On a 99 7.3 what is that little compressor deal that's right behind the battery that has vaccine hoses going to it? If I have the key forward it just stays running.

 

it is the compressor that controlls the air/heat and auto 4x4 (if you have that option) when and if it goes out you will only have defrost.. I would look for a leak somewhere..

 

Well I unplugged the power to it and it still switched over to the different settings fine. So it def doesn't control that. I have 2wd too

 

Without the pump it will stay on defrost, I know I just had to replace mine in my 2wd truck. Maybe you are looking at something else.

 

That's your vacuum pump, and it does connect to your HVAC selector control panel.

The vacuum system diagram for our trucks is located on Page 8 of the attachment referencing the Ford Ranger mod in the first post of the thread linked below.

https://www.ford-trucks.com/forums/8...materials.html