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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.
there are two solenoids on the firewall by the brake booster (in front of the driver). There is a vacuum hose running to the two solenoids. When you are in 2 x 4, one solenoid puts vacuum to the vacuum piston on the axle to move one direction. When you are in 4 x 4, the other solenoid puts vacuum to the vacuum piston on the axle to move the other direction. You can feel vaccuum on different hoses if you move the **** from 2 to 4 wheel drive and someone feel the hose vacuum. If you dont have vacuum on both, check the other end of hose at the solenoid to see if it is opening.