My A/c works good when I turn it on, But after some time (about 45 min) it will start blowing less and less. (its still cold) I looked under the hood after a long drive and the whole system is a chunk of ice. Its amazing because the under hood temp is terrible hot. Now when it starts to blow slow, I turn it off (the A/C) for ten minutes and it works again. I assume it is frozen in the duct also. Does anyone else have this problem? My A/C system is almost all new. Its a 90' Bronco w/351 and is converted to the 134 freon.

 

Air flow through evaporator is restricted or system is undercharged -- probably the latter. Brian

 

I have always thought that if there is ice on the outside of the condensor or the lines, that there is a leak.

 

I agree with n9lhm. Years ago an A/C specialist suggested 1.) never run A/C with lowest fan speed. It builds higher head pressure and is harder on the compressor. 2.) set it on Max to recirculate and dry the air more, unless you are in the desert, and the evaporator coil will not get dirty as fast. Undercharged systems will freeze your fanny until they freeze up.

 

The max setting will recirculate the inside air, reg. draws in air from the outside and runs the compressor more.
I shouldnt matter what fan speed it's on if it's properly charged.

 

I'd put gauges on it, hold the engine speed at about 1500 rpm (2-4 minutes), and monitor overall gauge pressures and in particular the low side pressure at the point in time the system cuts out and back on. In general, it should cut out at 32-35 PSI; the reason for this is because temperature and pressure are directly related. At the pressures I just noted, the temperature is approximately the same (and in particular, they're at the freezing point for water), thus if there is no low pressure cutout the evaporator will ice over and block airflow. However, if the low pressure cutout switch is operating correctly, any icing over of the evaporator results in a low pressure which in turn results in the compressor shutting off until the pressure rises to roughly 50 psi. The process repeats, more so on cool days, less so on hot ones, and additionally may be very rapid on a undercharged system. This is why modern systems don't (and should not) ice over on low fan settings, even in humid conditions. My first suspicion in your case here, assuming that you have factory A/C, is that the low pressure cutout is way out of adjustment, bypassed, or the switch has failed.

*Iced over automotive A/C is a bad situation since as the evaporator icing progresses, the low side pressure continues to drop. It is possible for the pressure to drop below 0 psi (I've seen it firsthand), at which time there is now the concern of drawing air into the system.

 

Well, if that happens then the system should be drawing a vaccum if it manages to suck in air, then you have a leak anyhow. And hence the problem of low freon.

 

I will have it checked. Those guages are one of the few tools I dont own. By the way, the freeze up accurs when the fan is on high. Everything but the front (A/C) rad is new in this system. If it draws a vacume, couldnt it draw air through the shader valves without there being a leak?

 

Maybe, but remember there is an o-ring on the caps for the gauge access ports and the schrader valves are spring loaded. Also, do realize that the system has a low pressure cutout that is there to ensure that the condition I noted never occurs (unless there is a problem with the low pressure cutout). From what I've seen on automotive A/C over about 30 years or working on them is that it takes a lot of miles of driving with an iced up evaporator to get them down to 0 psi and on a system using an orifice tube the low pressure cutout will not allow this to happen (correct operation assumed). Ancient automotive A/C which uses an expansion valve as opposed to an orifice tube and low pressure cutout are WAY more vulnerable to being able to being operated in a manner that can pull a vacuum.

Interesting and a good point that you brought up schrader valves as if you have a suspected leak the schrader valves are sometimes overlooked. I've had a few cases where I have found them in good order but just not screwed in tight enough and thus leaking. Leaks can often be found on the schrader valves (ports) and elsewhere by using some diluted dish soap and applying it to joints. bubbles = leak. Traces of oil are also a tipoff to leaks. I hope this helps.

*Oh, and since this issue of *possibly* drawing air into the system came up, it's worth noting that air in A/C can frequently be detected by a wandering high pressure side reading as well as an unusually high reading on the high side. Air, unlike freon, does not condense. But back to the situation you're facing, I think your best bet for now is to put gauges on it and ensure that the charge makes sense as well as verify operation of your low pressure cutout.

 

If ice is covering the whole compressor, there is a possibility that the system is overcharged. When the system is overcharged, you dont get the correct pressure drop at your metering device (before the evaporator) allowing your refrigerant to boil (evaporate) and remove heat. When this happens, the suction valve on the compressor becomes the metering device and and the compressor itself becomes the evaporator and it will be covered in ice. If it's your suction line and evaporator that are iced up, it is probably an airflow problem across the evaporator coil or you are undercharged as n9lhm was saying...

 

I think its just the suction line and evaporator as you said. I will have it checked next week.

Thankyou all.

 

I had the system checked and they replaced the front A/C rad. (the only thing that wasnt new) The guy said that cold air was leaking out of the box at the firewall also, so he sealed that. It seems to work good now. I take another trip next week, so I will find out if it ices up.