Honda Acty — Drivetrain & 4WD

4WD System Overview

The HA4 uses a real-time (part-time) 4WD system with no center differential. In 2WD mode the truck is rear-wheel drive only. Engaging 4WD locks the front and rear driveshafts together at the transfer case, splitting torque 50/50 between the front and rear axles. Because there is no center differential, 4WD should only be used on loose surfaces (gravel, dirt, mud, snow) where the tires can slip slightly during turns. Using 4WD on dry pavement causes drivetrain binding, accelerated tire wear, and stress on the transfer case and axles.

2WD / 4WD Selector Bolt

Many HA4 trucks imported into the US have had their electric 4WD engagement switch fail or are otherwise stuck in one mode. A common and durable fix — popular in the owner community — is to replace the electric 4WD actuator with a manual selector bolt that physically locks the front differential in either 2WD or 4WD.

The selector bolt threads into the transfer case in place of the electronic actuator. By rotating the bolt head, the owner manually engages or disengages the front axle. This eliminates dependence on the wiring and actuator motor, and tends to be very reliable. The bolt is a popular modification among owners who use their trucks primarily for off-road or farm work where frequent 2/4WD switching is not needed.

Driveshaft U-Joints

The front and rear driveshafts are connected to the axles via standard universal joints. Worn U-joints produce a clunk or shudder during acceleration or deceleration, particularly at low speeds. They can also produce a vibration that increases with road speed. Inspect U-joints by placing the truck in neutral, getting under the vehicle, and attempting to rotate the driveshaft while holding the yoke — any rotational play or binding indicates a worn joint.

U-Joint Replacement

1. Mark the orientation of the driveshaft to the differential and transmission flanges before removal — this preserves the factory balance.
2. Remove the driveshaft by unbolting the flange bolts at each end. Support the shaft to avoid stressing the rubber center bearing (if equipped).
3. Use snap ring pliers to remove the snap rings retaining the U-joint bearing cups.
4. Press the old U-joint out using a vise and appropriate sockets — a socket slightly smaller than the bearing cup drives the joint through, while a larger socket catches the cup on the other side.
5. Press the new joint in the same way. Ensure the bearing cups seat fully in the yoke bores and the snap rings seat correctly in their grooves.
6. Reinstall the driveshaft aligned to the marks made during removal.

Front Axle CV Joints

4WD models have CV (constant velocity) joints at the outer ends of the front halfshafts. The CV joint allows the front wheels to steer while transmitting drive torque. Signs of CV joint wear include a clicking or snapping noise during low-speed turns, particularly when turning sharply at parking lot speeds. The most common failure mode is a torn CV boot — once the boot tears, grease is lost and contaminants enter the joint, rapidly accelerating wear.

Inspect CV boots regularly, especially after lifting the suspension (which increases joint angles). A torn boot can be replaced without replacing the entire joint if caught early. Once the joint itself is clicking, full replacement is necessary.