A customer brought in a four-wheel-drive wheel loader complaining of incredibly aggressive tire wear on the front axle. The front tires were being scalloped and shredded within a few hundred hours, and the machine was practically fighting the operator when trying to turn on asphalt. The operator mentioned that the "diff lock" light on the dash wasn't illuminating when he pressed the button, but he assumed the feature was just broken.
When a diff lock light doesn't come on, most mechanics assume the system is disengaged. In this case, the exact opposite was true. On this particular loader, the differential lock is engaged by a hydraulic piston pushing a sliding dog clutch inside the front axle. When you press the button on the dash, an electric solenoid sends pilot hydraulic pressure to engage the lock. When you turn it off, a return spring is supposed to push the piston back and disengage the clutch.
Because this machine spent its life in a sandy, muddy environment, the fine silt had packed its way into the cavity where that sliding clutch lives. The silt mixed with the axle oil and turned into a thick, abrasive paste. This paste had packed behind the return spring and completely immobilized the dog clutch in the locked position. When the operator pressed the button, the solenoid tried to send pressure, but because the clutch was already physically locked together, nothing moved, which is why the dashboard sensor didn't trigger.
We had to drop the front axle cover, chisel out the packed, concrete-like mud, and manually free the sliding collar. We cleaned the bore, replaced the return spring (which had rusted solid from the moisture in the mud), and put it back together. The key takeaway: if a four-wheel-drive machine is eating tires and the diff lock light isn't working, never assume the axle is unlocked. Always manually check the mechanical linkage or pull an inspection cover. Driving a loader on hard ground with a permanently locked front differential will destroy the tires, the axle shafts, and the ring and pinion gears in a matter of weeks.
