First thing to make sure of is to check for a blown fuse. The fuse location is marked 4WD and it should be a 25 amp fuse. If the fuse is bad, the lights on the 4wd console wont light up. If the lights do light up, then you can move to the transfer case. On top of the transfer case, toward the rear of the truck is a switch that has a 2 wire connector. That switch sends power to the front axle actuator when you try to enagage 4wd inside the truck. That could be bad. The switch is threaded into the transfer case and is still available through GM. If the switch is ok and the wiring that attaches to it is ok, then you can move up to the axle. Threaded into the front axle is the front axle actuator. These actuators are prone to failure in older trucks and is very prone to cold weather failure. You can test the actuator by removing it and then WITHOUT the truck running, put the truck into 4wd and then turn the key to the hot position. The plunger inside the actuator will extended outwards. More than likely it will be one of those 3 things. If it still wont engage, you may have a short in one of the wiring harnesses associated with the 4wd system. Also, you may have a damaged transfer case. To check the transfer case, put the truck in 4wd without it running and then crawl underneath and see if you can turn the front shaft by hand. If not....it's good. If you can....then the shifter forks inside the case are worn or damaged. I hope this helps.....start small and work your way up!
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There is an electric switch on the front axle toward the passenger side. These are notorious for failing and they aren't too expensive. If you have a test light you can check for power at the wires before you replace it.
Go to napa or autozone and ask. I do know that the 73 - 89 axles take the exact Gear Oil that is put into the SM465 Bulldog 4 Speed Manual Transmission as well as the NP205 from 1973 GMC Super Custom 2500 4x4 rigs
unplug thae actuator,unscrew it from the axle,plug it back in and engage the 4x4 ,plunger should extend,if it does the trouble is in the housing(washer out of place)
actually that first soultion is wrong it is correct on smaller pickups but on the full size pick ups they use a differnet sytem
there a electric actuator on front axle
there be two elctrical plugs on front axle u want the bigger of the two other one is just to turn the 4 wheel drive light on
un plug actuator then remove actuator after it removed plug it back in but leave it out of axle turn key on and shift down to 4 wheel drive the actuator should move out if it dont that your problem if i does move this isnt your problem it either the tranfer case or in front axle
If Your Front CV shaft is broken it will not engage...Shaft can be in visibly good shape, but you might hear a knocking sound or a shutter when turning sharply. I had this problem this last winter. Get under the truck and try and turn the CV by hand. if it can be rotated, it is bad.
Re check your wiring it souds like you may have crossed some wires, or wired it where the switch is always on. The electrical componet you have is much different than the original vaccum type 4 wheel drive engagement.
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