A customer brought in a medium wheel loader that was experiencing a violent shudder and a loud clunk from the drivetrain whenever the operator tried to drive the bucket into a compacted pile of dirt or gravel. It felt like the transmission was slipping and then violently grabbing, but the fluid was clean, full, and less than a hundred hours old.
When a loader exhibits this kind of behavior under a heavy load, a lot of technicians immediately suspect the torque converter or the transmission clutches. However, we started by putting the machine up on jack stands and locking out the parking brake. When we put it in forward gear and manually spun the rear wheels, they turned smoothly. But when we tried to spin the front wheels, they were completely locked together.
This loader had a front-wheel-drive assist that was supposed to engage automatically only when the rear wheels slipped. The problem was in the front axle's differential. Inside the front axle, there is a cone clutch or a wet disc pack designed to slip when the machine turns corners, allowing the left and right front wheels to rotate at different speeds. The hydraulic piston that applies this clutch had seized in the fully engaged position due to contaminated axle oil and microscopic metal shavings from a failing pinion bearing. Because the front differential was permanently locked, the moment the loader turned slightly to enter a dirt pile, the inside front wheel was forced to drag. The torque would build up until the tire finally lost traction and skipped violently across the ground, causing the whole machine to jump. We disassembled the front axle, replaced the seized apply piston and the damaged pinion bearing, and flushed the axle housing. The key takeaway: if a four-wheel-drive loader shudders under load, put it on stands and check the front axle lock-out before you tear into the transmission.
