I recently installed a used central AC unit in my garage that a friend gave me. After trying unsuccessfully to install the thermostat (I couldn't figure out where the 3 wires went) I bought a new Honeywell ct31 manual thermostat at Walmart figuring that I would be getting a schematic or wiring diagram for my money. The installation instructions simply said to "mark the position of the old wires"... Well. DAH! I don't have any old wiring.
I need wiring help for this AC/heating. My unit is an old Intertherm central AC with electric heating coils. It is a 3 wire 24 volt system (I think). When I put my green,white,and red wires on the corresponding posts ("G" "W" and "R") on the ct31, the AC will only run with the thermostat in the "heat" position and the heating will not work at all. With no thermostat, I can operate the ac by jumping the white and red wires. Can anybody help?
The AC is imperative, the heat is a plus later on but doesn't have to work right now. Thanks in advance!

If the tstat is for heating, it will not work properly with AC. Heating stats open on temp rise, and cooling close on temp rise.

The HAVC unit should need at least a four wire stat. Your stat is Red is 24 volt power to the stat , white is switched return to heating relay, green is switched return to fan relay, and missing on the stat is Yellow switched return for cooling relay.

Not sure why your heating/cooling system only has three wire control, cannot control both heat and cool with only three wires.

On the inside of an access cover should be a wiring diagram, see if you can find it and determine how the unit needs to be wired.

On the older units (I've been told) the fan does not operate independently of the cooling or heating. There is no schematic inside the cover because it's an old unit. From what I can see from visual inspection, red comes off the transformer (power), green goes to the heating relay and white goes to the cooling relay. Again, jumping the white and the red get the unit blowing cool air. The thermostat is a dual use unit with a switch from heating to cooling, but when I wire it the way it is, the COOLING relay is activated with the thermo in the "heating" position. (and yes, that will not work because the thermostat cuts off backward).I need a schematic on a Honeywell CT31 and the folks at honeywell.com have not been that helpful.
Thanks!