Hi all, hoping for a little troubleshooting direction on this one. I have a C425 which has an electric AC system. It's pretty simple. There's a dual shaft electric motor which drives both the compressor and a fan which blows air past the condenser. The system has both low pressure and high pressure interlocks on the compressor discharge line. This switches are in series and if either one reaches their respective set points it removes power to the relay coil and the motor stops. There are some other interlocks in the loop too such as relays that shut off the motor if you turn the electric heater on, when the engines are starting, if the cabin blower is off, etc. I have the temperature control rheostat disabled so it calls for cooling any time the system is turned on.

In 2 of the last 3 flights the AC cycles on and off for a while, then it stays on. On 6/23 I was taxiing and it was super hot (over 105F) and the AC was cycling on and off every few seconds. The period was pretty constant. The cycling continued while climbing out and I took a video. By luck as I was videoing the AC started to work and stayed on for the rest of the flight. It also worked today for the entire flight from startup to shutdown. Here’s the video from 6/23:

https://youtu.be/V_QTNZKMGQA

One theory is that either the high or low pressure switch is tripping which stops the motor. My guess is that it is the high pressure switch on the compressor discharge. If the condenser is not condensing the refrigerant then the compressor discharge pressure will rise until it cuts out at 350 psi or whatever. Then the pressure decreases, the switch resets, and the motor comes back on. Maybe the condenser needs to be cleaned? I took at look at it today and it seems clean enough. There’s no crud or anything blocking the fins.

Any idea where to start troubleshooting given that I can’t reliably reproduce the problem on the ground? Big time kudos and karma for anyone that solves this one. I hate intermittent problems...

You will need a guage set attached to check the pressures when the system acts up.
The unknown is why the cycling? Is the system over pressurizing due to a restriction in the high side and the safety switch is working properly or is the switch bad and it works but opens at too low of a high side pressure.
Another possibility is the system is low on freon causing the low side switch to open as the pressure goes too low.
As you climb to altitude the temps cool off and the workload on the system is greatly reduced. This could cause the system to appear to work "normally" at altitude.
I suspect low on freon.

Second the gauges. Would also bet a tad low on refrigerant.

When the compressor kicks in, the low side will drop. If there isn't enough refrigerant, it will keep dropping till it kicks out. When it kicks out, the high and low start to equalize and the low side is satisfied again (rising pressure in the low side). Gauges is only way to know.