Paul Standaert

I went on my first road trip with my Jeep and determined that my a/c evaporator is freezing up. Loss of airflow, weak performance, and it drips for hours later. All are indicators of ice buildup.

I replaced the heater core and went through everything behind the dash during the build. I do not recall there being a thermostat in there to cycle the a/c off but that doesn't mean there isn't one. I used a scale to add the exact amount of refrigerant that the sticker calls for.

Airflow is plentiful prior to freeze-up.

Any tips?

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Saudade

How long ago did you change the core? Did you evacuate the system, then recharge using a set of manifold gauges? If so, what were the pressures then? Have you checked the pressures now?

Sirsyc0

I had this problem with my 99

Paul Standaert

I will need to wait until I get home to check pressures. I do not remember what they were, but they were where I would expect them to be with the charge given and ambient temperature that day. I checked superheat of the suction line and... I don't remember. I am assuming it was around 10 with the fan on high because that's typically where they want it on other refrigeration systems and I didn't freak out or question it. Also keep in mind that the temp readings were from an IR gun, so it's less than ideal.

I pulled a vacuum for over a half hour. I did not bother to use something that reads microns of vacuum because... Well, I am lazy I guess.

I only replaced the heater core and left the evap core in place after a thorough cleaning of the fins... And a cleaning of everything else in this 22 year old grime bucket on wheels.

When the a/c works, it is COLD. But like I said, once it ices up, the airflow goes in the toilet and it becomes wimpy.

I will also need to verify that the low pressure cutout switch is working. I honestly don't recall it cycling. I was hoping for there to be a temperature sensor in the air box that I forgot to plug in, but there is no such item.

tech

My guess would be its the low pressure switch or maybe the a/c compressor relay . because it got cold enough to freeze the evap. the pressure should of been low enough so the switch would have cut off the compressor.

Sirsyc0

It may cycle, but not as it should if it’s freezing up with extended use.

IJM

I had the same problem on my '98, and replacing the low-pressure switch fixed it. The switch is very easy to replace.

Paul Standaert

Well, I am not sure what to say. I have to provide some kind of follow-up. Keep in mind that most of my refrigeration equipment is at work and I am on vacation.

I must not have checked pressures with higher engine RPM when I originally charged it. The gauges said, at 3500 engine RPM and about 75 degree ambient temperatures, about 350 PSI on the liquid line and about 35 PSI on the vapor line. The radiator fans are running great. The condenser is not clogged up with stuff on the outside. The vapor line would never get close to the ~18 PSI cutout, give or take since the switch and gauges aren't in the exact same spot. Experience suggests that an over-charge of an A/C unit does not result in evaporator freeze-up, as the saturation temperature is usually increased, but with these kinds of pressures, the clutch would never cycle off.

I reclaimed about four ounces. I have no scale to verify that amount.

The vapor line under the hood is now not nearly as cold, and nor is the air out of the vents inside. The compressor is quieter now. I cannot get over 275 PSI on the liquid line and the suction line goes as low as about 20 PSI, and will now trip the low pressure cutoff.

Maybe I have a restriction of some kind in the system or there is too much oil taking up room where refrigerant should be.

I still have some messing around to do.

agreen

So what are your at idle pressures?

4.3L XJ

If it is freezing up, it is undercharged. If it worked before, the question now is where is the leak?

Paul Standaert

I do not like leaving posts hanging, so I am following up now. I just got back from Colorado and ran the air conditioner through all the heat and humidity of Nebraska a couple days ago and I did not experience freeze-ups, at least not to the point where I lost significant airflow through the evaporator.

On a typical refrigeration system, a low system charge will result in the saturation temperature going below freezing at the evaporator and any moisture in the air will turn to ice and then you get the freeze-up. However, in all of my years, I have never had an automotive air conditioner freeze its evaporator due to a low charge. I attribute this to the low pressure cut-off switch cycling the compressor on and off which keeps the evaporator warmer. However, if the low pressure cycling switch does not function, it will never cycle off and a freeze-up is imminent as long as there's enough refrigerant to flood the evaporator.

What I do not understand is that I weighed-in the refrigerant according to the sticker under the hood. I think it called for 16 ounces? I pulled a vacuum on the system for over half an hour with a real actual vacuum pump hooked to the gauge manifold and then purged the line from the refrigerant tank and let it go in as a liquid until the tank weighed 16 ounces less. I believe that I charged it with the engine not running.

With that refrigerant amount, the compressor never cycled on and off. The clutch is a bit worn and makes the screeching sound (probably should take out a washer shim in the clutch), but I would never hear that more than once with the 16 ounces in there, but I do with the lesser amount that I have in there now, suggesting that it is cycling on and off now. At about 3,500 RPM, with the full 16 ounces of refrigerant, the liquid side pressure went over 350+ PSI. The vapor side would never drop below about 30 PSI, which the pressure/temperature chart for 134a puts it at a freezing temperature, and since it will never cycle off at those pressures, a freeze-up is imminent. Less refrigerant means less pressure there, thus the saturation temperature getting colder, but since it cycles and/or doesn't flood the evaporator, it doesn't ice over, at least not completely.

I suspect that there's too much oil in the system or there are aftermarket parts that are reducing the capacity of this A/C system, or maybe even some kind of unwanted restriction somewhere. However, it is still plenty cold and I drove across Nebraska in 88 degree temps with some significant humidity and sun and never had to raise the fan speed over #2 and actually had to mix in some hot air to keep from being too cold. I am calling it good. I also do not hear any compressor noise anymore.

I also swapped the low pressure switch from my '98 and they both cycled on and off at about the same gauge reading. That A/C never froze up, and I have to charge it once every year or two because it does leak. That a/c system, like every other automotive a/c system I've worked with, just looses its chooch as the pressures get low. I never lost airflow with it and I had that thing for over 10 years now.

To answer the question of what kind of pressures I have now.... I don't remember. The low side gets into the teens now. I remember that much.

350+ PSI is a lot of fricken pressure for that compressor to create. I think it was compressing a bit of liquid and I had no superheat.