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Discussion Starter #1
Okay so this started out a couple of months ago with the Blower problem. I replaced the resistor and the wiring harness.

Last week I was going down the road and the blower just stopped. When I got home I pulled the plug out to check and make sure it wasnt happening again. I pulled the plug out of the blower motor to make sure that I was getting power to the blower (checked all the fuses and relays). I was getting power. I removed the blower and hooked it directly up to power and the blower worked so decided to check the ground side and once it grounded it started working again with no additional help. I just caulked it up to make a mad connection.

Yesterday my wife was driving the truck and the A/C stopped again and this time there is no power to the A/C switch on the dash. Now I did not notice if this was the issues when it happened 2 days before.

Attached is a wiring diagram and I have checked the Ign fuse, the A/C fuse, Blower fuse. There is no power to the hot side of the blower motor fuse (the 30amp big fuse)from the relay and there is no Ign power to the HVAC logic as well. The A/C relay has power. I have checked the ground at sp202 on the fender and sp200 to the HVAC control and the ground is good.

It almost seems to me that the BCM is not sending a signal to cut the A/C on. Do these trucks have issues with BCM's. Any help would be appreciated

Thanks in Advance
 

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Please clarify if the HVAC Fan is inoperable or if the Air conditioning is not switching on.
There is a lot of talk about the fan on one mention of A/C shutting off.

I ask this question because in the winter, the heat goes out and in the summer the A/C goes out when in reality, it's the fan that stop. Each of the 3 scenarios require different troubleshooting efforts.
 

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Discussion Starter #3
Neither work. There is no power to the circuit from the BCM to the HVAC 3 Relay. There is the two circuits that are hot all the time but nothing from the BCM which powers the Blower motor and the HVAC control module.

If you look at that wiring diagram you can trace the path. kinda at the top left
 

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:welcome to C'Fans

If you are getting 12 volts at two of the relay terminals the power side is good. Have you tried swapping the relay with another like relay in the fuse panel? Depending on model year the Run/Crank, VSES, Starter, and RAP relays could all be the same as the IGN3 HVAC.

The BCM is a pretty reliable component. I would suspect something other than that to be the cause of the issue. The BCM provides the ground side of the circuit for relay activation.
 

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Also if you are saying that there is no power on the IGN 3 voltage line (F10 C7 orange wire), that line should not have power on it, It is a ground supplied by the BCM to the IGN HVAC relay.

If you are missing power to both fuse 5 and fuse 41, then your problem is before that. Are you getting plus and minus 12 volts across the relay coil? You can pull the relay and probe those connections (pins) direct in the relay socket. They are diagonal pins in the relay socket.
 

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Discussion Starter #6
Okay I was able to get my hands on a Tech 2 today. There is no A/C request being picked up by the BCM when cutting it on. I can jump the HVAC 3 relay and everything comes on. I can connect my Fluke meter to the ground that is coming from the BCM and there is nothing. It stays open. I have gone back and rechecked all the grounds and everything is pointing to a BCM. I need to find a BCM pin out diagram so I can make sure that there is no open in that circuit and I have one most test I want to do before I replace it. I will keep you updated
 

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Discussion Starter #8
So I have gone through this diagnoses chart from GM and it all points to the BCM. I have checked all grounds. The only thing I havent checked is an open in the BCM to the HVAC 3 relay yet. I know it is not providing a ground and it is not shorted to ground as it would run all the time so if anyone else can think of anything. Please let me know.

Attached is the chart I used
 

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I guess I got caught up in the "BCM" reference and wasn't alert. The wiring schematics in the Service Manual indicate that the ECM (PCM) provides the ground for the A/C relay. Does your reference indicate the BCM?

What model year truck do you have?

Edit: Never mind, was looking at the compressor relay!
 

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Okay, maybe I have been looking at schematics too long and am getting cross-eyed, but it appears that there are two separate sources for HVAC power.

It looks like control power for the compressor relay comes through the RUN/CRANK relay and the ECM/PCM provides the ground to actuate the relay. Compressor power comes through the 10A AC Comprsr fuse.

The schematics show a separate circuit for the blower, with the power going through the IGN3/HVAC relay which feeds two fuses, the BLWR fuse and the AC fuse. The AC fuse feeds power to the HVAC Control Panel. The BCM controls the IGN3/HVAC relay.
 

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Discussion Starter #11
Here lies the problem. The BCM is not providing the ground to that HVAC 3 relay. I can just the relay and everything works fine so its not a power issue. I have gone through every power and ground circuit I can find and it all works other than the BCM providing the ground to the HVAC 3 relay to energize it to allow the power to flow through that relay to the blower and the HVAC controller
 

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Okay, my bad. I thought you weren't getting power to the compressor.

I found the circuit (300) coming out of connector 7, pin F!0 at the fuse box. I also found the circuit going into the BCM in connector X1, pin A26. The problem with the source that I have, the Service Manual, there is no indication of the routing of the wire for the circuit. The schematics do not show routing through any of the inline connectors, so the assumption is that the wire goes directly to the BCM without break. I checked the pinouts for all of the possible inline connectors, but that turned up nothing. Not much help. Verifying the continuity of that circuit would eliminate it as a possible source.
 

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Discussion Starter #13
I have now checked all the wiring and everything is good so I guess I will replace the BCM. The truck does not have power anything so all I should have to do is program "Passlock" I am assuming LOL We will see Now to find a BCM. The dealer wanted 220 bucks UUUGGG
 

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Discussion Starter #15
Well I received my BCM today and learned that the ole key on 3 times for 15mins each part doesnt work so will have to take to dealer to get programmed
 

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Well I thought I would give an update on this situation. If I can stress anything to anyone that works of vehicles please dont forget the basics. I search what I thought was every possible scenario that would cause my panel to loose power. I went through the diagnosis directly from the GM service manual and it led me to replacing the BCM. I not only did it once I had another guy I know check behind me because I didnt want to spend almost 200 bucks on a BCM when it was not a common problem. Well I am here to tell you that it is still not a common problem.

I put the BCM in and had it reprogrammed at my local dealer and it still didnt work (so I am now over 300 dollars into this repair) so I came home shaking my head as to what it may possibly be so I went back to the beginning. I removed the Ign/HVAC 3 relay and testing power and ground. Went I placed the relay back it I pressed down on it kinda hard as I was a little on the upset side and low and behold the fan came on. So I am like okay WTF. so I tried to remove the fuse block and it seemed to be a pain so instead of breaking something I tried to get a pick into the openings of the connectors so I could try and bend them back and that didnt work so I went old school on it and took a pair of needle nose pliers and twisted the relay prongs just a little and put it back in and it works like a champ so dont be like me and waste 300 bucks and living in Florida for a couple of weeks with no A/C for a simple repair.
 

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Well I thought I would give an update on this situation. If I can stress anything to anyone that works of vehicles please dont forget the basics. I search what I thought was every possible scenario that would cause my panel to loose power. I went through the diagnosis directly from the GM service manual and it led me to replacing the BCM. I not only did it once I had another guy I know check behind me because I didnt want to spend almost 200 bucks on a BCM when it was not a common problem. Well I am here to tell you that it is still not a common problem.

I put the BCM in and had it reprogrammed at my local dealer and it still didnt work (so I am now over 300 dollars into this repair) so I came home shaking my head as to what it may possibly be so I went back to the beginning. I removed the Ign/HVAC 3 relay and testing power and ground. Went I placed the relay back it I pressed down on it kinda hard as I was a little on the upset side and low and behold the fan came on. So I am like okay WTF. so I tried to remove the fuse block and it seemed to be a pain so instead of breaking something I tried to get a pick into the openings of the connectors so I could try and bend them back and that didnt work so I went old school on it and took a pair of needle nose pliers and twisted the relay prongs just a little and put it back in and it works like a champ so dont be like me and waste 300 bucks and living in Florida for a couple of weeks with no A/C for a simple repair.
But what exactly did you fix?

You went from not wanting to remove the fuse panel to using needle nose pliers to twist relay prongs.
 

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thanks guys

I've read a lot of the threads on this subject here and fixed my truck. I would like to thank everyone that has posted for all the help. It ended up the ground box on the fender which I didn't think it was. My a/c light passed the test suggested in one thread. The light was off in the off position and on in all four of the on positions. So, the only thing this test tells me is the switch is good. So, thanks again for your help and I know where to look for help when I need it.
 
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