I own a 1992 Ford F150 Custom 4X4 and my heater works when i put it on HI but when i turn it down or put it on LO it doesn't work at all any suggestions?

 

the switch might be going out

 

you probably have a bad resistor for the blower motor, although your switch is another possibility. If its the same resistor as my 86 had, you will find it on the firewall on the engine side near where your heater hoses pass through. I think it has 3-4 wires and was about half the size of a post it note, with 1-2 small screws holding it in. Mine was so packed full of leaves in there it had nearly set my truck ablaze! hopefully someone can chime in to confirm that is the same location on the newer trucks.

 

95% chance it is the blower motor resistor. Look under the hood, in the blower motor housing; has a couple wires running to it... Right where your coolant hoses run into the fire wall...

Oh and the resistor part number is...
# 973011 Dorman
# YH1698 Motorcraft

 

thanks for the help