great white
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Something that's always bugged me since the OEM instituted it has been auto AC with any defog setting. So I usually disable it on my vehicles.
My thoughts have always been I'll decide what the bloody car is going to do or what it needs based on the present conditions. I don't need some engineer making the decisions for me on something simple like defog.
There was a time when they left it to the operator to turn AC on if needed for defogging and just made a nice simple note in the owners manual. Then some idiot decided everyone else needed to be idiot proofed 100% of the time. Probably because he was the idiot sitting in his car with a steaming hot coffee who didn't turn on his AC one morning and had a recirc door in recirc and wondering why his windows won't clear.....
Truthfully, it's there because of the way new cars are designed: smaller bodies, smaller HVAC components, bigger windshields and tighter weather seals. Today's HVAC just can't keep up with it. So in order to stave off all the "my car is always fogged up" or "the HVAC sucks" warranty service, they defaulted to "on" all the time. Makes up for shortcommings in design and it doesn't cost the OEM more than the cost of designing the HVAC to come on.
So yeah, I know why it's there (dehumidify, etc). This is a thread about "how" not "why" so let's keep it there please. If you don't think you need or will use it, then don't do it.
This is for the 96-01 gmt400's only. 95 will have to take their chances with it as I cannot confirm it will work. Probably will, but 95 is a weird animal sometimes. 88-94 are SOL, you'll have to figure it out yourself if you want it.
So, on to the job.
AC compressor driver control
Pull the HVAC head out and go to this part of the PCB:
We will want to cut the circuit trace that ties the selector to the AC enable:
I use and exacto knife and make two cuts close together and then remove the foil from the board. You can meter across the break if you want to be certain it's cut.
That's it. Your AC compressor is now fully under your control through the AC button. It will not run unless you turn it on. The auto mode is disengaged.
No muss, no fuss and no extra switches in the cabin (for example, if you install a switch in the compressor clutch wire to achieve the same effect). If you try it and prefer it the way the OEM built it, then it's just a simple jumper across the break and it's just like factory again.
Recirc door driver control
The same idiots that idiot proofed your AC compressor with defog went and idiot proofed your recirc door with defog/defrost. Select any of the defog/defrost modes and the recirc door opens. Now, I'll admit it's not a bad thing to be designed this way, but I still want control over it. I'm not an idiot and I'm perfectly capable of making the right selection and there are isolated times where defrost and recirc would be useful. For example: it's -30C outside (-10C to -30C is not unusual in Canada, not much humidity in the air at -30C), you run out and start the truck and run back in the house (or remote start if you have it). You have the recirc door closed, the AC off and defrost/defog on so it just keeps reheating the interior air instead of mixing in -30C ambient air. Your cab warms up quicker and the windshield melts sooner. Since you're not in there breathing, there's no humidity jump and the windows don't fog. Win-Win.
My recirc door is almost always in "outside" anyways, but I'll decide Thank-you-very-much momma GM!
This modification is more in the "optional" column, but it can be useful in extreme cold conditions. You decide if you want it or not.
While you have the HVAC apart, install a jumper wire across these two terminals:
And just like that you're done! You can now command the recirc door open and closed regardless of the selector position.
Now, go out and use it wisely.
My thoughts have always been I'll decide what the bloody car is going to do or what it needs based on the present conditions. I don't need some engineer making the decisions for me on something simple like defog.
There was a time when they left it to the operator to turn AC on if needed for defogging and just made a nice simple note in the owners manual. Then some idiot decided everyone else needed to be idiot proofed 100% of the time. Probably because he was the idiot sitting in his car with a steaming hot coffee who didn't turn on his AC one morning and had a recirc door in recirc and wondering why his windows won't clear.....
Truthfully, it's there because of the way new cars are designed: smaller bodies, smaller HVAC components, bigger windshields and tighter weather seals. Today's HVAC just can't keep up with it. So in order to stave off all the "my car is always fogged up" or "the HVAC sucks" warranty service, they defaulted to "on" all the time. Makes up for shortcommings in design and it doesn't cost the OEM more than the cost of designing the HVAC to come on.
So yeah, I know why it's there (dehumidify, etc). This is a thread about "how" not "why" so let's keep it there please. If you don't think you need or will use it, then don't do it.
This is for the 96-01 gmt400's only. 95 will have to take their chances with it as I cannot confirm it will work. Probably will, but 95 is a weird animal sometimes. 88-94 are SOL, you'll have to figure it out yourself if you want it.
So, on to the job.
AC compressor driver control
Pull the HVAC head out and go to this part of the PCB:
You must be registered for see images attach
We will want to cut the circuit trace that ties the selector to the AC enable:
You must be registered for see images attach
I use and exacto knife and make two cuts close together and then remove the foil from the board. You can meter across the break if you want to be certain it's cut.
That's it. Your AC compressor is now fully under your control through the AC button. It will not run unless you turn it on. The auto mode is disengaged.
No muss, no fuss and no extra switches in the cabin (for example, if you install a switch in the compressor clutch wire to achieve the same effect). If you try it and prefer it the way the OEM built it, then it's just a simple jumper across the break and it's just like factory again.
Recirc door driver control
The same idiots that idiot proofed your AC compressor with defog went and idiot proofed your recirc door with defog/defrost. Select any of the defog/defrost modes and the recirc door opens. Now, I'll admit it's not a bad thing to be designed this way, but I still want control over it. I'm not an idiot and I'm perfectly capable of making the right selection and there are isolated times where defrost and recirc would be useful. For example: it's -30C outside (-10C to -30C is not unusual in Canada, not much humidity in the air at -30C), you run out and start the truck and run back in the house (or remote start if you have it). You have the recirc door closed, the AC off and defrost/defog on so it just keeps reheating the interior air instead of mixing in -30C ambient air. Your cab warms up quicker and the windshield melts sooner. Since you're not in there breathing, there's no humidity jump and the windows don't fog. Win-Win.
My recirc door is almost always in "outside" anyways, but I'll decide Thank-you-very-much momma GM!
This modification is more in the "optional" column, but it can be useful in extreme cold conditions. You decide if you want it or not.
While you have the HVAC apart, install a jumper wire across these two terminals:
You must be registered for see images attach
And just like that you're done! You can now command the recirc door open and closed regardless of the selector position.
Now, go out and use it wisely.
Last edited:
