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Hvac Blower Control Module, Pulse Signal Reading from Control Head

knightgang on Tue January 11, 2011 10:23 AM User is offline

Year: 2001
Make: Chevy
Model: 1500HD
Engine Size: 6.0
Refrigerant Type: 134A

2001 Chevy 1500 HD with Automatic Climate Control.

***Please note*** this truck has automatic climate control with a Blower Motor Control Module and it does not have a Blower Motor Resistor in it. Please do not reply with anything having to do with a resistor as they are two different animals...

That said. i am still having problems with the Blower fan not working. I have replaced the AC Control head and the Blower Motor Control Module. Before hooking up the module, I metered the signal from the new control head. Orange Wire, 12-14 volts as it should be. Black Wire, ground and has continuity as normal. Small grey wire, measured voltage was 0.01 volts at the lowest speed setting and 0.07 volts at the highest speed setting.

I figured, good deal, this thing will now work. So I plugged in a new Blower Control Module and measured the purple and black wires coming from it and got the varied voltage out (up to 12V) depending on the position of the fan switch. So, I continued and plugged up the module to the blower fan and everything worked great for a while, until I went for a drive...

Once I pulled out of my neighborhood, the fan motor stopped blowing. Here is where I cannot figure out what happened. I have retested the signals form the control head, no change. I am not getting any voltage out of the blower control module regardless of the fan speed setting. Again, the same signals tested earlier and still being measured at the lead from the control head to the blower control module. So, I figure that the module has gone bad again, but something must be causing it but I cannot find any chaffing, short or burnt wire anywhere in the truck, much less in a circuit that might actually affect this system.

I have taken the Blower Control module apart and examined it and cannot find any signs of failure. No burnt circuits, no busted resistors, no swelled capacitors, nothing.

So, to my actual question. What type of signal does that Blower Control Module look for form the Control head. Is it a variable pulse and if so, at what voltage? Is it a constant pulse at a variable voltage? Is it a set pulse at a set voltage with a variable frequency? etc....

I have a service department at my work that can test it if we know what type of signal that module is looking for.

Additionally, if anyone else has any ideas as to what is causing this issue, please chime in. This has not worked properly since May.


Additionally, this was posted on another forum and a member there indicated that even though this is auto climate control and has the Blower Motor Control Module, there is still a blower resistor in the system. Is this true? I really want to find this problem and get my HVAC system working properly again. It kills me not to be able to drive this truck...

bohica2xo on Tue January 11, 2011 6:38 PM User is offline

There is no resistor in that system. The +12v comes straight from the 30 amp fuse in the underhood distribution block.

The signal to the BCM should be a variable pulse width signal around 5v - you really need a scope to look at things like that, a DMM will not do it. If your DMM has frequency / duty cycle, use that setting.

One of three things has happened.

1) You have a bad BCM, and it failed. Solid state stuff either runs for decades or craps within minutes.

2) You have a failed blower motor. Perhaps the original BCM was OK too. Ten years of operation may have worn the brushes out on that motor. Check the motor for continuity.

3) Your blower motor has failed shorted or locked. The fuse may be blown, or the BCM may have died as a result.

Put the module back in the truck. Check the module output at the fan motor connector. Check the condition of the plugs between the module & fan motor. Check the fuse.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Tue January 11, 2011 7:16 PM User is offline

Nothing like having a circuit diagram, doesn't show any waveforms or voltages but at least shows the basic interconnection and what you have. I have never worked on your type of vehicle. Climate control panel isn't really a climate control, its more like a computer keyboard that sends commands in serial digital form to the BCM. Some differences in system occur as to whether your blower module is controlled directly by the BCM or the climate control programmer. Here a circuit would be nice. Generally fires a 5 volt PWM voltage to the module that is basically a power switching circuit.

In just about any GM vehicle I have worked on, that damned module will blow before the always hot 30 amp fuse will blow, could be yours is the same. Yes I have seen more of my share of PCBs that were hit by lightning and a visual check easily determines the broad was blown clear to hell. But that is the exception, not the rule, a power MOSFET transistor can be blown open internally, but look perfectly fine externally, has to be tested.

Whenever a module blows, and they do.

First thing I check is the blower motor, this way overpriced cheap brushed permanent magnet motors with a cold roll steel shaft that rusts like crazy can bind in bronze sleave bearings. Heat causes expansion in the armature windings where a pair can short out. Just a couple of thing that drastically increase the load current on the blower module that is not a piece of nichrome wire resistance.

Typical free air blower current, motor out driving the press on squirrel cage blower wheel is 22 amperes maximum. Also call this wheel a leaf and debris chopper, seen many where debris buildup stalls this wheel causing greatly increased current draw. Yet another cause is rain, either frosting up the blower wheel so it rubs against the housing, or plugged up drains in the fresh air container. Use to repair these motor, but no more, just buy a new one, all welded together with a one way blower wheel impossible to remove.

Do that before replacing the module, even if it means removing the fresh air intake grille and doing a lot of janitorial work first. Another area of severe debris buildup is on the face of the evaporator core.

Just sounds like you have motor problems that popped your brand new module, here the blower motor is the culprit and your module is the victim, actually your pocketbook is the victim. See your module only runs around 35 bucks, some are as high as 300 bucks.

knightgang on Tue January 11, 2011 9:55 PM User is offline

I should have stated earlier, I believe that my blower motor is good. It works fine with 12V applied. And, when the module and control head were installed, it ran fine for 30 minutes in the driveway before shutting down on my test drive...

Secondly, I have tested the output from the module looking for any sign of voltage, did not get any.

Thank you for the feedback on the variable pulse signal. This is the info I was looking for. My service department can replicate the 5v variable pulse to test the BCM. Once I know if it is good or not, I can continue to trouble shoot the issue and see what else I can find...

Of course, it is quite possible that the fan is bad, just not entirely inop and that could be causing problems with voltage feedback, running short or binding. If it is a binding, then it is possible that is was ok sitting still and something shifted causing it to bind once I began driving... Does this sound like a fair statement...

NickD on Wed January 12, 2011 6:45 AM User is offline

Good point on intermittent motor brushes or even an intermittent ground, can also toss in a corroded fuse holder, fuse, intermittent PWM signal. Should have also stated that, but darn near was writing a book again, having a GM SUV in the family with countless unplated connectors and using bare copper for switch contacts. Kept me constantly busy. Can measure 12V with the high impedance of a voltmeter, but drops to nothing under the slightest load. I put the module on the bench, have high current power supplies, various loads, scopes, and a PWM pulse generator. Want to make darn sure before getting my credit card out.

Hand turn the blower motor, and can even peek inside the brush cooler hole to see how much brush is left, always cleaning terminals. But do know, if on the bench by holding the blower wheel and applying 12V and a PWM signal to these modules, will blow the module. Probably why I design my own.

mk378 on Wed January 12, 2011 6:46 PM User is offline

The module pulls the orange wire to ground (black wire) to run the motor. The other side of the motor has constant battery voltage from the fuse. If you unplug the module and jump the orange wire to the black wire, motor should take off at full speed. That proves the motor, fuse, and wiring are OK. Though it is common for the plug to overheat where it plugs into the module and lose contact, requiring a new plug and module.

kathyy on Thu January 13, 2011 11:49 AM User is offlineView users profile

I’m hoping someone can help me out since I’m new to any type of forum. Sorry if I’m not posting this in the right spot and maybe I could be directed to the right spot. I own a 2007 Chevrolet Trailblazer LT 4.2 L/V6 and I am having issues with my air conditioning. I already tried to add R-134a but the gauge was already in the red. I added some anyway just to see what would happen. Now I’m also trying to find out how to release some of the fluid in a safe manner. My main problem is that the AC works fine when I’m driving but blows warm air when idling or at a traffic light. I hear a click every time the idler pulley tries to kick in but it doesn’t seem to catch. So I’ve been told it could be anything from replacing the idler pulley or even the compressor clutch. I am hoping someone has a simple solution to this problem because I’ve had simple solutions already to a broke temperature gauge and other issues.

Happy shopping

JACK ADAMS on Thu January 13, 2011 12:51 PM User is offline


best to post a new Post with your car info for more forum supporters to view. From the information provided you may have a bad fan clutch or cooling problems. You may just have to have it looked at by a professional. Adding refrigerant without gauges can do more harm than good..

Good Luck!

P.S. I see you are in Az, give us a call and we can see if we can help you out.

NickD on Fri January 14, 2011 9:47 AM User is offline

Heard that a million times, has to be low on "free-on", then with kits available with that crazy sealer on the inside to even make a further mess. Then to post after the fact when the damage is already done.

Good news so far, apparently you didn't blow your system or more important, yourself apart.

You stated, "broke temperature gauge" that indicates ETC problems that could well be your problem, or the cycling controls. Surprised there isn't a ton of liability issues with these kits, unless they are keeping them quiet.

Suggest you call Jack.

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