front diff problem
 

hey guys
i have a 94 toyota pickup with the 3L v6 4x4 and im having a problem with my front diff not engaging im getting the light on the dash coming on so im assuming that the actuator in the ADD is engaging the switch and i have checked and i do get vacume from the right hoses at the ADD what else could be wrong if the actuator is engaging the light, the transfer case is good front drive shaft engages when in 4wd
does there need to be a certian amount of vacume to engage the diff and less vacume to engage the switch?
help would be nice

cheers
frank