Some reasons for a four-wheel drive not engaging include vacuum leaks, a malfunctioning actuator, a faulty transfer case shift motor and a bad transfer case control module. Vacuum leaks tend to be the most common reason for a malfunctioning four-wheel drive system.
Vacuum power is free energy created by the internal combustion engine, and many elements of a vehicle's workings are powered by vacuum. Four-wheel drive systems are no different, and many are controlled by vacuum power. Vacuum leaks become more problematic as the vehicle gets older, and hoses exposed to high heat in the engine are likely to become damaged first.
Vacuum line leaks and disconnections are two common reasons for a four-wheel drive system to not be working properly. A ripped actuator or one filled with transfer case fluid can also be a problem. For electronically shifting systems, the transfer case control module, transfer case shift motor and interior dash buttons can all be sources to troubleshoot. The dash buttons trigger the transfer case control module, which then instructs the transfer case shift motor.
If something in this line has a fault, the dash buttons may blink on and off, communicating a diagnostic trouble code for what is wrong with the system. Problems may include anything from a corroded electrical connection to a faulty part, such as the shift motor.