Disable AC in Defrost Mode

Well, minor mystery....there are no wires hooked up to anything that resemble what is described in that link in my Tundra.

Nothing on the console, nothing on the sub assembly with the gears and cables.

Also, there is a mystery third cable, (mechanical cable) not connected to either the temp knob or the mod nob. Just hanging out in there, not attached to anything. You can see the end of it sticking out under the white temp selector gear.

 

Okay, I found the Defrost switch. Its hiding underneath the control assembly. Found it, killed it. Can confirm that it kills the automatic AC-defrost function....but the ability to manually turn on AC in all modes remains. AC switch lights up appropriately.

Discovered the third cable is supposed to attach to arm on the heat control gear, an arm that has broken off my truck. Oh well, It doesn't seem important as the heater control still works.

 

Pics by chance?

 

I did not happen to take more pictures. The switch is on the backside (bottom side) of the black gear assembly on the right of my previous picture.

You kind of need to preform some mild yoga to see it without detaching the assembly from the dash. The switch simply has two, single wire connectors on it, pull of either one and its disabled.

Here is the diagram from the FSM:

 

Fantastic!

Thanks @Aerindel

 

I vote for a sticky. @Pinay

 

You want this thread in the 1st Gen info dump thread?

 

Fogged interior glass is moisture that has condensated and defrosting releases this humidity back into the air, your AC is your dehumidifier. You can't get rid of the moisture without the AC running.

 

I've spent some time cyferin' on it, and I can't for the life of me figure out what that cable would control. Clearly it's something that changes when you change the temp setting. It can't be fresh/recirculate as that's controlled electrically by a switch. Hmm. Any thoughts?

 

Huh....what did people do back before they had AC?

Anyway...the moisture holding capacity of air goes up with temperature. Warm air can hold more water vapor without condensing than cold air. Bring in cold exterior air and warm it up and its relative humidity plummets.There is the same amount of water in it, but now it can hold a lot more so the effect is that it becomes drier. Blow that warm, dry air over the moisture condensed on the inside of your windshield and it absorbs some of that water....and then it gets blown out the back of your cab through the vents, taking that water with it...which is why you don't use recirculate in the winter.

Works pretty good actually.

AC is just a booster, it takes that warmed exterior air and runs it over ice cold vanes to condense more moisture out of it and make it even drier before it hits your windshield. But really, you don't need it unless your windshield is very cold, and your cab is very wet. Handy option to be sure but you don't need it all the time. I haven't had any fog issues since disabling the automatic AC yet. My old truck didn't even have AC but warm air was always enough to clear the glass.

Well, one cable controls the hot water valve to your core, the other is called 'mix' which I don't understand why thats a separate control.

I have not been able to figure which is which yet, you can see the water valve under your hood on the firewall, If I had two people I could just turn the knob and see if that valve moved or not and then I would know which is which.

My only guess is that for some reason the temp control moves the water valve and an air flap at the same time to regulate temp, although why I can't tell you as I haven't dug deep enough into the climate control to see what flaps do what, and even with one of mine broken I get hot and cold air appropriately.

As I haven't been able to find just that gear for sale yet, and even on ebay the whole assembly is $150, it hasn't been a priority since I've been driving the truck for a year without even knowing anything was wrong.

 

Run the AC while in defrost and gradually increase the temperature as you like, not only does this defrost the windows it warms and dehumidifies the cab.

 

That's true, but for people like me here in west TX where the avg humidity is 25% and the last precipitation we got was before Halloween, this mod makes a lot of sense.

 

We covered this earlier. This mod makes sense for anyone who wants to be able to manually control when the AC compressor turns on, but admittedly, it would get more use in dry and/or cold climates. I fully understand the role of AC in the defrosting process, but it's not needed at all times. For instance right now, it's winter in the North East and there's very little moisture in the air. Any moisture inside the cabin is coming from the breath of the occupants. If it's just me in the truck, I don't need the AC to keep the windows clear -- I just need outside air warmed by the heater core blowing on the windshield. Even in warmer months when it's say 70 - 75 degrees, I might only need air to be blowing over the windshield to keep it clear. It's great that the AC is there in case relative humidity is high or there's some heavy breathing going on in the cabin... but I'd prefer the option to control when it operates.

So that covers defrosting. Then there's simple climate control/comfort. 90% of the time I'm driving, I don't want the climate control air blowing directly on me through the vents. I prefer that it be blowing a mixure on the windshield and the floor. To me that's more comfortable, and keeps the cabin at a more consistant temperature... but others may prefer a different mode. That's fine, and neither is right or wrong.

The key takeaway is this: The mod doesn't disable the use of AC when in defrost mode, it simply allows manual control over whether AC runs in defrost mode.

 

I like the added power / responsiveness I get from the engine when the fan is turned off, but I have to turn it on from time to time because cracking a window open is not always an option. It all depends on the atmospheric conditions at the time, and here in Georgia that changes hourly. Good luck with your mod.

 

Really?

 

Cool stuff! Curious about this mystery cable. Suspense is getting me.

 

Really!

Power drain from accessories is real, and you can feel the compressor stealing power when it's turned on.

 

Frozen Tundra here, so hurry back with pic

 

Maybe, but today is steering rack bushing install day at the Hand Model Compound. I like my OEM defrost mode just the way it was designed so there is no motivation other than the intrigue!

 

There are two for temperature, the air mix cable and the water valve cable.

 

The AC compressor running in defrost mode does two things. 1. helps remove moisture from the car to defrost the windows quicker on damp days. 2. It makes the compressor run during the winter to circulate the oil in the AC system and to exercise the AC compressor.

 

OK. Now that you mention it, that makes sense. With the temp turned all the way to cold, one cable shuts off the flow of coolant, the other cable closes off air flow from the heater core.
Is that correct?

 

Maybe a picture will help? See yellow arrow. Picture taken from brake pedal area looking towards pass side.

 

Fundamentally.

The heater box allows air to go through the heater core, or around it, with an air diverter flap controlling this.

The coolant valve shuts water off to the heater core, as well letting water in and regulating how much. Again, by temperature selection.

If there was only a coolant valve, turning the heat off would be slow, taking several minutes to cool down the heater core. If there was only an air diverter the hot heater core would create a good bit of heat under the dashboard. Together, you get a most effective climate control.

 

Back to the a/c defrost mode; you could still manually turn on the a/c if needed to aid in defrosting, alternatively, a switch could be added to the dash in place of the unplugged one in the selector.
I find the factory setup is working well for me, but I live in a wet climate and get in and out of my truck a lot in wet gear, on the other hand, I like to just leave my fan on low all the time in the defrost/foot well position and don’t need it always running the a/c, I’m tempted to add a switch for that.

 

There already is a switch on the dash to manually turn on the A/C -- it's conveniently labeled "A/C". As @Aerindel pointed out, with the mod done, the A/C button allows control of the compressor while in the defrost or drefrost/foot modes.
I also prefer to have the mode set to defrost/foot. With this mod, I will now have control over whether or not the A/C compressor comes on while in this mode. No other functionality is lost.

 

Well yeah, I mentioned that too. Does anyone know, it seems like the a/c on defrost is only sort of on, like not full blast a/c, could it be partially on like that?

 

The compressor is either all the way on or off. But how often it cycles is based off the pressure from the condenser side of the system and so obviously in the winter it doesn't have to work very hard so its run cycles are a lot shorter.

 

Cool, so yeah, disabling the switch in the selector and just manually turning the a/c on with the button is the perfect solution.
Thanks for figuring this out