Transmission, Driveline, and Axle Assemblies
Power to the wheels
Driveline diagnosis follows the path of torque: engine, torque converter, transmission, driveshaft, differential, axles. On an automatic-equipped transit bus, the transmission fluid check has a procedure that must be followed exactly, typically at operating temperature, engine running, transmission in neutral, on level ground, after cycling the ranges. Checking with the engine off gives a false high reading and leads to overfilling, which causes foaming and overheating.
Inspect for leaks at the pan, cooler lines, output seal, and retarder. Note fluid color and smell on the dipstick; dark, burnt fluid with clutch material signals internal wear, and a fluid sample is better evidence than opinion. Verify the selector, neutral-start interlock, and retarder operation during a road test.
Underneath, check U-joints for play and dry, rusty bearing caps; slip-yoke splines for wear and adequate grease; the carrier bearing and its mount; and driveshaft phasing, since yokes must be in line or vibration results. Grease U-joints until clean grease purges, but stop before seals are blown.
At the axle, check differential oil level at the fill plug, full when lubricant is at the bottom of the hole; look for leaks at the pinion and wheel seals; and inspect the magnetic drain plug for chips and paste at oil change.
Worked rule of thumb: a vibration that changes with road speed but not with engine rpm points to the driveline, not the engine.
Trap: overgreasing is a real failure mode, pushing seals out and contaminating brake linings.