1. Torque Converter Issues

• Typical symptoms: Engine runs normally under load, but the machine feels weak or does not move.

• Key checks:

  • Is the torque converter output shaft rotating?

  • Does the engine produce black smoke (indicating overload)?

  • Do tires/tracks slip (is torque actually being transmitted)?

• Practical tip:

  • If the torque converter slips or fails, the machine usually works under light load but stalls under heavy load.

  • The CAT988B case is a classic example of torque converter internal failure.

2. Transmission Issues

• Typical symptoms:

  • Certain gears fail (e.g., reverse gear disappears or is weak).

  • Gear pressure is low, but the engine does not bog down.

• Key diagnosis steps:

  • First check the transmission pump suction line (air leaks, clogged filters, cracked hoses).

  • Then check system pressure and flow. Having sufficient pressure but insufficient flow can cause misdiagnosis if flow is ignored.

  • If only certain gears fail, most likely due to piston seal leakage or clutch pack internal leakage.

3. Suction System Problems (High-Frequency Faults)

• Typical symptoms:

  • More noticeable when cold or in winter.

  • Long delay in building pressure after start-up, or no pressure until much later.

  • Weak transmission, with air bubbles in the oil circuit.

• Case examples:

  • CAT D7G: After replacing the propeller shaft, the aged suction hose cracked → air ingress.

  • Non-genuine hose replacement → insufficient flow in winter → similar “oil starvation” problem.

• Practical tips:

  • Air only, no oil → suction line air leak.

  • Problem worse at higher throttle → insufficient oil supply.

4. Pressure Judgment Techniques

• Overall low pressure → first check suction line, then pump wear.

• Large hot vs. cold pressure difference (>10%) → pump or valve clearances are worn.

• Low pressure in specific gears only → internal leakage (seals, clutch packs).

• No visible external leakage → most likely overlooked during inspection (a reminder not to get careless).

5. Value of Experience and Test Equipment

• Relying only on part-swapping is costly and often inaccurate.

• Self-made or modified test benches (mobile hydraulic test stands, common rail benches, intercooler power test stands, etc.) greatly improve diagnostic efficiency.

• Many seemingly “major” failures can actually be caused by small issues like loose bolts or aged suction hoses.