Technical AC Condenser Freezing and stopping air flow

[SailorBob]

AC Evaporator Freezing and stopping air flow

This is actually a problem from the summer that I'm just getting around to addressing. The basic problem is that air stops flowing out of the vents after the AC has been operating for a while.

The vehicle in question is a 2010 Fiat Ducato 2.3L Multijet with the most basic manual climate control system.

I can currently reproduce the problem after about half an hour of operation. Ambient temp as measured at the vent with fan speed at 4 and AC off is 10*C. Setting fan speed to 4 and recirculating on, engaging the AC brings the temp down to about 2.5*C after a few minutes idling. The temp eventually got down -1.2*C and air flow almost completely stopped.

If I turn off the AC and set re-circulation to outside air, the air starts flowing again after a minute or two. I verified by sound and touch that the blower motor is in fact running the whole time.

Scan tool shows AC pressure constantly oscillating between about 11800 mbar up to 16000 mbar. I notice when the compressor cuts out at 16000mbar the system voltage drops momentarily to around 12 volts. These oscillations happen fairly quickly.

I couldn't find any PID for a temp sensor in the cab. Only temp sensor I found in the instrument panel was the outside temp sensor which shows a constant 216*C. Don't thinks that's related but thought I'd bring it up anyways.

Shouldn't there be some kind of cabin or evaporator temp sensor that prevents the AC compressor from operating below a certain temperature? I'm assuming that would be the root problem, but can't find any reference to such a sensor.

My assumption is that the evaporator is freezing over preventing air from flowing and that this is being caused by the system not detecting the low temp to cut out the compressor.

 

[SailorBob]

I see in the x250 training manual that there is a 3 pin 47 kOhm linear pressure switch that reads from 3.018 bar up to 29.508 bar. It has ground, 5v ref and the third wire is the signal which reads from 10% of the vref up to 90% vref.

I just read in Tim Gilles Automotive Service in the Evaporator Icing Control section that:

Sometimes a pressure cycling switch is used instead of a thermostatic switch. The temperature in the evaporator can be predicted by the pressure of the refrigerant inside. The pressure cycling switch is mounted on the accumulator at the evaporator outlet.

So is that how evaporator icing is controlled on the Ducato? Is it possible the pressure switch is defective?

 

[SailorBob]

So as far as I can tell the problem was a dirty evaporator. The previous owner didn't have a cabin filter installed and this is how the coils looked:

And this was after half an hour of running the AC:

Now that I've cleaned the evaporator using some electronics cleaner it seems to be doing OK and isn't freezing up or experiencing restricted air flow...