If vacuum built during the first step of the test releases when you ground the solenoid, it indicates a normally closed solenoid that's OK. If vacuum didn't build during the first part of the test, but then builds with the solenoid grounded, it indicates a normally open solenoid that's also OK. Replace any solenoid that fails to hold or release a vacuum during the transition of energizing and de-energizing.
Many vehicles with shift-on-the-fly 4x4 systems use an arrangement of vacuum controls to activate the front axle. When troubleshooting this type of system, it's important to first divide out the electrical/electronic side of the system from the vacuum-control side. Then, ensure that the electrical/electronic side is doing its job, and is not the culprit of the inoperative 4x4.
Troubleshoot the vacuum controls exactly as prescribed in the shop manual, but keep one thing in mind. The vacuum controls and interconnecting hoses often live in an environment that's not exactly friendly for this kind of technology. A thorough visual inspection may reveal cracked or damaged vacuum hoses, or damage to the vacuum servo.
Reservoirs and more
Most cars have a vacuum reservoir to keep the vacuum supply steady to all vacuum circuits under periods of heavy engine load. Sometimes these reservoirs split at their molding seams and leak vacuum. Since many reservoirs are tucked away underneath the fender or buried below something else, a visual check is almost impossible. The best check, of course, is with your vacuum pump because you can always get access to the vacuum ports. Most have one or two.
To check the reservoir for leakage, just remove its hose and connect the hose of your vacuum pump. On two-port reservoirs, connect your pump to one and plug the other one. Build vacuum with the pump until it's around 25" Hg. The reservoir should hold the vacuum for several minutes without leaking.
As you can see, these examples prove that uses for a hand-held vacuum pump are almost endless. Vacuum technology may be slipping away, but there's still plenty of it around to keep you busy.
Technical Editor Dave Cappert answers your questions about borescopes and more.