AC Cond.No Start, 2-wire heat-only tstat & split sys

[plumducky]

MGH-065-ABWL-06, 27 years old. 2-wire heat-only t-stat connects to split system at control panel mounted on front of blower housing: Heat/Cool Switch on left, Fan-On/Auto switch on right.

With Fan switch on Auto, Heat/Cool switch on Cool, condenser will run, along with Fan in furnace, but only when tstat is calling for Heat. When calling for Cool... nothing.

There are only what appears to be two relays...one small/square shaped with red dot which the diagram on blower housing calls "low voltage relay", and one larger/rectangular shaped not called-out on diagram, but a part number found online calls it a Fan Relay. Got the part number from a MIller Illustratons/parts list online: http://mobilehomedepotmi.com/mh_catalog2/mgh--mgb.html