and the hatch open which caused the interior lights to stay lit. Also, while recording the voltages, I didn't start the TB but, rather turned the key to the ON position. Needless to say, the battery, which was almost at the end of it's life, died. I found this out when I tried to start the TB and it wouldn't start. I replaced the battery and it started but, began idling very roughly. When the AC was turned on, the TB died. This happened several times. Then I remembered, from previous perusing of other threads, that this is the classic symptom of a dirty throttle body. Mine has never been cleaned. By this time, it was 8:00 pm and I didn't feel like working on it anymore.

It's been driven yesterday and today and will be until Friday (with the AC turned off at every stop). Good thing it's only 2 miles and 3 stops to work. I'll be able to clean the throttle body Friday. I have to finish the cleaning before I can work on the rear blower problem again. Every time I start the TB and let it idle, it dies when the AC is turned on. There's no way I can make it to the back to read the voltages before it dies. Also, I don't have anyone who can help me right now by pressing the gas pedal and keeping the RPM's up. So, I'm going to clean the throttle body Friday then jump back into troubleshooting the rear blower problem.

Good news is someone let me borrow a Fluke 105 Scopemeter. So, I'll be able to really see if the PWM signal is getting to the BCMP. I'll post pictures of the scope display once I'm finished.

While testing, I'm going to leave the module plugged into the BCMP, as you suggested. The scope leads will be connected between Pin 3 (Gray) and Pin 2 (Black).
Is this correct?

Thanks

 

[joee]

Cleaned the throttle body, changed the plugs and air filter. All is good. Now, back to the AC.
I plugged the wiring harness into the BCMP and went through the same routine as I mentioned above. The measurements:

DIAL BY SHIFTER
Position VAC VDC
0 = 00.00 VAC//00.00 VDC
1 = 05.72 VAC//-07.32 VDC
2 = 04.66 VAC//-09.57 VDC
3 = 03.16 VAC//-10.95 VDC

DIAL ON REAR OF MIDDLE CONSOLE (when dial was on FACE only position)
Position VAC VDC
0 = 00.00 VAC//00.00 VDC
1 = 05.73 VAC//-07.33 VDC
2 = 04.67 VAC//-09.56 VDC
3 = 03.18 VAC//-10.93 VDC

DIAL ON REAR OF MIDDLE CONSOLE (when dial was on FOOT only position)
Position VAC VDC
0 = 00.00 VAC//00.00 VDC
1 = 05.70 VAC//-07.48 VDC
2 = 05.11 VAC//-08.92 VDC
3 = 03.72 VAC//-10.56 VDC

There was a slight voltage difference on Position 2 when measuring DIAL ON REAR OF MIDDLE CONSOLE when blower position was changed from FACE to FOOT.

The scope pictures show the waveforms for the DIAL BY SHIFTER measurements. I didn't post the DIAL ON REAR OF MIDDLE CONSOLE pictures because they are very similar to the ones posted due to the voltages being almost identical.

There's a glare on the screen but, the voltages and waveforms can be clearly seen. The voltages and waveforms change as the dial position changes. So, does this mean the rear HVAC module is functioning properly?

 

[The_Roadie]

Those are spot-on classic PWM waveforms. Your control module is OK, so either the blower or its speed control module is bad. Or the fuse - rear fuse block #13 or the in-line 30A one somewhere in the harness back there (never saw a photo of it that I remember).

 

[joee]

I put an ohmmeter across the fuse and it checked out good. Then I put 12V across the rear blower motor and it started blowing. So, I replaced the blower speed control module (ACDelco 15-72813 Control Module Kit for $63.11 @ Amazon.com). Now the rear blower works as it should and the wife and kids are happy.
Thanks for the help.

 

[The_Roadie]

A happy wife is a frisky wife. :wootwoot:

Happy kids are only a passing state - just wait a minute.