air conditioning vent help!!

[ktmracer]

i kno w some of you gurus know the answer so i`m just gonna skip trying to diagnose it myself.
when i first start out on my truck a/c works fine. after a few miles you feel less and less air coming out of the vents. what little air is coming out is good and cold mind you, but the a/c-heater fan sounds like it should be frosting you,but only minimal cold air coming out.
where/what do i need to look at for the problem? this mostly happens when the truck is having to "work for it`s supper." i know a heating engine is less likely to blow cold air but the air that still comes out of the vents is very cold leading me to belive the problem is in a switch or vaccum problem.any help much appreciated.

[JohnC]

Originally posted by ktmracer:
after a few miles you feel less and less air coming out of the vents. Sounds like it's freezing up. Try shutting the compressor off and see if the airflow doesn't return in a few minutes.

[ktmracer]

what are you referring to freezing? the a/c system itself or a switch that switches it from one function to another? thanks for the help.

[JohnC]

Condensation on the evaporator core freezes up and blocks the airflow. I'm no AC expert, but I do get freeze-ups on the hottest, most humid days. I understand low (or was it high?) charge can be one cause.

[ktmracer]

thanks. could one check this by vaccuming the system down and refilling per the manufacturers rec. amount? also, when you get freeze ups,are the symptons you get the same as i described? lots of fan noise but only minimal (cold vent airflow?)forgot to mention if the air is directed say, to the upper body only, when this problem starts, you also feel a lot of air coming out in the floorboard area and it is kinda warm.thanks again.reminds me of a condition on gas powered vehicles that have vaccum ported switching,when under acceleration ,it`s almost like you turned off any place for the cold air to go to and the fan is still screaming at full speed.

[DmaxMaverick]

Sounds like you have a freeze up. When an evaporator freeze up occurs, the system (on later models) will go into a "de-ice" mode, which usually switches the A/C into an on/off cycle, and varies the vent direction. On some models, it will actually flow heater core air through the evaporator to speed up the process, perceived as "hot" air from the vents. A blocked evaporator drain makes this worse, as the moisture that is normally drained from the evaporator box stays to get refrozen. The evaporator box drain is what causes most of the water puddle you'll see under the vehicle while using A/C. If you don't see any, or much water dripping from the passenger side, behind the wheel well, the drain is probably blocked and needs clearing. There were several TSB's for many models regarding this. The dealers remedy was to clear the drain, and in some cases, route it differently.

Doing an A/C service on a reletively new system should not be necessary unless something has broken or starts leaking. I suggest starting with the drain.

Another thing to check for is condensor blockage. It should be pretty obvious if something is blocking the radiator area.

[patrick m.]

check the "max" air door, sometimes called "recirculate" door. many times, something has gotten sucked up to the door, blocking air flow (such as small plastic bags) and as we all know, "no air in, no air out".

[HowieE]

I will agree with the above statements that you most likely are freezing up. Any auto AC system will leak down over time and eventually get to a point were the evaporator is so cold, from lack of refigerant, that it will freeze the condinsate right to the coil. This time of year there is not much load on the system and that will happen.
Run the truck till you lose air flow and then pull off the road. Within 5 mins. look just behind and slightly inside the left front tire. If a puddle of water starts to forms after this period of time your coil was frozen and the system needs a little gas to top it off.
If left unattended and the system leaks down somemore the compressor will cycle for a short period each time you start the truck and then shut down against the low pressure switch on the accumulator.
Don't bother to evacuate the system as ref. is expensive. However check along the lines and around the compressor for any signs of oil. Refridgant leaks will often push out some oil and mark the spot of the leak. A common place for a leak is the bolt that holds the 2 lines to the top of the compressor. There are 2 O rings seals under that bolt that often leak. Another common place for a leak is the front seal on the compressor just behind the clutch.

[ktmracer]

thanks to all.i`ve already tried everything except for ADDING refrigerant.

[jimcreek]

hay there I've seen alot of this type of problems an you are really playing the guessing game if you don't know what your high side pressure is and low side pressure is and vent air temperture is and outside air temperture is.
all the responses are posibalities one quick way of elimanating a problem is putting a pocket thermometer in the vent perferble a digital one to see if vent air temperture gets below 35*F if it does your cycling switch that is mounted on accumalator tank (aluminum tank close to firewall that has suction hose attached to one side evaporater to the other side) is stuck in the closed position and can be replaced by unscewing old one off putting new one on.
NOTE:switch port has a schrader valve and should not release refrigerant keep eyes away just incase shrader valve does not shut.
If you have high head pressure you have other problems that could be anything from restricted air flow across condesor(in font of radiator) to much refrigerant, air or contamination in system, restricted orfice tube

[Polar Diesel]

I am also in the refrigeration buisness, and refer man has the best answer. Oh and i did some stuff with my signature so we'll see if it works.