Using your Jeep's four-wheel drive is the safest way to travel in dangerous weather conditions, especially snow. Unfortunately, four-wheel drive may not always work. Shifting into 4H will not engage the front axle. The reason for this is a faulty vacuum system in the axle actuator. The only way to find out the precise problem with the axle actuator vacuum system is to get underneath your Jeep and inspect the vacuum lines. They are behind the passenger-side wheel.
Place your car on a lift and start the engine. You'll need the lift to get the best view of the axle actuator axle system. Shift the car into 4H.
Raise your car high enough to comfortably inspect underneath.
Check the vacuum lines located behind the passenger-side front wheel. Look for lines that are hard, brittle and/or cracked. These are indications that the vacuum lines are old and need to be replaced. Additionally, cracks in the lines will prevent a sufficient amount of suction to engage four-wheel drive.
Inspect the vacuum lines to make sure they are all there. It is possible for fragile vacuum lines to break apart while driving. You'll find remnants of the missing vacuum tube or vacant vacuum tube fittings if this is the case.
Check the vacuum actuator diaphragm to see if it's working. This is at the back end of the vacuum tubes. If the vacuum actuator does not engage or otherwise appears to be malfunctioning, you will need to replace it.
