(k1500) 4wd not engaging......please help

i got a 89 k1500
when you put it in 4wd, nothing happens, the light doesnt illuminate, the front wheels wont turn, nothing
first i checked the fuses, all good
then i changed the thermal switch in the front axle, that telescopes when you put juice to it.... and still... nothing
all connectors are plugged in also

please help me troubleshoot

is this a manual shift TC or push button. also what setup do you have? tranny, Tcase, etc...

check for power to the thermal switch wires at the T-case when the 4x4 button is pressed. If there's power, it may be a mechanical problem. If no power, keep checking until you find wires that are hot. Also (this is a long shot) if the lights burned out it may have caused the computer to think something is wrong. I saw a car on which the anti-lock brakes stoped working due to the third brake light being burned out.

The problem is the linkages to the shifter. Check where the shifter handle is and where it goes to the transfer case. I think it can be adjusted.

If the thermal switch and the front axle actuator are the same thing then this will not help u, but I had the same problem as your having and it ended up being the Front axle actuator, its a cylindrical shaped device that screws on to the outside of the front differential , its about 1 1/2" diameter by 4" long with 2 wires coming out of it. Its just a vacum acuator. I replaced mine. ended up costing bout 110$, very easy to do, just unscrew it with a 1" wrench i think (maybe 1 1/4). And disconeect the wires. and put the new one on. Also some times when u put it in 4wd and it doesnt engage or light up its cause theres not enough vacum power, while its in 4wd put the tranny in neutral and keep it revved at 2000rpms til it kicks in 4x4, that will build up vacum and engage it.

The front actuator on a k truck is NOT vacuum actuated. The actuator has wires going to it not vacuum lines and the actuator is oil filled and in very cold temps it could take a few mins to warm the fluid and expand the plunger to engage the 4wd