Evaporator freezing up

[SnowDrift]

On a 3 1/2 hour one-way trip this weekend to go on vacation/bring dad's non-starting Duramax home, I noticed about 2 hours into the trip that the air coming from the vents slowed dramatically. Once we got into town, I got hot enough that I turned off the A/C since it seemed to be barely working anyhow and after a few minutes, I noticed that the air was coming from the vents again, like it should be. I turned the A/C back on and we finished out our trip with it on. We had one short trip in town with the A/C on and had no issues. On the trip home, a similar thing happened where we were about 2 hours into the trip and the air from the vents nearly stopped again. I turned it off and after a few minutes, it started blowing again.

Second, I had noticed some water on the floor mat about 3 hours into the trip home and I traced it to the plastic box behind/below the glove box, which was sweating. My conclusion is that the evaporator is freezing over and not allowing the air to blow past it. It's rare I make such a long trip to where this is a problem, but I'm also aware that it could be a larger problem eventually. I've read the horror stories about 8-15 hours to replace the evaporator, as well.

Am I correct to assume that if it is freezing over, it indicates a blockage inside the evaporator core? If so, then I'm guessing I'll soon be amongst the dash-tipping club to get at it.

 

[robzombie4551]

First to check clogged condenser, second, clogged evap. I listed these first because they're visual checks

Low on freon?, or clogged expansion valve/ will indicate high lowside pressure and low highside pressure. This you will need gauges to check.

 

[Acesneights1]

The cycle switch is most likely a chinese knockoff and bad. I had this problem a few times. I only use GM factory ones. Use the vin from the truck. There are a few different ones. Low freon will not cause it to freeze up. That happens on Home/residential units. ON these the switch will make it shut on and off like mad and cycle to much. Evap could be clogged but most likely the cycle switch. Again, I would ONLY use GM.

 

[WarWagon]

Yes it is possible the issue is a low side switch on the evaporator is stuck on or cutting out below 32 degrees: this will always cause a freeze up. I have had this issue happen with a slightly low freon charge and only show up on long trips. I have also had this happen with a restricted accumulator - due to a shop skipping it's replacement with a new compressor - this is extremely rare even if a shop does skip this. The HVAC module specifically the blower switch can also contribute to this. The blower switch goes bad specifically the compressor switch and will run the AC with the blower motor off. This will ice the evaporator and send liquid to the compressor possibly de-lubing and ruining it. The system can't control it's temps with the blower stopped. Does the AC light flicker or come on with the blower in the off position indicating this failure? If you ran the vehicle and then turned on the AC - you are already starting with a partial ice up of the evaporator.

Testing the switch is easy. Set the blower to low speed, spray water on the accumulator, then rev the engine to 2000 RPM. The clutch should kick out in less than 1 min. If the water turns to ice - you have a bad switch. Otherwise I would guess the system is low, evac and recharge by weight.

The reason a system would freeze up with low freon - this system runs with a flooded evaporator. That is some freon in liquid form exits the evaporator in liquid form and boils in the accumulator. This design makes sure the evaporator is at an even temperature and allows oil to flow through it rather than get trapped in it as gas doesn't move oil very well. So when the system gets low the top of the evaporator is hotter than the lower part of the evaporator. The top part heats the gas over freezing while the bottom becomes blocked with ice. Thus the system pressure is over freezing and will not trip the cycling switch even though the bottom of the evaporator is frozen solid. After a certain amount of ice forms the only way to unfreeze it, even though the switch is now cycling the clutch, is to turn the AC off. The system works on pressure, not temperature, however the heating of the gas keeps the pressure over freezing even though some of the system is below freezing. The temperature is measured and regulated by pressure and the above shows the single flaw in a system that needs attention.

 

[robzombie4551]

^^^^^^X2^^^^^^

Thanks for being more detailed than I was.

 

[SnowDrift]

Thanks for the explanation. I think I'll get this back in to my A/C guy, then.

The evaporator isn't exactly clean on the outside, but it's as clean as I can get it without tearing the dash apart. Condenser is clean on the outside. I was thinking it was a blockage when freezing occurred since someone had told me a long time ago that the dryer on an old C10 diesel we had was frosted due to blockage inside it. It was an R-12 system, though.

 

[robzombie4551]

It will happen with 134A also. Same basic system/different refridgerant, that's all.

 

[THEFERMANATOR]

Low pressure switch or over charged is the 2 most common problems. I have never seen an under charge cause a modern auto A/C to freeze up. Over charge WILL cause one to freeze up with a low pressure cycling switch system. The low pressure switch should open at 18-20 PSI and close at 50-55 for a GMT-400 and R-134A systems.

 

[Turbine Doc]

Ever run any detector/sealant freon through it ??? That stuff gunks up orifice tubes also causing freeze-up

 

[SnowDrift]

Yes, the dye was put in when I replaced all the other parts a few years back. Everything was new, then, except the evaporator inside the cab, including the all lines. Never any sealant, though - I refuse to use canned repair jobs. Too many potential issues down the road.

 

[NVW]

i was told never to use the sealant, it reacts with air, that's how it seals. If you open the system up to repair it, it's screwed.

 

[Acesneights1]

I don't know if the stuff's really any better but I know years ago if a system came into my father's shop with tracer dye, he sent them somewhere else. We were an Authorized Mark IV , Coleman and Doutherm installer and service shop.