The secondary fan is controlled through a relay, closed by a temp switch or by the a/c switch. The ecm has only an input for this fan. Shannen Peter Gargano wrote: > > Shannen Durphey wrote: > > > I can't see any way that the ecm controls the heavy duty fan > > after looking through the schematics. > > It controls it through a relay - or did I miss something? > > Peter