The Tomy Big Loader (ビッグローダー) is classic automaton-type toy in the vein of shuttle trains in which a powered chassis car traverses a series of tracks switching between different vehicle bodies and interacting with scoops, hoppers, lifts, and other components usually with the goal of cycling several small balls around between the different vehicles and accessories.
The Big Loader power car is rather ingenious but with time is prone to failure. The most common issues are an electrical problem, usually when batteries have been left inside the chassis and they have corroded, or a physical issue with one or more of the gears in the geartrain or the traction tires being worn out.
Three screws hold the bottom of Big Loader's chassis on. Later on, the three cross-head screws were changed for triangular-headed security screws, seemingly in part because parents continued to unscrew the bottom of the chassis and drop all the gears on the floor when they just mean to put in the batteries.
The geartrain inside includes an interesting mechanical reverser that also uncouples the output of the wheels from the input, leaving the chassis free-wheeling when unpowered. The motor pinion drives a contrate gear which has an integrated smaller pinion. Another gear that rotates inside a slot in a drum which rotates freely on the shaft sits against this integrated gear. A small spring sits on the stem of the gear, keeping it pressed against the other cog integrated into the contrate gear. This drum spins around freely until the catches on its surface catch with the reversing lever - this stops the "planet" gear riding against the gear integrated into the contrate gear and either engages with the clutch gear on the drive shaft directly or through a reversing idler that spins freely on the shaft the powered wheels are on, reversing the direction of the output shaft. The drive shaft has brass bushings that I have seen in a few different orientations as the tooling and mechanism were adjusted over time.
The output shaft has a clutch mechanism to let the drive wheels slip if held in place while powered as well as the gear that drives the gear integrated into one of the rear wheels. The entire rear wheel assemble is pinned through a holder plate that holds two small gears that are positioned to engage with the racks and cogs sticking up out of the Big Loader tracks. The reversing idler is also free-spinning on the axle.
The most common gears to split are the smaller ones, namely the 8 tooth gear on the 2mm motor shaft and the 12 tooth gear on the 2mm shaft that drives the drive wheel cluster. Another problem on the same shaft as the 12 tooth gear that engages with the drive wheel is the clutch mechanism holding the other gear on the shaft in place - a curvy interface between a collar and the gear is under spring compression and if the collar ring splits the spring will push the gear up against the plastic casing and the chassis will not operate properly. Unlike the gears, which usually do not mesh properly if reglued, these collars can usually be glued in place to fix this issue.