4X4 help

I have a 92 b2600i and for some reason the 4 wheel drive will not engage. It was working perfectly fine last time I drove it, parked it for a few months and now it won't work at all. I've checked everything I can think of and my mechanic is also stumped. Any suggestions?

Maybe a mouse ate the wires while it was parked. If there is an electric solenoid not engaging, that could be your problem. Check for open fuses. Is there a light on the four wheel switch that comes on and now won't?

It’s a vacuum operated engagement. Lower driver’s side upper side of front axle. 2 vacuum hoses lead to it from the actuation assemblyman the passenger side fender . Stay outside the vehicle and listen for a click while a second person engages the 4x4 switch on the dash. No click, remove the vacuum switch and bench check it. 12 volts one side or the other, dosen’t matter. Still no click, replace the vacuum switch. If after that and you hear a click, pull one vacuum line at a time and check for leaks. If no leaks, go under the truck and remove the vacuum canister. Clean the plunger, remove and replace the cotter pin. Apply vacuum to the canister to check operation. If no movement replace the cannister (they’re not easy to find). Most of the time all that’s needed is a good cleaning and lubrications of the cannister plunger. Good luck!