a/c evaporator icing?

[5bucks]

1999 XJ, Air conditioning works great for about 20 minutes then slowly gets warmer. If I turn the air off for 10 minutes or so I can turn it back on and everything is great again for 20 minutes. As the vent air gets warmer, the volume of air through vents decreases. To me, this seems like classic sign that the evaporator is icing up.

In my research I've come across 2 potential problems. Either low refrigerant or a switch that cycles the compressor on/off to keep ice from forming on the evaporator.

Anyone aware of a common problem and/or way to test the compressor cycling?

 

[riverfever]

How many times does the compressor come on and cycle in a one minute period. IIRC...RichP had looked it up in the FSM and if it happens about 5 times in that one minute then it's likely low on freon. I just did my freon last week. NAPA has a kit on sale for 35 bucks. It comes with the gauge and gets connected to the low side line. Easy enough to do.

Welcome to by the way.

-river

 

[Fred85]

lol, a late model xj evaporator icing up??

more like not sealing up

 

[JJacobs]

yup, icing up. happens a lot on the stuff I work on. Too much/ too little freon, cycling switch, bad dryer are among the causes. If it cycles and does it frequently, as mentioned above, it's probably just low.

 

[creeperjeep]

From what the refrigerant techs tell me; if you over charge it the (something) will freeze up and eventually burn your compressor

 

[Ivan]

Do a search on testing / replacing the low pressure switch. It seems that 98% of the icing problems for late model XJ's stem from this switch. It happened to me, and that cure it.....

 

[5bucks]

Last night I ran the A/C on full for about 20 minutes. Never saw the compressor cycle it - was always engaged. Is this normal? Thought it had to cycle in order to prevent the evaporator from icing up. Think my problem is evaporator icing. Like I said earlier, the A/C blows very cold air for 20 minutes or so then gradually warms up. Turn a/c off for 10 minutes and it will blow cold air again. My understanding is the low pressure switch will cuase the compressor to disengage when it senses a presure that would cause a freezing condition at the evaporator. So I'm thinking the low pressure switch is bad. I searched "low pressure switch" on the forum and did not find any good test procedures. Also, if the low pressure switch needs replacement does this require opening the system/discharging it?

 

[Ivan]

Last night I ran the A/C on full for about 20 minutes. Never saw the compressor cycle it - was always engaged. Is this normal? Thought it had to cycle in order to prevent the evaporator from icing up. Think my problem is evaporator icing. Like I said earlier, the A/C blows very cold air for 20 minutes or so then gradually warms up. Turn a/c off for 10 minutes and it will blow cold air again. My understanding is the low pressure switch will cuase the compressor to disengage when it senses a presure that would cause a freezing condition at the evaporator. So I'm thinking the low pressure switch is bad. I searched "low pressure switch" on the forum and did not find any good test procedures. Also, if the low pressure switch needs replacement does this require opening the system/discharging it?

Here's some info I save from another post I stumbled across a while back regarding the replacement of the low pressure switch... I forget the author:

"It screws into the top of the accumulator. Takes about two minutes to replace. Unplug, unscrew, replace.

Standing next to the truck, hood open, with your knees at the passenger side front tire, to accumulator is to your left, near the firewall but not against it.

It looks like a black or gray fat sausage standing up straight with some hard lines going to it and a little relay-looking thing screwed into the top with some wires coming off it. That is the switch."