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HVAC Blower Just quit, HELP!!! Relay Location needed...

knightgang on Sun May 02, 2010 12:26 AM User is offline

Year: 2001
Make: Chevy
Model: 1500HD
Engine Size: 6.0ltr

I have a 2001 1500HD, Crew Cab 4X4, 6.0ltr with Computerized Climate Control. The AC Blower just stopped working one day with no indication that anything was wrong. The blower motor works fine when 12vdc is applied to it, but when plugged installed will not work. I think it is just not getting power. I thought it was the blower control module (my model has this instead of the blower motor resistor). However, after replacing the control module, it still will not work. All fuses and relays (that I can find) seem to be good. The Chiltons manual says that there is a blower relay (on the 99 to 02 models) attached to the blower motor housing under the dash. This must be where my issue resides, but I cannot find this relay. Can someone tell me where this is located so that I can fix my air?


Dougflas on Sun May 02, 2010 7:23 AM User is offline

Macs says it's located on the HVAC module near the blower motor

mk378 on Sun May 02, 2010 9:24 AM User is offline

The diagram I found shows no relay. Everything passes through the blower control module. There should be power at all times on the orange wire to the blower control module, supplied from a fuse under the hood (30 amps, "FRT HVAC"). The black wire is grounded, and the gray and black wire is PWM pulses from the control panel (which you already tested). Check that you didn't lose the ground. Also measure voltage at the motor leads from the control module with everything plugged in. Blower motors can get intermittent so they pass a test once but then still don't work.

knightgang on Sun May 02, 2010 9:24 PM User is offline

mk387, that is the most detailed information I have gotten yet. So, here is an update.

I tried testing the volate at the blower module input plug again and got nothing. I have not gone back through the fuses yet, but I bet nothing has changed. Anyway, I pulled a power wire from the battery, put an inline fuse and a switch on it and used it to supply power directly to the blower, and it works non stop. This is pure 12vdc, so this is basically setting the fan on high all of the time but at least it gives me air movement so that I do not burn up from no A/C.

Now, if I am getting intermitten power at the blower module input plug, then where do you think the problem is. I check the power at the blower input plug and have voltage one minute, and none the next. Where is this grounding at? Or should I tap into the black wire you referred to and create my own "GOOD" ground? Would this hurt anything?

Before reading your post, I was thinking that maybe my control head is going bad. I hope it is a simple ground issue like you suggest. If you have any other guidance on this, please let me know...

mk378 on Sun May 02, 2010 11:07 PM User is offline

No power on the orange wire means the fuse under the hood is blown, or there is a problem with the fuse box or wiring. I think the wire stays orange all the way back to the fuse. It of course goes through one of the big plugs at the firewall.

knightgang on Mon May 03, 2010 10:14 PM User is offline


Here is an update. I tested the voltage on the orange wire a couple of times today. The first time, I did it, I got 12v immedietly when on the orange and a door hinge bolt as ground. So, fuse is good and have power to the control module. Tested across the orange and the black, and got 12v, so ground appears to be good. But, I cannot seem to get any voltage from the grey/black wire, either to hinge bolt ground or black wire ground.

Also, I guess my metter might be bad, because about 20 minutes after this test, I tested again and I did not have 12v on the orange without really scrapping on the hinge bolt for ground. Tested across orange and black and got nothing. I am thinking that I could have a damaged harness somewhere that is grounding the voltage before it reached the plug, Or I could have a finiky meter, which is possible, it is old and has been left outside...

Anyway, how likely is it that the control head for the A/C unit is bad? Is there a way to test the ohms on certain pronks of the control head to rule out the fan speed control in the head is good/bad? I would love to be able to isolate where to focus attention to find the issue. If it is the control head, great I will get one, if I can rule it out, I know I have a wire harness issue to find. I do not want to waste time on the harness if it is the head. I do not want to buy a head if it is the harness.... What is most likely???


NickD on Tue May 04, 2010 6:48 AM User is offline

See you started a new thread, was a bit curious about your old module as to whether its using thick film or a PCB, the latter is repairable, former is not. Been reports here where when the module went, it took out the BCM that sends a PWM signal to this module, when off, zero volts. When mind went, the power MOSFET transistor in a thick film circuit shorted out, so the blower was running full speed even with the ignition off. Don't you just love always hot? I had to unplug my blower to keep my battery from running down.

First thing I did was to grab my scope to check for the BCM pulses, said thank you Lord because they were there. Hey, you need a scope, didn't before, was a time I didn't even need a circuit diagram, but one of my vehicles as just the electrical service manual with 486 pages in it, this is just electrical, that is a bit too much. Even a vehicle as simple as my 04 Cavalier has over 3,000 pages in three volumes, this is getting ridiculous. At one time I thought auto climate control was great, still have that in two of my vehicles, but would never buy another. Hell, I am strapped in a seat with no place to go, my finger can turn an old fashion blower motor switch, don't need a bunch of microcontrollers to do that.

My original circuit also had a voltage doubler switch that was history as GM is using N-channel power MOSFETS in the common source configuration, this requires a gate driver of greater than 12 volts, more like at least 18 volts to fully saturate that transistor. Since mind is hard wired, I switched to the more expensive P-channel types where just a simple inverter was required to drive them. Ha, more expensive, like a dime a piece, working for GM, a half a cent to them was like a million dollars.

Voltmeter is worthless for checking voltages when not under load, impedance is typically 10 Mohm, so can read full voltage, but the instant you put a load on the circuit, will drop to zero if you have a resistive connection. And a resistive connection can be caused just by a slight discoloration of those cheap unplated brass terminals.

Just about any module problem reported here ended up in a shorted module, your problems sounds like the opposite, may be possible your module is still good. Not sure about your circuit, typical is the only good ground you need is for the blower motor, module acts like a variable resistance in the high side. Digital voltmeters now are as cheap as 3 bucks, ten bucks will get you a fairly decent one.

knightgang on Tue May 04, 2010 8:16 AM User is offline

Hi Nick...

Well, I had gotten a new module and that did not fix it. My voltmeter is digital, but I guess that as you said, will not read the PWM pulses. Without measuring those, I cannot determine if the fan witch in the control head is bad of not. Is there a resistance test that can be performed to do this?

mk378 on Tue May 04, 2010 8:49 AM User is offline

Unplug the blower control module entirely and jump the orange and black onto the motor. If the motor runs then you know for sure the truck is getting power and ground to those wires. Finding a good ground for the meter is always a problem when testing car electrics. Inside the car I usually use the shell of the lighter socket.

There should be some activity on the grey/black wire from the control head. I don't know exactly what to look for though it seems it would be a simple pulsed voltage that will appear on a voltmeter. In your other thread you said the voltage reading varied from 0 to 12 volts, which sounds right. Try it with the module unplugged.

Edited: Tue May 04, 2010 at 8:53 AM by mk378

knightgang on Tue May 04, 2010 11:34 AM User is offline

All of the voltage tests have been with the module unplugged. I had variable voltage once, but have not been able to replicate it since. I wll try the tests you mentioned above and any other trouble shooting I can think up.

I feel like I am leaning to a control head problem or a wiring harness issue.

NickD on Tue May 04, 2010 12:29 PM User is offline

I don't have a circuit for your truck, every ATC unit I have seen, the climate control panel is like your keyboard on your computer, no power switches, but push button switches that send commands via a data link to the BCM, the body computer module, that in turn fires the pulses to the motor module. This system does have a control box with servo motors to open and close doors, but it doesn't sent out any codes, just receives them, BCM is the brains.

Now your firmware is stored in flashram that may have got corrupted with your motor failure, if a loose or dirty connection, that generates high EMF pulses that can really foul up these circuits. If that is the problem, could need a reflash.

Recall a woman coming here a couple of years ago with a late model Caddy with an inoperative blower, took it her dealer with something like a 1,500 buck bill, she just wanted to know if she got screwed or not, her dealer said her BCM was bad, but she didn't know if it was replaced or reflashed.

Not like the late 60's when you could buy a brand new Delco blower motor for six bucks retail, but then they had screws holding the motor together, remove a couple of screws, could replace the brushes for 25 cents, and oil the bearings, so why spend six bucks? Will take the old days anytime.

Feel today, you can't even drive a car with an inoperative blower!

knightgang on Tue May 04, 2010 3:17 PM User is offline

Nick, this control head has knobs. I know they are not replaceable, and yes, the inside of the control head is like a computer, with motherboard, resisotors, capicitors and such. Is it possible that dirty connections on the fan knob board could be causing the problem know. I will investigate and let you know.

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