i have a 1984 s15 gmc, the four wheel drive "seems" to be working some times but other times it just grinds and wont "lock/shift" could it be the "vacume switch? " or is it an electric switch. where is it located on the firewall? tranny? transfer case? can any one help me. oh yeah its an auto tranny , 2.8 v6 . let me know i need help

 

I believe there is a vacuum solenoid on the firewall, but your problem lies in the vacuum actuator on the axle. I had a lot of trouble with mine on my '85 S10 4x4. Much of my troubles were alleviated when I replaced all of the vacuum lines for that system. I know that on my dad's '94 K2500, they had an electro-magnet actuator that was replaced with a worm gear type actuator to lock in the front axle, but I don't know if something like that is available for as a replacement for the vacuum actuator. Probably wouldn't be a bad idea to pull everything apart and see if you can't clean up and lube everything. Replacing the vacuum lines would be my first course of action though.

Good Luck

 

Is it your transfer case, or front diff making the noise?