03 f350 96k miles. is it normal that when i put it into 4x4 with the hubs in auto and my A/C is on it switches to defrost all on its own???

 

no its not, it means you have a leak in your vacume lines, your hubs are not holding pressure, get a hand held vaccume pump and test both hubs to check for leaks, mines doing the same im just to cheap to buy new hubs from the dealer for 300 a side (thats what they quoted me)

 

It's a vacuum leak. First check for any vacuum hose damage or disconnected. I would first look to see if the gray hose that is by the oil fill tube. It's common for this gray hose to get bumped off, it will either go to the in-line heater hose valve or the nipple on front of the HVAC box, depends on what options your truck came with.


You don't need to replace the whole 4wd hub, you can just buy a replacement seal if your 4wd hub seal is bad. It's around $50.

 

i have one that holds pressure but wont engage, then the other wont build pressure.

 

I'm fighting with mine right now. Currently, every single setting on my HVAC controls comes out the defroster, whether I have the 4x4 activated or not... Ordered all new lines from Ford. Went to install last night, and noticed the line up by the pump is split apart. Cut the end off and reinserted, then unplugged the pump and plugged it back in to get it running again, seemed to be building a little vacuum, but I still can't get HVAC controls to do anything other than defrost and auto 4x4 doesn't work still. I have two new hubs on there too.

Here's a good video on diagnosing the vacuum system though... https://www.dieselbombers.com/ford-p...t-working.html

 

I watched that video, it's pretty to the point and a GREAT trouble shooting tool.

On the upside, all hands join in congratulating brondondolon for actually USING his front axle. We've all heard of 'that guy' who has the concrete 4 wheeler that has no clue what the front axle does.

 

I just went through and replaced ALL of my lines from the pump down to the hubs, and still have defrost only. Even better, I have defrost only in EVERY SETTING, even with the truck in 2x4. Auto 4x4 still doesn't work. I know my pump is good, and now I know my lines are good, so now I have to try swapping the solenoid, I've heard of this sticking too. Always something. I did find the line cracked coming from the pump headed down to the hubs, but I literally snipped the end off and slid it back in. That didn't work so I said F it, I have the whole assembly of lines, I'll just swap them in. Did that last night, still no dice. On to the solenoid!

Karl, I've pretty much got a concrete 4 wheeler too, but you can't do a boosted launch without the front axle functioning!

 

Sure you can, it's called a smoke show, or a slip and slide!