i own a 1999 ford f150 4x4 with 174000 miles on it. when i go to activate the 4 wheel drive, it will go through 4 hi, n, nd 4 low with some fight, and will sometimes give a slight grind/pop when shifting it into low, but the light on the dash wont light up and it wont engage into 4 wheel drive. the front drive shaft spins when it is in 4 wheel drive, but no power to the wheels. i looked at the vacuum lines and the driver side line sucks air in 2 and in 4 wheel drive, but the passenger does nothing. also, the "drum" the lines connect to wont hold pressure with a vacuum tester. would my best bet be a bad seal or a sensor? or something more? hopefully someone here has experience with the same problem and how to fix it. thank you for your time.
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do your heater controls work properly?
the loss of heater controls is an indication of a vacuum leak.
and the automatic 4wheel drive like you have is notorious for vacuum leaks.
if you turn the auto hubs to lock does the 4 wheel drive work?

 

Tom the F-150's dont have hubs like that...

 

Some of them do.

 

in 99? im pretty sure mine alwasy spins no hubs to it...

 

The 97 and up F150s and Expeditions use a CAD (central axle disconnect) style front axle actuator... similar to the GM IFS and 94 and up Dodge 4x4s. It's either going to be the vacuum switch on the t-case, lines, or the actuator on the front axle. The hubs are direct drive with no disconnect at the wheels. It seems like most electric-shift t-cases will bang and clang from time to time when being shifted into low range. From my own experiences this is due to not using low range regularly enough.
John

 

i always thought it was because electric 4X4 sucks.

i really don't know too much about ESOF other than the fact that i will never own a truck with it after seeing all the problems others have had with it.

 

glad my transfercase is Manual

 

97+ have a center disconnect dealio, no hubs on the ends.

 

squawk!

 

bagawwwk!

gdmf10cl

 

There are 2 vacuum switches on the right side fire wall that may be your problem
You can switch the two to see if that's where the problem is it will pull the accuator in or out if switching them doesn't work make sure the lines are on the accuator correctly
I had this on my 99 150 at 165000 mi this was the fastest way I found to do the DIY test
Good luck and I hope this helps you