Icing detection works without a sensor - but how?

I have here a mobile air conditioner with a single exhaust hose. TROTEC PAC 3500 E. Operating instructions here.

The unit has the function that if the evaporator gets too cold, an anti-freeze function turns off the compressor for a few minutes and reduces the cold fan speed for 30 seconds. So the unit can somehow detect how cold the evaporator coil is and de-ice the evaporator coil.


However, there does not seem to be a corresponding sensor installed. Only two sensors are mentioned in the circuit diagram, and these seem to be the room temperature sensor and the exhaust air sensor:

Circuit Diagram:
Attachment 16257

What is called "coil sensor" in the wiring diagram seems to refer to the condenser temperature, see possible error code of the sensor:
Attachment 16258

In the parts list and in the exploded view I only find the room temperature sensor:
Attachment 16259
Attachment 16260

The room temperature sensor is a simple NTC, I have found that its temperature does not seem to have any influence on the defrost function.

Can anyone tell me how the defrost function of the unit is triggered if not by a sensor at or on or near the evaporator coil? The De-icing seems to work well that it only becomes active when the cooled air has been around freezing point for a 5 minutes or so.

Thanks!

The wiring diagram seems to show a Pressure Switch?
If it does have one, maybe its on the low pressure side of the system. Then you could use it to activate a defrost on the pressure going too low when the coil ices up.

The parts list seems to show a item named "Induction temperature head bracket" no.44 thats near the evaporator? Maybe theres an induction temperature head that goes in that bracket?

If theres a temp sensor at the air outlet, maybe is looking for a temperature difference between air into and out of the unit?

Quote:

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Originally Posted by seanf

The wiring diagram seems to show a Pressure Switch?
If it does have one, maybe its on the low pressure side of the system. Then you could use it to activate a defrost on the pressure going too low when the coil ices up.

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Based on the documents, this seems to be the case. Is it common for mobile air conditioners to be built without a pressure switch at the hot, high pressure stage?

Quote:

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Originally Posted by seanf

The parts list seems to show a item named "Induction temperature head bracket" no.44 thats near the evaporator? Maybe theres an induction temperature head that goes in that bracket?

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I think this is a typo. Not "Induction temperature head bracket", it should be "Insuction temperature head bracket". This is the ambient air thermometer at the air intake.


Could it be considered an acceptable risk to deactivate the pressure switch on the low-pressure stage while keeping an eye on ice building manually?

Quote:

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Originally Posted by Juschi

Based on the documents, this seems to be the case. Is it common for mobile air conditioners to be built without a pressure switch at the hot, high pressure stage?

Cant say Ive worked on many mobile AC units so. But alot of the time the small domestic fridges wont have any HP switch, they just rely on compressor klixons.


I think this is a typo. Not "Induction temperature head bracket", it should be "Insuction temperature head bracket". This is the ambient air thermometer at the air intake.


Could it be considered an acceptable risk to deactivate the pressure switch on the low-pressure stage while keeping an eye on ice building manually?

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If the units been designed to use the pressure switch as the defrost monitor I wouldnt leave it unattended when deactivated.
If you leave the unit running with an iced up coil you risk letting liquid refrigerant get back to the compressor, so keep an eye on the coil frost and also the temperature of the compressor.

Is there a fault your trying to fix or are you just interested to know how its doing its defrost?

Thanks.

For a temporary project, I want to cool down a very small room in my basement to a relatively low temperature. With the anti-frost function and conversion of the unit to recirculation function, I can manage 14 degrees Celsius with the aforementioned unit. My goal is 10 degrees Celsius.