How do I trouble shoot my thermostat to rule it out?

[huron52]

Looking on the internet I think I have a Coleman Mach 7330-3322. When I turn it from off to either heat or cool I get the fan on the heater.
From my search I have found I have a Green wire- High Fan
Gray wire- Low Fan
Yellow wire- Compressor
Red wire - 12 volt positive
Blue wire- 12volt negative
White wire- Heat

How can I trouble shoot to see if it is my thermostat and not something else? Can I check voltage? Would it be safe to start crossing wires?

[huron52]

I have ordered a new thermostat. I guess I'll see if that fixes the problem. Will report back with an update.

[Old-Biscuit]

T-stat can not control just furnace fan.
Furnace fan should come on when you select 'heat' (and t-stat set point is higher than inside temp) but after purge cycle burner should light off

But for furnace fan to come on when you select 'cool' indicates problem is with furnace fan control (circuit board)

[Joe417]

If you disconnect the white wire (heat), only the fans on the AC have control. The heater should not come on.

If it still comes on, you have a problem other than the thermostat.

You may have a choice on the thermostat to select whether you are using electric heat or gas heat. If set to the electric or heat pump setting it will turn the fan on. I use that setting to control my basement fan when the gas furnace is on.

[sdianel_-acct_c]

Maybe shut power to the thermostat for 45 secs to see if it will reset. Found this on another post: "I set the control slide switch to "off", and removed the cover of the thermostat by gently prying on the left side of the cover, and then opened the fuse by pulling it from the right side of it's connection holder. Count to 15, put it back in. put the slide switch in "cool" et voila - all is well."

[n7bsn]

The Fan-hi and Fan-lo wires should go to the A/C unit
Only the white wire controls the heater.

It's not making sense to me, my guess is that the thermostat electronics might be hosed up

[Diesel_Camper]

This is a split system correct? We have a Coleman Mach and there is a roof unit for AC and the lower furnace unit. The furnace fan should not activate when the thermostat is in AC mode. My thoughts are something went bad in the thermostat and it's only commanding heat not and not AC no matter where the switch is.