A/C evaporator freezing up

[nelbur]

My '86 Bronco has an A/C with a fixed orifice tube. The pressure switch is supposed to be set so that the compressor cycles to keep the evaporator from freezing up. There is nothing on this truck to read the evaporator temp., like on earlier ford trucks with an expansion valve and a temp. sensor in the evaporator.

The pressure switch is adjustable with a ***** driver, but the plug has to be pulled to access the adjustment *****. Is there some sort of jumper that would allow one to run the compressor while turning the adjustment to get the correct cycle rate, and what is the right cycle rate. I have adjusted it by trial and error, but as you lose refrigerant over time it needs to be adjusted to keep the cycling rate from being excessive. There ought to be an easy way to adjust the switch with the A/C running. Anyone know how this should be done?

 

[Seabronc]

Generally with A/C an evaporator freezing up can be caused by low freon, dirty fins, and poor air flow across the evaporator fins. I'd get some coil cleaner, READ THE DIRECTIONS, hazardous to your health if you breath the fumes, and clean the coils. Then if it still happens I'd check the freon charge with a real set of gages, NOT the Auto Zone can, that means you may have to take it to someone who has a set. Also, you may have poor air flow for some other reason.

Good luck,

 

[Rons beast]

Ditto what Seabronc said, The main cause of evap freeze up is poor air flow. Check that. If you run with the system in recirculate too long that could make it freeze too.

Do you have a R134 System?

The switch on the Accumulator is to cut off the compressor if the low side preasure gets too low. Not to regulate the temp in the evap.

You really need to see both high and low side preasures, but if you want to try, you can use a jumper wire on the switch plug to activate the compressor. A system running R134 should run about 25 to 40 on the low side The switch should cut out at about 20. You really need gauges for this, as a full system should never run lower than that, with the ambient temperature at at least 75. Caution! If you adjust the switch too low you could ruin your compressor by oil starvation. Freon carries the oil....low freon= low oil. R134 molecules are larger than R12 so low preasures result in less molecules and less oil. That is why the preasures are higher on the low side than with a R12 system.

 

[nelbur]

Thanks for the comments. The system has been converted to 134a a couple years ago. I only filled it to 25 PSI since the R-12 pressure was in that ball park. I figured that the 134a would be less efficient and I doubted it would freeze up the evaporator if the compressor ran full time, so I adjusted the pressure switch to stop the cycling. I was wrong. The air is ice cold, and around town it does not freeze up because the frequent stops let the ice melt. On a trip the evaporator freezes up to the point where the air stops blowing.

So my problem is, how do I adjust the switch to get the correct cycling rate and what should the cycling rate be. I can do it by trial and error, but it seems like there ought to be a better way. I can only adjust the switch if it is unplugged. I thought there might be a jumper type plug that would give me access to the adjustment ***** while the compressor was running.

This fixed orifice system with nothing to measure evaporator temperature, was not one of Ford's better ideas. Back when this truck used R-12, when I would get an A/C shop to fill it up, it would freeze up for awhile until enough leaked out for the cycling to pick up to the proper level. As more leaked out the cycling would become so frequent that the A/C would not cool properly. The old expansion valve system in my E van was a much better system, as the valve was adjusted by the evaporator temperature. Not all "improvements" are improvements.

 

[Rons beast]

Expansion valve systems were more effective. Use a cotter key or jumperwire to jump the plug while you adjust the switch. Have the engine running at 1500rpm. Without guages , trial and error is the only way to do the adjustment. My system was filled to correct level and preasures, and I still sometimes encounter freeze up on the highway...and this is in Florida! Still better than not cold enough.