Hounddog

Thanks guys.At the end of last summer it started doing this and was told "slighly low"and recharged.Neighbor was told same thing same day.He made several trips to Texas etc. and said is was the same as before.He did say he could run his ac temp up from the coldest setting a few clicks and it would not do this.I found this to be true on mine also.It most definitely is freezing up.Everthing stops blowing from dash vents,leave controls the same but punch ac off and in 10 to 15 minutes slowly real cold air starts coming from vents.Then it gets warm and full blow from vents occur.Hit ac button and cycle starts over again.Stopped in a rest area and rec.drier and lines were heavy thick in ice.No water dripping but from them,then it starts pouring from drain hole and cools again etc.

CRXsi

By setting the temp up just slightly from cold you are allowing the air temp in the evaporator to be just warm enough to not let ice plug up the system so I would say this confirms there is a problem with your ac fin temp sensor.

MikeyB

Just because the tech says the freon pressure is in spec doesn't mean it's not the problem. Even on the low side of the range it can still freeze up. Always put the pressure near or at max for the given ambient air temp. Last year my wife's Isuzu Rodeo A/C evaporator froze up, and I check the pressure and it was in range (near the low point). Added about 1/2 lb of R134A to the system and no more freezing.

MikeyB

CRXsi

Sorry, but I am still having a hard time understanding how low amounts of R134a can cause freeze up of the evaporator. It can definitly cause poor ac performance but have never experienced an evaporator freeze up from low charge. I do agree that I have encountered where pressures are close to normal, but after recovering and evacuating the system have discovered the system is low on refrigerant based on how many lbs were recovered, again causing poor ac performance in general (not enough cooling) but haven't seen an evaporator freeze up as a result. Likewise if I were to recover and see more recovered than spec I would assume Evaporator freeze up to be the result. This is why I usually give pressures a quick check on customer cars now and usually still recover to make sure how much was in the system. There can be a big difference in performance between 26oz, 24oz and 28oz (26oz being what our trucks take). Don't get me wrong, pressures are still a very important diagnosis tool but this is just what I use.

MikeyB

CRXsi,
Low pressure freon entering the evaporator will drop below the freezing point that much quicker because of the lower density. Higher pressure will keep the freon in the evaporator above 32F. With Humidity present and the freon temp at 32F or below the evaporator will freeze.
Hope this makes sense.

MikeyB

tcr

CRXsi, understand that all refrigerent temps are directly related to the refrigerent pressures, when the pressure goes down so does the temp of the refrigerent. when an a/c system is properly charged, the coil is just above freezing, the air to be cooled keeps the coil warm enough to not freeze. When a refrigerent (R-22 for example)boils off @ the metering device of an evap coil it will drop from a head pressure of roughly 180-200 psi to a suction pressure of around 60-75 psi. At 60 psi R-22 is at +/- 32 degrees if you were to barely drop the level of charge that temp of 32 deg is going to drop as well causing the coil to freeze. This is only true to a certain extent, after you loose too much refrigerent you will loose all cooling capabilities all together.

CRXsi

Ah Yes, Thank you both for explaining it as now I remember and see what you are saying....I was thinking more from the stand point that if it was really low, not just slightly.

Hounddog

Real informitive threads!

CRXsi

And educational, I learned things I forgot I had forgotten!! Again thanks everyone for helping me to realize my errors

tcr

I am cursed w/ being in the HVAC biz, I'm glad if I am able to help.

CRXsi

That's Ok, I am cursed with being in the Automotive fixin business...how can you tell AC is not where my specialty is, I have to do it on occasion but electronics and electrical diagnosis is where I usually spend my time along with driveability issues. Either why it may be a curse for both of us but there will always be work. Again thanks for the explanation.

Lowco

Thanks for all the info. from everybody.
I took my truck in to the dealer, and he was talking about some part in the coil area needing to be changed and ALSO relocated to a different position. I dont remember what he called it, but it sounds like it is probably this AC fin Probe CRX found in the manual. I didnt let them do it at the time because he said he had to remove the WHOLE dash assymbly! I DONT have any squeaks or rattles yet and DONT want any, but my truck is coming up on 36K and I dont want this problem anymore!

Hounddog

The WHOLE dash assy is a piece iof cake to remove.No biggy.

CRXsi

Yeah, that's probably what they are refering to. Most cars nowadays the dash needs to be removed to access the heater core/ evaporator assy. It may be a pain but if you are still in warranty then it will be much cheaper then having it done after you are out of warranty.

tripper=t57

On my 2003 3500, the AC would freeze up when I ran it on low fan. It took several trips when it was new to figure out what was taking place, same as above ,freezing then when it was turned off or heat turned on, slowly return to blowing air. Also, the passenger side always blew warmer air by about 10-15 degrees than the drivers side. The dealer first said it was OK, just run the fan on high, "nobody runs AC on low fan", that didn't get rid of me, so they checked out the system, said they added refrig to it and it seemed to quit freezing up. The passenger side still blew warmer than the driver, they said that is normal, I guess normal for late model DC. My '95 2500 would freeze you out on low fan on both sides of the truck.