Help with HVAC issue

[Keith Seymore]

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

[hatzie]

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.

[Keith Seymore]

Originally Posted by hatzie

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

[hatzie]

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.

2nd gen GMT800 Digital HVAC control head bulb...

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.

Bill Herd Special guest: Adrian Black of Adrian's...

[Keith Seymore]

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

K

[Keith Seymore]

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.

[hatzie]

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.

[Keith Seymore]

Thanks Hatzie-

I'll take a look!

K

[Bob B.]

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.

[Keith Seymore]

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

[Keith Seymore]

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

[Keith Seymore]

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

K

[Bob B.]

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.

[Keith Seymore]

Originally Posted by Bob B.

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 -

Originally Posted by Bob B.

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