Does anybody know how the outdoor fan is controlled ?

In cooling mode compressor and outdoor fan working fine.

Then on heating mode the compressor runs away and outdoor fan does not cut in, to the extent that the condenser coil, compressor and pipework ices up. No 24v on coil of outdoor fan contactor from control board when heating, transformer giving said board 24v. Can bring fan on in heat mode by pushing contactor in.
System running on R410A, outside temp of 8deg celcius, indoor 23deg celcius.