Symptoms
The lever goes into 4H with no noise and no front drive. On ADD trucks the hubs are locked all the time and the front axle is only connected when a vacuum-operated collar in the driver's-side axle tube moves (4Runners.com).
The fix, step by step
- Confirm you have ADD: plastic hub caps instead of locking-hub dials (IH8MUD).
- Drop the front skid plate and check the two vacuum lines at the actuator on the front differential, which can get knocked loose.
- Then trace them to the electrically switched vacuum valves on the inner fender and check the valves and their wiring.
- Plugging or swapping the lines to force the actuator tells you whether the diff side works.
- If you'd rather lose the system, 4Runners.com members lock it out or delete it with a pre-1990 driver's-side axle tube and fit manual hubs.
- LC Engineering notes that ADD trucks don't need its auto-hub conversion kit for that swap.
Tools & part numbers
- Hand vacuum pump with gauge
- test light or multimeter
- sockets for the skid plate
Where to buy the parts
Vacuum hose assortmentAmazon ↗
Hand vacuum pump with gaugeAmazon ↗
Aisin manual locking hubs, 1986–95 IFS (for an ADD delete)LC Engineering ↗Amazon ↗
Front hub star gasket (Toyota 43531-60010)LC Engineering ↗Amazon ↗
Hunting a used or discontinued part? Search eBay Motors for Toyota 4Runner 2nd Gen parts →
Rated for a competent home mechanic with a decent tool kit. Confirm torque figures, fluids and procedures against the factory service manual for your VIN, support the truck on stands rather than a jack, and work at your own risk (disclaimer). Some parts links are affiliate links, which pay us a small commission at no cost to you.