bd0nalds0n

A couple years ago, I had my whole AC system replaced. The car blows super cold air. But after a while, it becomes warmer. When I park the car, I get a big pool of water under the car.

My suspicion is that 1) the freeze switch isn't working or 2) the freeze switch is mis-calibrated so that, for all practical purposes, it isn't stopping ice from forming.

Is there a quick tutorial or checklist I can follow to test to see whether the freeze switch is operational?

Anyone have symptoms similar to what I describe above? Are these symptoms consistent with an evaporator icing up (no more cold air) and then ice melting (big puddle) once the car is shut off?

AO

I have a similar problem, but don't know the culprit. Interested in the suggestions.

Dozman

If ones a/c blows 32 degrees or less, and warms up in awhile while unit is running and you notice a larger than normal pool of condensate on the ground that is draining from the evaporator....Your problem is an under charge of refrigerant.

A/C 101 for cars is you do not want to under charge the refrigerant or the the evap pressure will be less than desired. Thus giving you over time an evaporator that is icing up to the point of becoming a block of ice. If this occurs then eventually the ice on the evap will start to defrost and drain, leaving a larger than expected pool of condensate.

FYI...Not all freeze stats (a system safety) are exact where they cut off at in temp. Some have been seen to cut off as low as 26 degrees.

Some owners say they have air coming from their vents at 32 degrees or below. This is either an under charge of refrigerant or a restriction in the system.

WallyP

The anti-freeze switch is removable (no loss of refrigerant) and testable. Some are adjustable, some aren't.

A possible problem is that the temp sensor is not in proper contact with the evaporator.

dprantl

First test your freeze switch or replace it (it's not that expensive). My car still does this even with a new switch and it's probably because at some point in the car's previous life it had the freeze switch bypassed. I just have to get off my butt and go through the wiring to find out where it is.

Dan
'91 928GT S/C 475hp/460lb.ft

Dozman

Even if replacing/adjusting the freeze stat is not going to correct the problem, if in fact the real culprit is a low charge of refrigerant. One should diagnois the actual problem why the evap is freezing up. A low charge or restriction.

If the system is low on refrigerant, a freeze stat is just goinng to keep the system cycling on/off, on FREEZING TEMPS. The low pressure switch will cycle the unit on low pressure.

j.kenzie@sbcglobal.net

Pull your freeze switch, it's between the air cleaner and the windshield cowl, under the black plastic water diverter. Place the capillary tube in ice water and see if the switch opens (it's a normally closed switch). I think a new one is about $40.

dprantl

I'm not sure I'm understanding your reasoning? If the refrigerant system has enough capacity to remove enough heat energy from the air being passed through the evaporator to cool it to under 32 deg F and cause ice to form on the evaporator fins, why would this mean that the charge is low? The charge is low when the system is unable to drop the temperature of the air flowing through the evaporator enough. Ice cold air is more than enough, and in fact that is why the freeze switch is there, so it doesn't get colder than freezing and form ice which will block air flow and hinder the heat exchaning efficiency of the evaporator.

While this car may be low on refrigerant, the fact that the evaporator is freezing up has nothing to do with that. A properly charged system will have the compressor cycling very frequently as it hits the limit of the freeze switch often.

Dan
'91 928GT S/C 475hp/460lb.ft

j.kenzie@sbcglobal.net

I thought the same thing. Seems like low refrigerant and no refrigerant should act in a similar manner and not cool the evaporator. I could picture a very dirty evaporator with reduced air flow might ice up quite quickly. John D. may be much more expert and I would welcome the explanation since my knowledge is limited.
PS. My 928th post

Dozman

The freeze stat is a SAFETY device and is meant to shut the system down if the air gets colder than 32 degrees so that the evaporator does not freeze up too much that restrict the air flow, and keep from damaging the compressor. The Freeze stat is not a control device.

The low pressure is a SAFETY device. This cycles the system off when the low pressure is too low to operate the system without damaging it.

The temp switch in the cabin is a control device. This device cycles the system on/off due to a set point.

Why have a low pressure switch and a freeze stat? Simple they are safety's to keep the system from being operated outside the design spec to keep the system from damage. They are wired in series.

If an evaporator saturation pressure/temp was colder than 32 degrees ice would form on the evap, making for less air flow, which in turn would make the evaporation pressure lower, which in turn more ice build up in turn make for less air flow, which in turn... well you get the idea. Then when you turn your system off, the ice starts to defrost and makes a larger than normal water pool draining from the evap.

Lets put it a few ways.

First and foremost NO a/c system is designed to chill the evaporator below 32 degrees, this is impractical and could cause a vast amount of warranty work, as well as an inefficient system. This is why freeze stats are installed in systems to ensure the system can be shut down in case the system is slightly low on charge, but not low enough to trip the low pressure switch. An a/c system is meant to cycle on a temp stat not a freeze stat. This would be the design spec of the evaporator saturation pressure, this pressure is involved in the designing of the system.

If one goes to the parts store and buys a cheap all in one fill/and gauge for r134a, it gives you a low side suction pressure (manufacturer's differ on pressure due to many vehicles designed differently) of 30-45 psi, which corresponds to a known direct P/T relationhship (R134A) of 35-49 degrees. If one would hook up the guage upon start-up of the system, and let run for approz. 15 minutes and see the low side pressure below 28psi which is 32 degrees (for R134a) one would tell from the guage the low side system pressure is low on charge or there is a restriction causing the low side pressure. Although one should always check both low and high side pressures.

When the system is working properly, and charged correctly (have seen a complete evacuations (at 50 micron, which is a DRY system) and weigh in the charge, the low side pressure being 30-34 psi on the 928's making the saturation evaporation pressure correlating temp of 35/39 degrees, per a P/T chart.

If the system was slightly low on charge (low enough not to trip the low pressure switch), the evaporation saturation pressure would be less than a correctly charged system. This could make it less than 28 psi, which would be 32 degrees or less. If this happened the evaporator would start to ice up due to a lower than 32 degrees evaporation saturation pressure. (Knowing the 1st 2/3rds of a ton of refrigeration is the dehumidifcation process, the last 1/3rd is cooling the temp). Thus the humidity from inside the car drops on an evap that is colder than 32 degrees would freeze on the evaporator. Thus making for less air flow due to ice building up on the evap fins making for less air flow across the evap in turn making mor humidity drop on the evap again freezing and so on continously till the system shuts off on low pressure or by the freeze stat.

Making it an inefficient system. Also if the freeze stat was not calibrated correctly and or not operating would cause the evaporator to freeze up and possibly cause the compressor to fail.

Dozman

Dan I am interested in hearing your thoughts on why a system would be engineered to operate below evaporator saturation pressure/temp of 32 degrees? I understand cooling cabin air is a good thing. What evaporator saturation pressure/temp should the system be at when operating correctly and efficiently? Not not sure how efficient a system would be if it had to keeps cycling on and off on the safety freeze state (ice build up on the evaporator).

What symptoms would be incurred if a system had a lower than design pressure reading on the low side? What would be symptoms of a higher than design pressure reading on the low side/ What could be the possible causes of each? Dan, I am not trying to upset you what so ever. I am just just trying to understand your logic of how an a/c system works.

Ice cold air at or below 32 degrees is too cold for our cars a/c system.

The evaporator in cars were designed for heat removal, via latent and sensible heat. In other words remove heat via dehumidification (latent heat) and removal of measureable heat (sensible heat)content. If ice was to form on the evaporator area of the evaportaor would decrease it's efficiency.

If the efficiency of any system is decreased for some reason, there is a problem that needs to be corrected.

j.kenzie@sbcglobal.net

Thanks for the explanation, John D.

dprantl

In my opinion the most efficient refrigerant system in a car is one that is constantly bouncing off the freeze switch. Why is that? Because it takes quite a bit of power to drive a compressor and pump refrigerant around in the system. If your freeze switch is operating correctly (which it should), you will never have a problem with icing at the evaporator and the air coming out the vents will always be as cold as it can be. Is that not what the function of the system is?

The answer to your question is that you can't really design a system that always outputs 32 deg F air regardless of conditions; no refrigerant system can do this. You want the system to cool as best as it can in any condition. Why is that? Because if you put a car in 120 deg F heat in the desert, no matter how hard it is run the air temps will never get anywhere near freezing. This means that when ambient temps are i.e. 50 deg F, the system will continually bounce off the freeze switch, but I don't see that as a problem or inefficiency, in fact quite the opposite.

The freeze switch is designed to cut off the compressor at just slightly above 32 deg F. Water can never freeze in a system with a switch like this because the evaporator can never be at or below 32 deg F no matter how much cooling capacity the system has, so there can be no inefficiency due to an iced up evaporator.

It's easy to see this in action. Just recharge the A/C system of a 928 with the correct by-weight amount of refrigerant (or any car, for that matter), then run the A/C for a couple of minutes at i.e. 70 deg F ambient. The freeze switch will cut the compressor out pretty fast, and the system will continue to cycle on the freeze switch, providing that the fan speed is not too high causing too much heat load on the evaporator. My wife's 968 which still has the original R134A charge from the factory bounces off the freeze switch every couple of seconds unless it's over 90 deg F outside. It is the best A/C system I've ever felt in any car.

I think you are thinking of a house A/C system that does not have either a freeze-switch or even an expansion valve. In such a system it is necessary to very carefully determine the refrigerant charge (and also the size of the compressor/condenser coil and evaporator coil), otherwise it will very easily ice up the evaporator and cause the system inefficiency you state.

Dan
'91 928GT S/C 475hp/460lb.ft

bd0nalds0n

I don't know anything about air conditioning, but I was going to point out that our temperature sliders (the "control" feature) doesn't cycle the AC compressor on and off, but rather blends in hot air from the heater. AFAIK, the compressor runs all the time and doesn't cycle if the cabin gets to the desired temperature.

Am I wrong?

Oh, I also got a shoe full of cold water today when I turned a corner. WTF? Drain clogged?

dprantl

Yes, that is correct. The A/C side is always cold when the compressor is running, so the climate control system must have a way to regulate temperature in order to be able to set a desired interior temperature. So, it does this by mixing hot air from the heater core with the cold air from the evaporator with mixer flap that can adjust how much of each to let through. This system has nothing to do with the A/C freeze switch.

If you get water on your feet during turns with the A/C on, the evaporator drain just behind the bell housing and above the torque tube is probably clogged up, and/or the evaporator housing is cracked at one of the corners.

Dan
'91 928GT S/C 475hp/460lb.ft