I have a 99 expedition that will not engage into 4x4 when i turn the selector switch with no dashboard lights

 

Check your fuses.

 

I don't think that's the problem because when I turn the switch I hear a clicking like a switch attempting to switch but it can't so it's cicking over and over plus when I start the truck the low range light will illuminate during the initial light check when the key is turned on. To explain further I was having trouble with my air conditioning defaulting to defrost, the vacuum line from the one-way check valve to the firewall was fine with no brakes so I made an assumption that the vacuum problem was somewhere in the dash so instead of tearing the dash apart I bypassed that line entirely with another line finding the easiest way to route up to the controller which fixed the a/c problem. My question would there be a correlation between doing that bypass and my current four wheel drive problem. I'm sorry about my cryptic descriptions I'm not a mechanic I'm just guy trying to save money

 

I'm assuming an expedition and F150 are the same?

 

There may very well be. Check the hard vacuum lines behind the battery, along with the accumulator in the fenderwell.