A/C Problem - frozen evaporator?

The A/C blows nice and cold, checked the pressures and they seem good. Low side between 45-55 psi, high side between 225-250 psi. I changed out the orifice tube and accumulator.

On two recent trips, the A/C will cut out for a period of time, usually a few minutes. The vent temperatures rise enough to be uncomfortable. The air is still blowing out of the vents.

To me it sounds like maybe a frozen evaporator. I changed my cycling switch with a new Motorcraft and still get the problem. Seems to only happen on the highway.

Any ideas? Thanks all! Its my 2000 Sport w/ OHV engine.

What was the outside temp when you took those readings?

I usually charge a system with all windows down and A\C on (Max A\C) and target for about 35 PSI low side.

You could be slightly over charged which is not letting the Freon "Boil Off" in the evaporator.

This is in 85-90 F ambient temps.

I did a test today. Ran the A/C on max, fan setting 1 (low). Sprayed water on the condenser to simulate high airflow. I noticed that the low side pressure had dropped to about 20 psi and the compressor was still running. I could see ice forming on the accumulator.

I assume that the low pressure cycling switch should never let the pressure drop this low, correct?

EDIT: I thought that slightly overcharged would cause low side pressure to be too high, and thus reduce cooling?

About 27 psi is freezing, so your compressor is not shutting off as you suspect and freezing water on the evaporator. With a new pressure switch, I would not be sure why. Maybe a a stuck relay???

Checking the relay sounds like a good idea but I only see a relay for WOT cutout.

Standard Motor Products RY116 Wide Open Throttle Cut-Out Relay

Is there another one that actually runs the compressor?

High pressures are good range...

With a 90 degree ambient the high side should be 207-243 F...If the evaporator is freezing up, is the drain clogged up and the airflow to the evaporator blocked?? I don't usually use the low side pressures but 45-55 psi if the compressor is running seem awfully high...

What vent temps are you seeing before the temps change? I am betting you are seeing 45-50 F...When you vacuumed the system down did it hold a proper vacuum for an hour??? I really hope you do not have air or moisture in the system freezing up inside the evaporator instead of the condensation freezing on the evaporator...Normally when the condensation freezes the airflow is greatly reduced or stops until the ice melts off the evaporator...

I think the low side pressure is a bit high at idle because there's not much airflow through the condenser w/ a mechanical fan. I checked the drain and it looks OK, no debris.

I haven't tested it yet but I found that the schrader valve connected to the cycling switch was damaged. I removed it, vacuumed the system, and recharged it. Kind of annoying since it came with the new accumulator. The system seems to be cycling as intended now. Will try it out tomorrow on the road.

System appears to be working normally with the schrader valve core removed at the cycling switch. Frustrating that a new accumulator out of the box had a bad valve core.

Just for reference I found the pressure charts for the A/C system from the Ford FSM using the following setup procedure.

1. Drive the vehicle or run the engine until it reaches normal operating temperature.
2. Connect a manifold gauge set or refrigerant service center with high-pressure and low-pressure gauges to the refrigerant system.
3. Set the climate controls.
4. If equipped with manual climate control, set the A/C controls for normal A/C-PANEL mode, full COOL temperature, FRESH air, HI Blower. If the vehicle has a fresh air/recirc button, set it to FRESH. If the vehicle has an A/C switch or compressor on switch, set it to A/C ON.
5. If equipped with ATC, set temperature to 60° F (15° C) (lowest possible temp setting) with the dual function disabled (if equipped). Manually set blower on high. If the vehicle has a fresh air/recirc button, set it to FRESH. If the vehicle has an A/C switch or compressor on switch, set it to A/C ON.
6. If the vehicle is equipped with auxiliary climate control, set the auxiliary controls to full COOL in the PANEL mode at HI blower speed.
7. Open all vehicle windows and leave the hood open for the test. Open the rear hatch and/or rear doors (if equipped).
8. Confirm the compressor clutch is engaged and the engine cooling fan(s) are operating or engaged. Allow the vehicle to idle until the suction (low-side) and discharge (high-side) pressures are stable or fluctuate in a range that repeats.
9. Record the ambient (shop) temperature.
10. Record the discharge pressure. If the pressure is fluctuating, record the average value.
11. Determine if the discharge pressure falls within the normal operating limits using the Normal Refrigerant Discharge Pressures chart.

High side:


Low side: