I have a 1999 Ford, Turbo 7.3 diesel, F.350. My heater stays continually on high heat defrost and I am unable to change the settings or shut the heater off. I can only change the fan speed. I am unable to access my air conditioning and the heat runs all summer much to my dismay. I have personally replaced the entire switch assembly on the dash but there is still no change and the defrost continues to run at high temperature. Does someone have any idea what I have to do to fix this problem?

 

Hey Doc,
I thinking its got some thing to do with the heater core door in the duct assy. In most new cars the door is opened and closed electronicly. It might be that the door is stuck open. air is brought in from either the inside or out, it goes though the fan then A/C evaporator, then to a door that can divert the air to the heater core or not, this is the door that I'm am talking about. As for the defrost not working or working all the time, in most cars these door are still opened and closed useing vacuum from the engine, maybe you have a leak?

Here a website that might be able to help you out, it not that great, as it doesn't have your year module but in some cases it helps:

http://www.autozone.com/servlet/UiBr...3d800a39e2.jsp

 

Sorry I took so long to say thanks to CCTXFISH. I still haven't had time to track the problem but I did find that my vacuum pump is not working but have not replaced it yet. I will let you know in due course if that rectifies everything. Thanks again, Doc

 

Thanks Doc,
You know that just may fix all the problems, But I'm still not sure if the heater duck door is ran electronicly or not. Please keep us posted on how everthing turns out

 

Thanks again CCTXFISH. I was wondering where I could get a wiring diagram for the water temperature control for the heater. It makes no difference where I set the temperature control dial it stays hot all the time. Is there an electric thermostatic valve that could be malfunctioning. I thought at 1st. it was the temperature control switch itself so I initially replaced the complete control in the dash but that didn't help.

 

Doc,
The problem maybe that the electronic module that opens and closes the heater door. you can check this by opening your glove box, pull the corners in so that it drops open or you can just remove it. Just behind it you'll see a big black air duct box. On the top of that is a white box (module) it has some wires going to it and a shaft coming out of the top through the module. With the key in the on postion, and the A/C control switch on any position other than OFF will work. Then move the temp control noble through from max cool to max heat a little at a time if the shaft isn't moving, it may be bad. Don't replace without testing it first, it maybe someother electrical problem. Sometime you may get a salvage yard to let you try it before you buy it, some don't as there are afraid that the problem if something else will damage it. If it isn't working you can remove it, don't remove the screw that look like thier holding it, just carefully pry it up and off the mount that stay secure to the air duck.
As for the wireing diagram, some of those books at Autozone have the wiring diagrams, but they're hard to see and read. Good luck and keep us posted