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Old 06-06-2022, 10:05 AM   #1
Keith Seymore
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Help with HVAC issue

2004 Silverado 2500 HD with Duramax.

Auto HVAC controls.

The issue is that with the controls set cooler than the outside ambient temperature the system will occasionally change itself and start blasting insanely hot air out the driver's RH vent.

Last summer it was fairly infrequent. This year it can happen every 10 or 20 minutes during an extended road trip.

Recycling the ignition will we restore normal operation. I thought maybe if I placed the system on manual fan operation, rather that auto, that would help but I don't think it makes a difference.

After doing a forum search I performed a system reset, per the attached. That maybe(?) helped for a while but yesterday while driving over a three hour period it was doing it every 10 or 15 minutes.

My next step is to replace the HVAC control head, unless you guys have other ideas and talk me out of it.

Thanks in advance -

K
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Last edited by Keith Seymore; 06-06-2022 at 10:25 AM.
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Old 06-06-2022, 07:31 PM   #2
hatzie
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Re: Help with HVAC issue

Something on the HVAC controller board is flaking out on you.
It'd be entertaining to point a Flir camera at the bare controller board after it starts freaking out.

Our 20 and 30 and 40 year old vehicles have various embryonic to fairly sophisticated controllers onboard. Eventually they will start to fail.

I watched an entertaining vid on that subject this weekend. Bill Herd was talking to Adrian Black about MOS fabbed chips on 1980's 8 bit computers and the fact that they were really never designed to last longer than about 10 years. They're all well past the time they should've dropped dead. The controllers in 1980's & 1990's vehicles are similar but not so popular that FPGA replacements for the internal chips are being made like the 8 bit computers from the 1980's.
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Old 06-07-2022, 08:46 PM   #3
Keith Seymore
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Re: Help with HVAC issue

Quote:
Originally Posted by hatzie View Post
Something on the HVAC controller board is flaking out on you.
Thanks, Hatzie.

Where is this controller board located? Is it in the HVAC head, or mounted elsewhere?

K
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Old 06-08-2022, 01:51 PM   #4
hatzie
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Re: Help with HVAC issue

I only have passing experience with the digital display HVAC controller. It's CPU is buried inside the HVAC head in the dash.

Remove the control head from the dash, remove the knobs from the encoder shafts, Remove the through screws that hold the face in place, and separate the face from the back 2/3 of the HVAC controller case.

There were two circuit boards inside my HVAC controller.
Directly behind the face is the digital display/encoder circuit board with a high density wire connector plug wired deeper inside to the actual CPU board. The display/encoder board is the only one I've removed from the housing since I was replacing the burnt out panel illumination bulbs.

It's pretty obvious how the display/encoder board comes out of the rear housing. The white plastic housing is likely fairly brittle and the board is super thin in comparison to a standard thickness PC board so treat it with care.

I was only interested in the display/encoder board so I didn't pay a lot of attention to the CPU board and how it was attached to the housing.

If the CPU board in the back of the housing is as thin as the display/encoder board you'll need to be very careful with it. I don't remember any noticeable large diameter connector pins on that board so with luck it's not soldered in place to the connector molded into the HVAC control head like the 4wd switch bulb board. I had to drill out some 2mm Hakko desoldering tips to around 2.8mm remove that 4x4 switch bulb board from the housing.
If they melted plastic posts to fix the CPU board in place it could be miserable to liberate it from the housing.

Once you're sure you have the controller working again... It might be good to replace the tungsten display bulbs so you don't have to monkey with it again. My bulb bases turned to powder as soon as I touched them. If the new bulbs don't have them it's not the end of the world. My replacement bulbs had bases but the wrong color silicone caps. I transferred the original blue silicone bulb covers to the new bulbs so the lamp color doesn't stick out like a sore thumb.


Here's the Bill Herd and Adrian Black video I referenced where Bill is talking about the difference between consumer chips vs industrial chips and industrial vs consumer board design and how long stuff was designed to last back at the dawn of the consumer electronics age in the late 70's.
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1969 Dodge Polara 500 B383, A833 SOLD
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1986 M1008 CUCV SOLD
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8


RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.

Last edited by hatzie; 06-08-2022 at 02:06 PM.
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Old 06-08-2022, 09:29 PM   #5
Keith Seymore
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Re: Help with HVAC issue

Thank you; that should keep me busy for a while.

K
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Old 12-30-2022, 02:23 PM   #6
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Re: Help with HVAC issue

Ok - I'm back.

By way of update, I replaced the control head back in June and that seemed to do it.

Until this past weekend.

It was bitter cold (like 4 degrees) and we were heading out for a 20 hour trip (10 down to SC and then 10 back) and after driving for a while we noticed we were still freezing. Engine temp was about 185 deg.

I checked and there was no air flow out at our feet, even though the fan was making plenty of noise. I ended up switching the discharge to "face" (out the vents and the driver side vents were blowing nice hot air but the passenger side vents were both blowing ice cold.

Does this sound like a mode door problem? Where is that located? Can it be reset, or replaced?

Thanks in advance -

K


Also - pretty sure I returned the original HVAC control head, since there was a core charge.
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Intro from an Old Assembly Guy: http://67-72chevytrucks.com/vboard/s...d.php?t=342926
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Chevelle intro: http://www.superchevy.com/features/s...hevy-chevelle/
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Old 12-30-2022, 08:28 PM   #7
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Re: Help with HVAC issue

Temp not changing is a blend door problem. The blend motors are on top and bottom of the HVAC box. I've forgotten which one is the RH temperature blend motor.
The top motor may be accessible by removing the radio but you may need to pull off the dash top. The motor on the bottom can be accessed by removing the kick cover.

The mode motor is above the throttle pedal on the side of the HVAC box. This is the motor that changes airflow to the floor through the defroster vents.
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1959 M35A2 LDT465-1D SOLD
1967 Dodge W200 B383, NP420/NP201 SOLD
1969 Dodge Polara 500 B383, A833 SOLD
1972 Ford F250 FE390, NP435/NP205 SOLD
1976 Chevy K20, 6.5L, NV4500/NP208 SOLD
1986 M1008 CUCV SOLD
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8


RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.

Last edited by hatzie; 12-31-2022 at 04:27 PM.
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Old 12-31-2022, 06:38 PM   #8
Keith Seymore
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Re: Help with HVAC issue

Thanks Hatzie-

I'll take a look!

K
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Old 01-02-2023, 02:42 PM   #9
Bob B.
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Re: Help with HVAC issue

Driver's side actuator is on the bottom of the HVAC housing, passenger side on the top. The bottom one is easy to replace but you have to work around the console if equipped. The top one is tricky, manual says to move the dash out of the way but I have been able to do them by removing the radio on some trucks.
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Old 01-02-2023, 10:19 PM   #10
Keith Seymore
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Re: Help with HVAC issue

Thank you -

I've got to finish some stuff up on the Chevelle and then I'll pull the truck around into the shop.

K
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Old 01-08-2023, 07:12 PM   #11
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Re: Help with HVAC issue

Awright - after much diagnosis I replaced the bottom (passenger side) mode door actuator.

I have successfully gone from "heat only/driver's side vents only" to no heat at all. So I put it back the way it was.

Maybe I'm not understanding this correctly -

The passenger side door is for temperature? (flips from hot to cold)

And the driver's side (above and inboard of the throttle pedal) is for defrost/vent and floor?

What controls the split between driver side and passenger side?

By way of reminder the truck is RPO CJ2 "Auto HVAC".

K
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Old 01-08-2023, 07:15 PM   #12
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Re: Help with HVAC issue

Can I just pull the actuator motors off and flip the doors around manually, to make sure I understand the functionality?

K
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Old 01-09-2023, 03:34 AM   #13
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Re: Help with HVAC issue

Yes, the actuator that is above the throttle pedal on the left side of the HVAC box is 'mode' (floor-dash-defrost). The actuator at the bottom of the HVAC box above the transmission tunnel (behind the console if so equipped) controls the left (driver's side) temp.. The right side temp. actuator is on top of the HVAC box behind the radio. Yes you can flip the doors with the actuator off to check their function, and it's a good idea to do so when replacing an actuator as a stuck door can damage an actuator. The split between left and right heat depends on what position the door/actuator is in for its respective side. They are independent. There is a calibration procedure when an actuator is replaced, off hand I don't remember exactly what it is. I will take a look or maybe one of the knowledgeable guys here will comment.
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Old 01-09-2023, 11:54 AM   #14
Keith Seymore
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Re: Help with HVAC issue

Quote:
Originally Posted by Bob B. View Post
Yes, the actuator that is above the throttle pedal on the left side of the HVAC box is 'mode' (floor-dash-defrost). The actuator at the bottom of the HVAC box above the transmission tunnel (behind the console if so equipped) controls the left (driver's side) temp.. The right side temp. actuator is on top of the HVAC box behind the radio. Yes you can flip the doors with the actuator off to check their function, and it's a good idea to do so when replacing an actuator as a stuck door can damage an actuator. The split between left and right heat depends on what position the door/actuator is in for its respective side. They are independent. There is a calibration procedure when an actuator is replaced, off hand I don't remember exactly what it is. I will take a look or maybe one of the knowledgeable guys here will comment.
Thank you -

Quote:
Originally Posted by Bob B. View Post
The right side temp. actuator is on top of the HVAC box behind the radio.
Ah - thank you! This is the one I was not finding.

So far all of the doors have flipped easily when I moved them manually.

I assume the calibration procedure is what I have shown in post #1 of this thread?

More to come -

K
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Intro from an Old Assembly Guy: http://67-72chevytrucks.com/vboard/s...d.php?t=342926
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Last edited by Keith Seymore; 01-09-2023 at 01:59 PM.
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