I'm sure it has been asked a few too many times, but my 4x4 is not engaging. I picked up my 94 blazer full size last week and 4x4 will not work. The light on the floor doesn't light up nor does it make any sound when i shift into any 4x4 gear. I was told it could either be the actuator or vacuum seal. Rather know for sure before i buy one of the parts.

 

No one is going to be able to help you without actually seeing the truck for this particular issue. If you're not handy, you should probably take it in to get looked at.

 

Hopefully somebody knowledgeable about the full size trucks chimes in. All my experience is with the S series blazers

 

The most common failure on this type of 4wd system is a failed front axle actuator. This will prevent 4wd engagement and cause the lights at the floor shift not to illuminate.

OPERATION
When the Ignition Switch is in RUN and the four-wheel drive control lever is engaged, the transfer case switch closes and battery voltage is supplied across the front axle actuator. The front axle actuator energizes and the four-wheel drive gears are meshed. This meshing action of the four-wheel drive gears closes the front axle switch, and battery voltage is supplied through the WHT (156) wire across the four-wheel drive indicator lighting the bulb. Battery voltage is also supplied through the front axle switch contacts and PPL (420) wires to the rear-wheel-antilock brake module. This will keep the antilock braking system disabled in 4WD mode. The In-Line Diode (auto trans only) prevents the transmission converter clutch solenoid from engaging when the front axle switch is closed.

So put TC lever in 4WD and check for voltage at the front axle actuator with the key on. If no voltage, check fuses & wiring. If voltage is there, is the axle locked in? if not replace actuator. Not sure about the vacuum seal you refer to. Good luck and best wishes for a successful repair.

 

Look under the battery. There is a part that holds a vacuum
Was my issue the first time and then just recently as well.