Don't know if this is any
help, but if it saves you going down the route of the fan controller it may be...
I had problems with my auxiliary water pump a while back. Sometimes it would come on, sometimes not.
When checking for a power feed at the connection to the pump, sure enough when the pump wasn't running there was no power being fed there.
BUT... it turned out to be the pump that was at fault.
It seems the fan control unit would only supply the power when the load of the pump was there, and as the pump was playing about it wouldn't start the pump every time.
Almost seemed like you need to match the resistance of a fully functioning pump at the time it should be activated in order to get the power supply to test it, but I could never prove this so just took the plunge and took a chance with changing the pump, and sure enough it's faultlessly run every time since...