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Old 07-21-2012, 07:17 PM   #1
Boog
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blown heater core

Yeah in my 06 GMC cc. We were headed out on a day trip and all of a sudden there was vapor coming from the dash vents. Sitting idling it was fine but on the road at 70 the vapor came and went only slightly smelling of antifreeze. Luckily it was not a major blow out we were able to make our trip and get home. I turned off the ac and cracked the windows. It was HOT.
I already had made an appointment with the dealer to look at it come tuesday for a bad vent door actuator. Driver side would blow hot and the passenger side would blow cold. Turn the truck off and restart and it would reset and work fine. He'll get to take care of both at once. Yeah the carpet is soaked with coolant so the seats and carpet have to come out too. The truck just has 106,000 miles on it. From what I read heater core failures are more common on these late models than trucks of years back.
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Old 07-22-2012, 04:23 PM   #2
63burban4x4
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Re: blown heater core

I'll be very interested in the repair required on your A/C problem.....my '04 Silverado 2500 HD has had the same issue for several years now. Suddenly blowing blistering hot air from the driver's side only, pass side is fine. For a while it would, like you said, reset on restarting, and be fine, but eventually it did it MOST of the time. It's been diagnosed as: a) computer problem; reprogramed, (a bunch of times!) only lasted a short while. b) blend door actuator; $$$, only fixed it for a while. c: bad BCM; replaced (with a used BCM) stopped doing it for a while, (meanwhile that caused OTHER problems: Air Bag warning light, still not fixed, locked security code in radio requiring a $95 reset of code BY DEALER ONLY) and NOW AFTER A COUPLE OF MONTHS, THE DRIVER'S SIDE A/C is intermitently blowing HOT AGAIN!!!!!!!!!!!!!!!!) This is driving me crazy, and I have had 2 different shops try to fix this. Both have pretty decent mechanics, and it's driving them crazy too!! Worst thing is, you cannot close the center A/C vent in the dash, so you have to suffer.....the only way I've been able to deal with it, is to make the wife drive, so I can sit on the cool side -Ed
oh, and it only NOW has 91,000 miles, started this crap at 60-70,000 miles!
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Old 07-22-2012, 07:48 PM   #3
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Re: blown heater core

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Originally Posted by 63burban4x4 View Post
I'll be very interested in the repair required on your A/C problem.....my '04 Silverado 2500 HD has had the same issue for several years now. Suddenly blowing blistering hot air from the driver's side only, pass side is fine. For a while it would, like you said, reset on restarting, and be fine, but eventually it did it MOST of the time. It's been diagnosed as: a) computer problem; reprogramed, (a bunch of times!) only lasted a short while. b) blend door actuator; $$$, only fixed it for a while. c: bad BCM; replaced (with a used BCM) stopped doing it for a while, (meanwhile that caused OTHER problems: Air Bag warning light, still not fixed, locked security code in radio requiring a $95 reset of code BY DEALER ONLY) and NOW AFTER A COUPLE OF MONTHS, THE DRIVER'S SIDE A/C is intermitently blowing HOT AGAIN!!!!!!!!!!!!!!!!) This is driving me crazy, and I have had 2 different shops try to fix this. Both have pretty decent mechanics, and it's driving them crazy too!! Worst thing is, you cannot close the center A/C vent in the dash, so you have to suffer.....the only way I've been able to deal with it, is to make the wife drive, so I can sit on the cool side -Ed
oh, and it only NOW has 91,000 miles, started this crap at 60-70,000 miles!
To fix this you have to know how it works! Most mechanic's are lazy, SOLEY dependant on a scan tool to tell them whats wrong....Though a Tech II is mandatory to retrieve codes from the HVAC control head.

Put the original BCM back in....It should have never been replaced.

If they replaced the Blend door actuator......I hope it was The Left Temperature Blend Door Actuator?


1. Gain access to the left air temperature motor for circuit testing. There are 5 wires at the motor.

2. The Brown wire is ignition power.

3. The Black wire is a ground at all times.

4. The Gray wire carries a 5-volt reference from the control head to the temperature motor.

5. The Light Blue wire is the feedback circuit to the control head and should measure between about 1 volt (cold) and 4 volts (hot). This feedback circuit operates similarly to a Throttle Position Sensor (TPS).

6. The Dark Blue wire is the motor control circuit sent from the control head.

7. When the control head sends out 5 volts on this wire, the motor moves towards cold.

8. When the control head sends out 0 volts, the motor moves towards hot.

9. When the temperature motor reaches the correct position (as measured by the feedback voltage on the Light Blue wire), the control head sends out 2.5 volts. This voltage signals the temperature motor to STOP and hold its present position.

10. Check for correct operation of these voltages.

11. In some cases, the air temperature door does not fit correctly in the plenum and will cause the air temperature motor to travel too far. This will cause the control head to become confused and operate erratically.
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Old 07-23-2012, 02:24 PM   #4
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Re: blown heater core

Thanks for your opinions.
1st tech, (about 20 years experience in a Chevy dealership, before GM pulled the franchise in 2009) tried reprograming the computer several times, eventually replaced the LEFT blend door actuator.

2nd tech (owner of very sucessful shop for 30+ years) checked voltages, as you describe, first thing. When everything checked out fine, and knowing the blend door actuator had already been replaced, suspected problems with the BCM, as the truck had exiibited prior problems with that computer. He happened to have one from a wreck on his shelf (probably explains the air bag problem) and decided it would be a good idea to install this in my truck as a loaner, and send mine out for bench check, and reprogram or repair if needed. He ALWAYS planned to reinstall my BCM. This IMMEDIATELY fixed the A/C problem, and it was fine for several months, at least. Meanwhile, he suffered some serious medical issues, was hospitalized, and out of work for about 6 weeks. During this time, the A/C problem returned. Now that he is back at work, and aware the problem is back, he indicated he thinks it could have a fitment problem with the door itself. We simply haven't had a chance to take it apart yet to check, as this is my work truck, and he's also waiting for the computer to come back from the vendor.
I would hardly call either of these techs "lazy mechanics"

thanks again.
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Old 07-23-2012, 07:37 PM   #5
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Re: blown heater core

You didnt mention they checked voltages, Only the parts that were replaced, I deal with trucks with your problem all the time, Alot are trucks the "dealer" couldnt fix. A good portion of them have the temp door "overtravel" intermittently. Reading the door posistion curcuit, Anything above 4vdc on full cold I consider overtraveling.
The only reason replacing parts seems to fix it....Is because disconnecting the battery resets the HVAC control head.
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Old 07-23-2012, 08:28 PM   #6
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Re: blown heater core

I took it into the garage today. The guy looked at the antifreeze soaked carpet and said "well you're gonna need a heater core and a blend door motor on the left side". Ya THINK! What tipped ya off? " gonna run ya bout a thousand". Not! Wife said thinkadamngain! It is supposed to be covered under warranty. We'll see.
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Old 07-25-2012, 10:44 AM   #7
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Re: blown heater core

Got the truck back. Wife's happy. They did the repair under warranty. Didn't want to but in the end they did. New left blend door motor and heater core + labor over $1200. I've never purchased an extended warranty on any of my vehicles before but this time I did. Good thing.
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Old 08-26-2012, 02:13 PM   #8
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Re: blown heater core

One month since the repair and the vent temp problem is back. It will be going back to the shop tomorrow.
Do you think it is only a door misalignment causing it to go warm on the one side? Is the door actuator adjustable?
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Old 08-26-2012, 05:25 PM   #9
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Re: blown heater core

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One month since the repair and the vent temp problem is back. It will be going back to the shop tomorrow.
Do you think it is only a door misalignment causing it to go warm on the one side? Is the door actuator adjustable?
Its most likely the blend door is not fitting correctly in the plenum, Or its broke, Unfortunately the HVAC module will have to be removed again. & no the actuator is not adjustable

If they will not fix it....You can get by with "recalibrating" it when it acts up....

Place the ignition switch to the OFF position.
Remove the HVAC/ECAS fuse for a minimum of 10 seconds.
Install the HVAC/ECAS fuse.
Start the vehicle.
Wait 40 seconds for the HVAC control module to self-calibrate.

IMPORTANT: Do not adjust any controls on the HVAC control module while the HVAC control module is self-calibrating. If interrupted, improper HVAC performance will result.
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Old 08-26-2012, 08:12 PM   #10
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Re: blown heater core

Thank you CB.
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Old 08-29-2012, 06:13 PM   #11
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Re: blown heater core

They put on another left side actuator. Lasted one month to the day I think.
I wonder if the blend door is broken or misaligned how hard is it to replace it. This truck has the dual climate control. We've been getting some small bits of foam out of the vents on the right side. The right side functions correctly so I'm not too worried over the loss of door seal foam.
I just don't want to have to keep replacing this left side actuator.
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Old 08-29-2012, 06:36 PM   #12
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Re: blown heater core

Quote:
Originally Posted by Boog View Post
They put on another left side actuator. Lasted one month to the day I think.
I wonder if the blend door is broken or misaligned how hard is it to replace it. This truck has the dual climate control. We've been getting some small bits of foam out of the vents on the right side. The right side functions correctly so I'm not too worried over the loss of door seal foam.
I just don't want to have to keep replacing this left side actuator.
The dash & HVAC box will need to be removed & disassembled.

The actuator isnt bad or going bad, It only "seems" to fix it because of the recalibration.

My old 04 was doing the exact same thing.....I just kept recalibrating it every month or so.

My 06 is doing OK....Knock on wood
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Old 08-29-2012, 06:43 PM   #13
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Re: blown heater core

Dern, I don't want them or me to have to pull the dash out for this. I saw a video of a fella replacing his own heater core. Thats deeper than I want to get into it.
How long did you put up with it on your 04? The next time ours does it I will try your recal sequence.
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Old 08-29-2012, 07:17 PM   #14
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Re: blown heater core

About 2 years, Didnt bother me really, I was going to fix it But someone wanted to pay more money than it was worth.
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Old 08-29-2012, 07:30 PM   #15
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Re: blown heater core

So if they put a new one in they had to calibrate it?
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Old 08-29-2012, 07:34 PM   #16
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Re: blown heater core

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So if they put a new one in they had to calibrate it?
Yes, Or had the battery disconnected, When you install a new actuator, It wont be in the same position the old one was.
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Old 09-01-2012, 09:50 PM   #17
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Re: blown heater core

I figure this might be an appropriate place to comment on the fun times anyone would have when trying to replace a heater core in a GMT800 truck. I had to replace the heater core in my 2000 GMC. Out of the available GMT800 trucks, this one should have been the easiest due to the lack of column shifter and centre console.

You need to remove all this:


So that you can get the dash to look like this:
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