I was taking a long drive back from New England to DC today, and the evaporator kept icing up every 15-20 minutes while driving at speed on the highway. It was about 85 degrees out for reference.

I'm pretty sure it was the evaporator, because it was giving me all the classic signs: reduced airflow, compressor still running, etc.), and when I shut the A/C off (put it on regular vent), it would blow cool air for several minues while gradually blowing more air as it melted. Good sized puddle under the car when I parked as well. A/C blew nice and cold when restarted, whether driving or idling.

It seems that the compressor is alwas on and not cycling off, even when the fan is on 1 and it's not recurculating. I stopped for gas at one point and let everthing melt. I then let it idle with the A/C on and the fan at 1. I left it like this for about 5 minutes, and not once did the A/C compressor cycle off.

I'm speculating it might be a bad low pressure sensor, as it's the primary control for the compressor cycling. Does this sound like a good place to start? Any other things to check?

 

Replace the low pressure cycling switch on the accumulator. It just screws off and on, piece of cake.

 

Sounds like it's low on Freon to me.

I would check that first before replacing any part or parts.

 

How do I check that?

 

With an A/C machine. You suck the refrigerant out and see how much was in the system. But I know you're not low on charge, it's the low pressure cycling switch causing the problem. I had an XJ at work that did the exact thing yours is doing, it was the LPC switch.

 

Not yet. You don't "suck" the refrigerant out and weigh it. An A/C tech puts gauges on it and checks the high and low side for proper pressure.

If the pressure is where it needs to be, then you can start thinking about replacing parts. Other people's experiences aside, it's not a good idea just to start throwing parts at something hoping to fix it.

Diagnostics before parts.

 

The machine I use tells the user how much refrigerant was removed, more accurate than looking at the pressures to determine if there is a low charge. Too many variables effect the pressure readings. I'm sticking with the LPC switch as being the problem. If the system was low on refrigerant the compressor would cycle rapidly if the LPC switch was good and there would not be a lot of condensation draining from the HVAC case. The OP is probably seeing a lot of ice build up on the accumulator and suction hose as well.

 

On my '87, low charge caused mine to do what his is doing. If some more leaks out past that point, it will short cycle when the amount of Freon is very low. In that case, it doesn't run long enough to ice.

I didn't have to replace the switch on mine to fix it.

 

On my 98 I had it freeze up when the freon was low. I added a little and it stopped freezing up. I would start there as Firestorm mensioned.

 

UPDATE: I finally got around to replacing the low-pressure switch and took it for a long drive on the highway in hot weather. The A/C works great now, and I could tell it was cycling properly at highway speeds. I did have a shop test the pressures in the system while running last summer, and they said it was fine.

My wife's 1st-generation Mazda3 is icing up in the same way, so maybe it's the low-pressure switch there too.

Thanks for the pointers, guys.