Daihatsu Hijet — 4WD System

The S110P engages the front differential via engine vacuum routed through electromagnetic solenoid valves (VSVs) to a diaphragm actuator on the differential. The factory service manual notes that 99% of 4WD engagement failures are a vacuum leak — not a mechanical gear failure. Check the vacuum system thoroughly before assuming a mechanical fault.

How It Works

When the cab selector switch is moved to 4WD, the VSVs open and direct engine vacuum to the actuator diaphragm. The diaphragm pulls a fork that engages the front axle. The VSV array is color-coded: a brown solenoid and a blue or gray solenoid control routing to the actuator.

If vacuum is insufficient — from a leak, a failed VSV, or a ruptured actuator diaphragm — the front axle will not engage or will slip out of engagement under load.

Actuator Test

Use a hand-held vacuum pump (Mityvac or equivalent) to isolate the actuator before condemning anything else:

1. Disconnect the vacuum line at the actuator and connect the pump directly to the actuator port.
2. Draw vacuum to 450 mmHg. The actuator arm should move — you will hear and feel it engage.
3. Hold the vacuum for 20 – 30 seconds and watch the gauge.
4. If pressure drops below 450 mmHg within 15 seconds, the internal diaphragm is ruptured — replace the actuator.
5. If the actuator holds vacuum, the fault is upstream: check the vacuum hoses for dry rot or cracks, the check valve for blockages, and the VSV solenoids for continuity.

VSV Solenoid Test

Each VSV can be tested with a multimeter. Remove the electrical connector and measure resistance across the solenoid terminals. A functional VSV reads approximately 30 – 50 Ω. An open circuit (infinite resistance) or a shorted solenoid (near-zero resistance) indicates a failed VSV that must be replaced.

You can also apply 12 V directly to the solenoid terminals (from a battery or jump leads) and listen for a click — a working VSV clicks audibly when energized.

Common Failure Points