I salvaged a Coleman (model # 7975a856) Heat and Air unit from an old trailer. I got rid of the air portion with the intention of using it to heat my shop. It has four wires that ran to the old thermostat. (I don’t have the old thermostat.) They are red, white, green and blue. I do not know what the blue wire does. When I turn the furnace on… the burner will kick on and start to burn. I have found that it should take 90 to 150 seconds for the fan to kick on. The fan does not start automatically. I can turn the fan to the on position from my thermostat and it runs fine. I assume that it is the upper temperature limit switch is not working. I don’t want to replace it if the blue wire should be kicking the fan on. Any help or suggestions would be much appreciated.

Rule of thumb R- red wire 24 volt from transformer, G- green wire blower circuit, W- white wire heat circuit. The blue wire may have been the cooling circuit. Tie the red to the green this should bring the blower on. Tie the red,white,green this should bring the heat on with the blower.

This works. I was not sure why the fan was not coming on in auto. I am still working on it.

Thanks.