Transmission & Drivetrain
Automatic and manual transmission operation, torque converter, clutch systems, driveshaft, U-joints, CV joints, differential, and drivetrain diagnostics.
- Explain automatic transmission operation including the torque converter and planetary gearsets
- Describe manual transmission components including the clutch and synchronizers
- Identify driveline components and their functions
- Diagnose common drivetrain vibrations, noises, and fluid conditions
Lesson 1
Automatic Transmission, Torque Converter & TCM
How an Automatic Transmission Works
The automatic transmission uses planetary gearsets, clutch packs, bands, and hydraulic circuits to select gear ratios without driver input. The Transmission Control Module (TCM) or the Powertrain Control Module (PCM) determines shift points based on vehicle speed, throttle position, and other sensor inputs.
The Torque Converter
The torque converter connects the engine to the automatic transmission. It uses fluid coupling to transmit power and can multiply torque during acceleration. The three main components are:
- Impeller (pump) - bolted to the engine flywheel, spins with the engine
- Turbine - connected to the transmission input shaft, driven by fluid from the impeller
- Stator - redirects fluid flow to multiply torque at low speeds
The torque converter clutch (TCC) or lock-up clutch mechanically locks the impeller to the turbine at cruising speeds to eliminate converter slippage and improve fuel economy. A faulty TCC causes shudder at light throttle or engine stalling at stops if it fails to release.
ATF Condition
Automatic transmission fluid (ATF) serves as hydraulic fluid, lubricant, and coolant. Normal ATF is red or pink and has a slightly sweet smell. Dark brown or black ATF with a burnt smell indicates overheating and internal damage - the transmission likely needs major service or rebuilding.
Exam Fact
Always use the manufacturer-specified ATF type. Using the wrong fluid causes harsh shifting, clutch slippage, and potential transmission failure. Never assume all ATF is interchangeable.
The torque converter uses an impeller, turbine, and stator to transmit and multiply engine torque. The lock-up clutch eliminates slippage at cruising speed. Burnt-smelling dark ATF indicates internal transmission damage.