GMT800 climate control automatic (CJ2) to manual (CJ3) conversion

[DAVe3283]

People have been complaining about the lack of tech here lately, so I thought I would completely misunderstand the assignment and share a project I am wrapping up for a friend. Is it tech? Technically yes. Is it stupid? Also yes. Behold:

Converting Automatic Climate Control to Manual: The Hard Way™​

On the GMT800 (specifically 2003-2007 classic) platform, the climate control hardware is nearly identical between CJ2 (fancy/automatic) and CJ3 (low-end/manual) controls. One of my friends has a Tahoe with the auto climate and wanted the manual climate controls. (This boils down to preferring to set vent temperature over cabin temperature.)

The critical difference is blower motor control. On the CJ2 (auto), there is a digital speed controller that provides variable voltage to the blower. On the CJ3 (manual), a blower motor resistor pack is used to drop voltage to the motor.

The easy way to do this would have been to get a blower resistor and wiring from the junkyard, and spend a few hours pulling wires through the dash. But the resistor pack wastes a lot of energy as heat (hence being downstream of the blower to get cooled), and is FAR more likely to catch fire and burn your truck to the ground. Plus I hate taking the dash apart, so instead, I chose to do it the hard way: build a module to run the CJ2 (auto) blower speed controller from a CJ2 (manual) control.

With the help of another friend, we created a PCB, 3D printed enclosure, and firmware to make a completely plug-and-play conversion.
It is all open source, available here: https://PigLab.ReaperLegion.net/automotive/gmt800/cj2-to-cj3

Assembly​

If you want to build your own, we used JLCPCB to fabricate the PCB and all the parts are commonly available (e.g. from Mouser), minus what you have to scavenge from the junkyard. The full details are in the link above.

​

Installation​

Despite great effort being put into making the module as small as it can practically be (given hand assembly was a requirement), it is too large to fit behind the climate control module. Mostly, the factory plugs take too much room. Fortunately, if you remove the radio, you can tuck the module down behind the dash.

Remove the CJ2 (auto) climate controls and the radio. Tuck the factory climate wires up through the original hole and pull them up behind the radio cavity:


With the factory CJ2 (auto) harness out of the way, you can feed the module's CJ3 (manual) harness through the same hole:


Tuck the module down behind the radio and plug the factory CJ2 (auto) harness into the module. Finally, plug the module's CJ3 (manual) harness into your junkyard CJ3 (manual) climate controls:

Calibration​

Once the install is complete, set the fan speed to 0 or 1, and turn on the ignition but don't start the engine. You will hear the actuators cycling through their range so the manual climate controls can learn the limits. This takes around 30 seconds, don't touch anything and let it finish. Then you can start the engine and enjoy manual climate controls!

Diagnostics​

The converter module can be diagnosed with USB if needed (see the link above for details and warnings).

The CJ3 (manual) controls work just like factory as far as the vehicle is concerned. You can talk to it with a Tech 2, the rear climate controls (e.g. Suburban/Tahoe) will still work, etc.
In fact, viewing the actuator data with a Tech 2 is the best way to diagnose worn actuators.

Worn Actuators​

If your actuators are worn out, the manual climate controls are far stupider than the automatic one, and it will give up trying to control them at the end of their range. I think GM did this on purpose to make the manual climate worse than the auto climate, since it is an intentional choice in the firmware, not a hardware limitation.

You can either replace all your actuators, or move the adjusters a click or two from the end (see what it takes for your particular unit), and it will start controlling them again. This is true for the mode as well as the temperature. Thanks, GM.

Edit: My friend replaced the junkyard CJ3 (manual) climate controls with a Dorman 599-210XD and it solved the actuator drift. So yes, 100% GM firmware holding it back. I would HIGHLY recommend just buying the Dorman rather than a junkyard module given the firmware doesn't suck.

Early-Model Warning​

Early GMT800s (2003 at least) have a different part number for the recirculation door actuator for CJ2 (auto) and CJ3 (manual) climate controls. If you still have the original CJ2 actuator, it will stick in recirculate mode when using the manual climate controls.

Around 2005, GM created a new part number that works for both CJ2 and CJ3.

So if you do this conversion and your recirc doesn't respond, you will have to update to the new part number (or a junkyard CJ3 (manual) actuator, but I recommend just getting the new updated part).

Potential Capability: HVAC Afterblow​

On later model cars (e.g. the Corvette and the Volt, probably many others) there is a feature called HVAC Afterblow that will turn the blower back on after the car has been parked for a little while to dry the condensation off the evaporator. This will help prevent mold/mildew/smells in the HVAC system.

I designed the module to read all the vent temperatures, so it is possible to infer when the A/C has been running, and do the same thing. I haven't found the motivation to write said firmware yet, but it is certainly a possibility.

It is also possible to choose any speeds you want for the 5 fan positions on the manual climate controls. It doesn't have to go 1→5=slow→fast, it could do whatever you want.

 

[2004LB7]

that is pretty sweet. I've never liked the automatic climate controls in these vehicles. and now I have one in mine . Having to take your eyes of the road to fiddle with the automatic controls vs being able to feel the knob and knowing what you are doing without looking at it. that's why I'm not a big fan of the touch screen stuff going into the newer vehicles

any chance of hacking the newer 7.5+ vehicles for the same conversation?

 

[1FastBrick]

I have to say that's actually pretty cool. Most people want to go the other way but after having a few GMT800 trucks with it, I would rather have the manual controller.

 

[TheBac]

That is pretty darn cool, Dave. Thank you.

 

[DAVe3283]

that is pretty sweet. I've never liked the automatic climate controls in these vehicles. and now I have one in mine . Having to take your eyes of the road to fiddle with the automatic controls vs being able to feel the knob and knowing what you are doing without looking at it. that's why I'm not a big fan of the touch screen stuff going into the newer vehicles

I am with you 100% on this, it is ridiculous to have to look down and interact with a screen to change simple things like temperature, radio volume, or (looking at you Tesla) windshield wipers.

any chance of hacking the newer 7.5+ vehicles for the same conversation?

A quick scan through the GMT900 schematics makes me think it might just need a custom jumper harness, not a custom module like the GMT800.
The GMT900 manual climate already uses the blower motor control module instead of the horrible resistor. The connectors are different between the two modules, but they appear to have most of the same signals.

The problem is I don't know anyone local with a GMT900 (all my friends have GMT800s) to test any theories on.

Edit: I take it back, it appears the manual GMT900 setup only has one temperature zone, while the auto has two, so there would have to be a module to take the single zone and duplicate it to the 2nd actuator. Never easy...

 

[2004LB7]

I am with you 100% on this, it is ridiculous to have to look down and interact with a screen to change simple things like temperature, radio volume, or (looking at you Tesla) windshield wipers.


A quick scan through the GMT900 schematics makes me think it might just need a custom jumper harness, not a custom module like the GMT800.
The GMT900 manual climate already uses the blower motor control module instead of the horrible resistor. The connectors are different between the two modules, but they appear to have most of the same signals.

The problem is I don't know anyone local with a GMT900 (all my friends have GMT800s) to test any theories on.

Edit: I take it back, it appears the manual GMT900 setup only has one temperature zone, while the auto has two, so there would have to be a module to take the single zone and duplicate it to the 2nd actuator. Never easy...

that dual temp zone never made much sense to me. we are both sitting right next to each other in the same cabin. the air from one is going to mix with the air from the other and mess it all up. plus with the ability to direct and turn off each vent it seems overly redundant and complicated. I would be completely fine with a single zone.

I wonder how hard it would be to just duplicate the zone temperature. the fan speed and mode is the same for both so it appears to just be temperature

 

[JoshH]

I can't say I understand why people hate the auto climate control so much. I much prefer it to fiddling with the controls all the time.

 

[2004LB7]

I can't say I understand why people hate the auto climate control so much. I much prefer it to fiddling with the controls all the time.

I can see that for someone who may have it on and set to their preferred temperature all the time. I don't run it much, or any A/C for that matter but when I do need it then I'm usually already on the road and then have to figure it out while driving. for something like defrost I have to find the button then watch the display and cycle through the modes to get it. with the manual controls I don't even have to look at it

 

[Bdsankey]

I can't say I understand why people hate the auto climate control so much. I much prefer it to fiddling with the controls all the time.

I agree 100%.

It's never been a big deal to me but I guess people hate it?

 

[2004LB7]

or or or... what about when we have weather like now in fall where the morning temps are cold enough that it's nice to have a little heat going to work but at the end of the day it's 90 out and you want cooling. sucks to have to push the temp button down for 10 seconds to get it to where you want. with a knob I can quickly rotate it one way or the other in less then a second and get the temp I want

 

[Bdsankey]

or or or... what about when we have weather like now in fall where the morning temps are cold enough that it's nice to have a little heat going to work but at the end of the day it's 90 out and you want cooling. sucks to have to push the temp button down for 10 seconds to get it to where you want. with a knob I can quickly rotate it one way or the other in less then a second and get the temp I want

I don't know what system you had but mine looked like below where I could easily set the temp via a rotary dial. I had to push up/down for the speed control but that's it.

 

[2004LB7]

I don't know what system you had but mine looked like below where I could easily set the temp via a rotary dial. I had to push up/down for the speed control but that's it.

View attachment 110270

I wouldn't mind that one. seems much better then what I have

not actually mine but same type. and yes this is what happens to the buttons because they get pushed so much. buttons are too small and flat to be able to feel for them. writing or images are tiny and hard to read at a glance. such a stupid design

this is the one that would fit the console. simple knobs for the often changed temp and fan speed. everything can be set by feel without having to look at it or read anything while driving. unfortunately as mentioned above it is a single zone so not directly compatible and needs the signal/ controls duplicated

 

[Bdsankey]

I wouldn't mind that one. seems much better then what I have

not actually mine but same type. and yes this is what happens to the buttons because they get pushed so much. buttons are too small and flat to be able to feel for them. writing or images are tiny and hard to read at a glance. such a stupid design

It is a GMT800 thread not GMT900

I certainly agree on GMT900 it's a crap design but I personally don't see any issues with the GMT800 auto controls.

 

[2004LB7]

It is a GMT800 thread not GMT900

I certainly agree on GMT900 it's a crap design but I personally don't see any issues with the GMT800 auto controls.

I get that. that's why I asked if there was a possibility of doing the same for the newer vehicles. then we went off on the merits of the auto controls

 

[Bdsankey]

The manual GMT900 control is lightyears above the autos, I don't know who thought they were a great idea but they hopefully learned their lesson.

 

[JoshH]

or or or... what about when we have weather like now in fall where the morning temps are cold enough that it's nice to have a little heat going to work but at the end of the day it's 90 out and you want cooling. sucks to have to push the temp button down for 10 seconds to get it to where you want. with a knob I can quickly rotate it one way or the other in less then a second and get the temp I want

See, that's why I like the automatic climate control. If you let it work like it's designed, it's like the thermostat in your house. It warms up when it's cold and cools off when it's hot.

 

[2004LB7]

See, that's why I like the automatic climate control. If you let it work like it's designed, it's like the thermostat in your house. It warms up when it's cold and cools off when it's hot.

mine doesn't seem to be that intelligent and never gets the temperature I want right. normally I just turn it off and only turn it on when I get too uncomfortable. I can see it working for many. it's just not how I like to use it. even my AC at home. I rarely turn it on until it's in the 80s inside. otherwise it's set to 80~82° so it barely runs.

in my truck the fan speed roars when you first start the truck when it's slightly warm. blows cold air until the engine warms up. times when I'd rather have it off and only turn it on when I really need it. because it doesn't run how I'd like to use it I end up with it off 99% of the time.

if I take my Mom somewhere she always like to have AC on but it's too complicated for her to figure out and the text is too small for her to read easily so I have to set it for her. a big knob with red and blue would be perfect.

 

[DAVe3283]

My friend replaced the junkyard CJ3 (manual) climate controls with a new Dorman 599-210XD from Amazon and it solved the actuator drift! So yes, 100% GM firmware holding the manual controls back. I still think they did it on purpose.

I would HIGHLY recommend just buying the Dorman rather than a junkyard module given the firmware doesn't suck if you do this conversion.

 

[DAVe3283]

mine doesn't seem to be that intelligent and never gets the temperature I want right. normally I just turn it off and only turn it on when I get too uncomfortable. I can see it working for many. it's just not how I like to use it. even my AC at home. I rarely turn it on until it's in the 80s inside. otherwise it's set to 80~82° so it barely runs.

in my truck the fan speed roars when you first start the truck when it's slightly warm. blows cold air until the engine warms up. times when I'd rather have it off and only turn it on when I really need it. because it doesn't run how I'd like to use it I end up with it off 99% of the time.

if I take my Mom somewhere she always like to have AC on but it's too complicated for her to figure out and the text is too small for her to read easily so I have to set it for her. a big knob with red and blue would be perfect.

It sounds like the GMT900 auto controls are way worse than the GMT800. I quite like the GMT800 (and older, like in my daily beater '98 Pontiac Grand Prix) auto controls. Especially now in the fall because when I head to work it runs the heat, and when I head home it runs the A/C, and I never touch it. In fact, I don't think I have touched the temperature setting in months.

One thing you mention jumped out at me: blasting cold air in the morning. That has to be a GMT900 regression, because on my GMT800s (and even the '98 Pontiac) the fan won't ramp up in the morning until the coolant temp comes up. The fan stays at 0 (near as I can tell) until I see the coolant gauge just start to creep up, then the fans slowly ramp up.

And the defrost is a dedicated button, no scrolling through modes. Just press defrost, and get defrost. When you're done, hit it again or press auto and it is back to normal. What's more, most of the time the auto system will guess that I need defrost and do it without me even touching the button. I only have to tell it to defrost (or not to defrost) a few weeks of the year, mostly it guesses right.

 

[Bdsankey]

It sounds like the GMT900 auto controls are way worse than the GMT800. I quite like the GMT800 (and older, like in my daily beater '98 Pontiac Grand Prix) auto controls. Especially now in the fall because when I head to work it runs the heat, and when I head home it runs the A/C, and I never touch it. In fact, I don't think I have touched the temperature setting in months.

One thing you mention jumped out at me: blasting cold air in the morning. That has to be a GMT900 regression, because on my GMT800s (and even the '98 Pontiac) the fan won't ramp up in the morning until the coolant temp comes up. The fan stays at 0 (near as I can tell) until I see the coolant gauge just start to creep up, then the fans slowly ramp up.

And the defrost is a dedicated button, no scrolling through modes. Just press defrost, and get defrost. When you're done, hit it again or press auto and it is back to normal. What's more, most of the time the auto system will guess that I need defrost and do it without me even touching the button. I only have to tell it to defrost (or not to defrost) a few weeks of the year, mostly it guesses right.

And yet here you are making a thread on how to remove it