4wd won't work.. 1990 chev. :(

I checked the front diff plug and it is getting power.. with the actuator out , I turn the key forward and it after about 1 second moves outward... so
I aasume this is working... I plug it back in and screw the thing back in the diff but stil no 4wd.. could the actuator be weak and not strong enough to engage 4wd?.... Any info would be great.. I don't wanna spend $250 to buy a new one and the old one be fine.. thanx
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