This is more of an annoyance. Turn on the AC and it blows cold air just fine after the compressor kicks in and builds up pressure. On typical hot days around here over 15-20min the airflow will slowly die down, even on high. If I turn off the ac the air flow will slowly come back, and it will stay cold for a while. To me its acting like the evaporator coil is freezing. Anything I should do/be worried about. From what I have read, this does not appear to be the blend door issue. Defrost, heat mix all work properly as long as the evap coil is "thawed".

 

I just had this problem with my interstate beater I drive back and forth work. It was the Thermo Control Module. No it wasn't on my mega but it might start you in the right direction. Total out the door was like $125.00 Hope that helps.

 

If it is low on refrigerant it will freeze up and stop blowing cold until it gets turned off and thaws. If it's doing that then you likely have a leak somewhere....

 

Thats what I thought might be the issue.

 

Have the system recharged and you will be fine. This issue is more common on the 03-04 trucks.

 

just had my evapator coil replaced (leaking refrigerant freezing up) $800.00 +

 

yeah, I fought the freezing up thing on my then new '03. Had my local dealer add freon expecting to pay for it but since it was a know common issue there was no charge. That was in '04 or '05 and no issues with freezing up sinse.

 

The refridgerant is 134A correct? I have a can of that left over from recharging another vehicle I had. I am pretty sure this has been acting up for a while. It did it last year too, but it wasn't this hot last year and I didn't have to use it as much.

 

is there a way to test the "thermo control module"? also the location? mine will initially blow cold then about 3 or 4 minutes it blows warm then starts to cool down. had the system completely evacuated and recharged and it still does it. it's not freezing up and the blend door seems to be operating okay.

Mike

 

As far I know there is no way to test it. We came up with through process of elimination. Freon(sp) was at right levels and everything else was functioning properly, but would stop blowing cold after about 20 min, blow warm then start blowing cool. Changed the Thermo Control Module and once again ice cold A/C. Like I said wasn't on my Mega Cab, but I have heard of many other brands of vehicles doing this.

 

My charge is fine, but it still exhibits those same symptoms

 

Where is the Thermo Control Module located?

 

I just read about a mod that may help also. You may have to search for it, but it is simply a ball valve placed on the heater core supply and return lines in the engine compartment. Apparently our 3rd gens circulate hot water through the heater core even when the heat is not on. By shutting off the supply and return lines you can eliminate the source of heat in the heater core, which helps your A/C cool better. I plan to do the mod on mine tomorrow.

Parts required:

2 - 1/2" brass ball valves
4 - 5/8 x 1/2 barb fittings (heater hoses are 5/8" id)
2 - hose clamps

A couple times over the summer months you should open the valves to allow coolant to circulate through the heater core and lines to keep it fresh and guard against any corrosion. Trucks with the 6.7L will also require a bypass so you don't shut off coolant to the turbo.

Just found the link: https://www.dieseltruckresource.com/...d-t209533.html

 

The thermo control module is located in the evaporator. Just to reiterate this issue was NOT on my Mega Cab, However should still point in the right direction.

 

Freezing up is a common occurrence on a system that is overcharged, not low.

Low refrigerant usually shows as inadequate cooling and long compressor run times.