my 90' jeep yj will not engage into four wheel drive. i bought the book on the jeep and did all the tests with a vacume pump and the actuator and shift motor holds a vacume which is all the book says about trouble shooting the system. i unhooked the vacume line from a "thing" mounted on the firewall that is just to the side of the battery. i hooked my vacume pump up to it and put it in 4x4 and the fourwheel drive engages. i went off road with the vacume pump attached to that line..... i have yet to find a single person that can tell me what that "thing" is or what it does. the book does not mention it at all, it has no part numbers on it, and every parts shop looks at me with the "what the hell is that" face when i try to find a new one. it has two wires attached to it, a hot wire thats putting out 14.4 volts, and a ground wire, which makes me believe it is not a switch, and way to small to be a vacume pump. if anybody out there knows what the hell i should do i would appreciate it. i have ordered the vacume actuator? that is mounted to the top of the transfercase w/four vacume lines coming out of it. in hopes that that will fix it. any help is appreciated.