Some history....back in the winter (March '10) The AC compressor would not engage in the defrost setting to de-humidify the air to defog the windshield. After diagnosis the AC circuit, the low pressure switch was the cause. Being in a bind, I ran to the auto parts store and got a 4 Seasons (Made in China) brand. Installed it and everything worked fine.

May '10 I was on a road trip from NJ to OH, and I turned the AC on and put it on the MAX setting. MAX re-circulates the inside air. Within 1 hour, there was no air coming out of the vents. Didn't think much of it, thought maybe I had a bad blower resistor or blower motor, no biggie.

Then another problem arose. With the AC on, the AC compressor would randomly cut out. I would touch the harness by the compresser/high pressure switch then the compressor would kick back in. I replaced the high pressure switch connector and high pressure switch.

July '10 I was driving down the NJ Turnpike and same thing, within an hour airflow stopped coming out of the vents. Got home, took out the blower and blower resistor. Everything was fine. After the truck was sitting for an hour in 90* temps, the evaporator was freezing cold......hmmmmm. Since I had the blower out, it cleaned out the evaporator. There was one only pine needle in the airbox.

Started the truck up and the AC compressor is not cycling. So I'm thinking it must be that POS 4 Seasons switch. I replaced the switch with a Motorcraft piece and the AC compressor still does not cycle.

My friend put a set of gauges on it. The outside temp was in the mid 90s.
At idle, the low pressure was at 40
At 1300 rpms, low pressure was 27ish and the high pressure was 250

Everyone I talk to, are stumped over this. The evaporator still freezes and the AC compressor still does not cycle. When the air stops blowing out of the vents, I turn the **** to vent to melt the ice. As soon as I get airflow, I put the **** back to AC.

This happens in the MAX setting. I did not try the regular AC position.

Anyone have any ideas?

Billy T.
gnxtc2@aol.com

 

i have this same problem in the mack trucks at work. there is a thermometer in the evaporator box that cycles the compressor on and off to prevent evaporator freeze up.
when the thermometer probe goes bad, it will not cycle the compressor off, allowing the evaporator to freeze. when the drivers tell me the AC blower fan stops working after a half hour of driving, i change out the probe, and no more freezing.

i am not sure if the fords have the same thing, but is worth looking into.

looking at the service DVD for the 2000 superduty, i se a section for testing the electronic automatic temperature control module.
do you have the auto temp control system, or the basic heater AC system??

 

Sorry, forgot to post truck info

1997 F250HD 7.3L Powerstroke.

Just a manual HVAC controls, nothing electronic. No thermometer probe inside the airbox.

Billy T.
gnxtc2@aol.com

 

Using the "MAX" setting is worst case here. You are continuously recycling air/humidity which does bring the vent temperatures down to or below freezing. I can't say I have ever run any vehicle in "MAX" for extended periods of time.

Nonetheless because your low side pressure is running inside the window of operation for the clutch cycling switch the compressor is going to run continuously. A few things come to mind on your issue. First is the condensate drain plugged for the evaporator case? Second, has there been any recent service to the A/C system such as a refill or other components changed?

Your pressures look to be normal (high side is a tad high) for the conditions you stated so I do not think the system is overcharged, common cause of evaporator icing. A few things come to mind for why the continuous compressor operation: 1. Air in the lines, improper evacuation. 2. Flow restriction in the suction line from the evaporator to the compressor. 3. Poor compressor performance. Meaning the compressor may have a problem.

 

Clear

Never had AC work done to the truck. I'm the original owner

Never added anything to the system

Would a partially clogged orfice tube/screen cause my issue? If so, how does it become clogged if it's a sealed system?????

Billy T.
gnxtc2@aol.com

 

The orifice tube came to mind too but the symptoms do not quite line up with what I have seen before, but there is always a first! It can get clogged with debris from the compressor when a bearing or some other particles are let loose. Some debris is normal, but when it becomes a case of "black death" it is catastrophic.

 

More thoughts.....your pressure readings on the low side actually look high for the conditions you stated. The high side may be on the high because of poor airflow. I assume your vehicle was not moving and combined with the high ambient temperatures it is artificially raising the pressure. The most common cause of evaporator freezing is faulty low pressure/clutch cycling switch, poor evap. airflow or low charge. You have addressed the first two, it's the third I am now leaning towards.

Two way to address a potential low charge. First is to evacuate the system, then recharge by weight. The second would be to add some more refrigerant to see if the low side can now draw down enough to deactivate the clutch cycling switch. My preferred method is the first. Evacuate, then I suggest you pull the orifice tube and inspect it for signs of something bad happening inside the system. If it looks good, install a new OT which then also leads to a perfect opportunity to replace all the o-rings, then recharge by weight. If there are signs of significant build up, you need to replace more parts.

 

Does the compressor cycle when in the normal a/c mode (not on max)? I have been told that max a/c and fan on high will keep the compressor engaged, I assume until the pressure cut-outs kick in.