What determines AC Unit to go on defrost

[Crypton]

I have an old R22 Mitsubishi electric Puh/ZVK3. Outdoor unit covered in ice so much that fan is tiping off ice. Ambient temp 1deg. Air off the indoor coil about 15deg. Will not heat properly because of such an ice build up on the outdoor unit. The unit obviously is not defrosting properly. Can anyone tell me what way these particular units operate on defrost?

[Thermatech]

The outdoor defrost is activated by the outdoor coil temperature sensor which you will find located on one of the cappliary tubes.
A defrost is started when the circuit board gets minus 5 deg C data from the coil sensor after the compressor has been running for minumum 30 mins.
or
The outdoor coil temp sensor is 8 deg C lower than it was 10 mins after of compressor operation following last defrost.
The defrost is terminated when coil sensor reads +8 deg C & max deforst time is 15 mins.

If the deforst take the max 15 mins then the heating operation time is limited to 30 mins before next defost.
10 to 15 mins deforst = 40 mins heat
7 to 10 mins defrost = 60 mins heat
3 to 7 mins defrost = 80 mins heat
less then 3 mins defrost = 120 mins heat.

So if the coil temp sensor has correct resistance & therefore provides correct temp data to the circuit board then the coil will completely defrost within the 15 mins provided the refrigerant charge is correct.

Suspect outdoor coil sensor or system short of gas.

[Crypton]

Brilliant. great answer thanks.