4 Wheel Drive/SUVs: 1990 GMC 4X4 3/4 TON, gmc 4x4, transfer case

Question
FOUR WHEEL DRIVE WAS WORKING 3 MONTHS AGO.FOUR WHEEL DRIVE WILL NOT ENGAUGE.CHECKED ACTUATOR ON FRONT DIFFERENTIAL NO VOLTAGE.CHECKED ALL FUSES.IS THERE A RELAY SOME WHERE?PULLED ACTUATOR AND IT WILL WORK,IF YOU APPLY VOLTAGE.

Answer
James,

There is a transfer case relay (on the firewall), a front axle actuator and a front axle switch.  You need to check all 3 of these for voltage/continuity.

I would suggest that you purchase a repair manual for you GMC as they have great trouble shooting sections and diagrams/pictures of the actual locations of relays and actuators.

Hopefully, it will be a simple fix.

Carl