4x4 problems

i have a 94 4-runner sr5 v6 the 4x4 wont engage. the 4x4 is all auto locking and ive tryed to blow air into the lines and have tryed with my limited automotive skills to try and figure out why it wont engage. it is driving me insane and winter is approaching and i would like to have it fixed before it does.

any help would be greatly appreated

Probably in your ADD control system. I don't know if the following instructions apply to your system, but this is a similar truck:

Check resistance between contacts of ADD solenoids (driver side fender, 37-47 ohms)

Apply battery power to each solenoid. Air should flow from one port to the other, but not to the air filter (large outlet).

Without battery power, air should flow from the inboard port (E) to the air filter and not to the outboard port (F).
Replace solenoid if not functional.

Check continuity of ADD relay (passenger side near firewall)
Facing contacts with three horizontal contacts on top, upper contacts are 1, 2, 3 from left to right, and bottom contacts are 4 and 5, left to right.

There should be
continuity between 1 and 3
continuity between 2 and 4

continuity between 3 and 5 with (+) battery power on 4 and (-) batter power on 2
Replace relay if not functional.

Chech continuity of ADD switch (on ADD actuator)
There should be continuity between the terminals when the switch is pushed and no continuity when the switch is released. Replace switch if not functional.

Check ADD actuator by applying vacuum to port A on end. Sleeve fork should move to actuator side. There should be no vacuum leak.
Sleeve fork should moves away from actuator when vacuum is applied to port B on the side of the actuator. There should be no vacuum leak. Replace actuator if not functional.

There is also a transfer case 4WD switch - no information.

Also, I have heard that you may have trouble engaging 4WD if the front differential has 90W gear oil instead of 75W90. Perhaps changing the gear oil in the front differential is the easiest 1st thing to try.