Coleman Mach AC runs on low but wont shutoff

[Rich Wildman]

I have a 1998 Coleman Mach air Condtioner that is ducted on a Class C. It is a 7330 model series. It will run and cool when it is set on cool and low fan, not auto, but it will not run on high fan and fan continues to run even in off setting. I have already changed out six wire Coleman thermostat with another six wire Coleman thermostat and no difference. Any Clues. Thanks

 

[Henry J Fate]

If it is ducted, up inside the filter grille is the control box. The control box should have wires to it that are identified with print on the control box.
The thermostat wires should be obvious and sperated from the 120v wires.

The control box uses 12 volts signals from the thermostat to switch the compressor and the two fan speeds via 12 volt relays.

It may be a bad control box or possibly wired wrong.

Look up inside there with a good light. You may need to carefully clean the box to read the wire labels. Once you figure out the control box wiring, you should be able to perform some tests to gain some clues as to where the problem is.

 

[regval]

To expand on what Henry said, The wire colors from / to the thermostat should be:
Yellow- turns on air conditioner compressor
Green - Hi fan speed
Gray - Lo fan speed
Use a volt meter to determine if there is +12V at the appropriate wire. If the thermostat hi fan switch
Is on, there should be 12v on the Green wire and 0 volts on the Gray.
If you disconnect the green and gray wires, the fan should shut off. If the fan continues to run, you likely have a defective control board. The board is usually housed in a rectangular metal box visible from inside the coach below the AC unit.

 

[Rich Wildman]

Thanks so much. So I shut off the 30A breaker, and then tested all the 12 v wires like it was suggested above. I did have current on the correct wires based on the thermostat settings. Talked to two RV techs and they both said it was likely control box like it was suggested above. Looks like it might be available on Amazon. Any suggestions on changing a control box or buying one.