Hello, Defrost is the default position when the system looses vacuum. The 7.3 diesel has a electric vacuum pump located on the passenger side of the engine compartment between the vacuum reservoir and the heater box. If you turn the key to the run position do you hear this pump run?
I don't have the vehicle here. I did check the vacuum pump fuse and it is good. I will check it tomorrow but what would be the next step ?
Yes, the vacuum pumps are pretty common to fail and cause this concern. When you get to the truck, if the pump does not run, disconnect the electrical connector to it and check for voltage using a volt meter if you have one or you can use a test light on the pin with the light blue and pink wire. You should have 12 volts with the key in the run position. If voltage is present the pump is most likely bad and will need to be replaced. After replacement turn the key to the run position and let is run for a minute or two and it should shut off once it builds enough vacuum. If the pump continuously runs and never shuts off then there is a leak in one of the vacuum lines and will need to be repaired or it will burn out the new pump.
Alright, let me know if there is anything else.
can i keep this chat open until tomorrow until I check it out incase there is something else to ask you?
Yes that's fine. I usually check a few times a day, so my response may not be immediate, but I will get back to you.
ok no problem
Earl,the vacuum pump is working. I checked the hoses and everything seemed fine. There are two fittings in the pump (black and white). The end of the white one is open so I put my finger over it and I could feel the vacuum then the pump shut off. Amazingly the climate control selector started working. Is there a check valve in that fitting that can be replaced? If it happens again....what is the fix? Thanks again.
There is a check valve in that connection internal to the pump. The white end is supposed to be open, that is the exhaust side for the vacuum pump. Buy placing your finger over it, you may have allowed the check valve to reset. Unfortunately the vacuum pump is not serviceable and can only be replaced as a assembly.
thanks for the help