5.2 Automatic Transmission & Transaxle Parts
Key Takeaways
- The torque converter acts as a fluid coupling between the engine and transmission, utilizing an impeller, turbine, stator with one-way clutch, and a lockup clutch (TCC).
- Planetary gear sets produce varying gear ratios, reverse, and direct drive through selective application of multi-plate wet clutch packs, brake bands, and sprag clutches.
- Electronic valve bodies route pressurized fluid using pulse-width modulated (PWM) shift solenoids, line pressure solenoids, and speed sensors monitored by the TCM.
- Modern automatic transmissions require strict fluid compliance (e.g., Dexron VI, Mercon LV, ATF+4, CVT NS-3); using incorrect fluid causes friction material degradation and shift shudder.
Automatic Transmission & Transaxle Parts
Torque Converter Assembly & Hydraulic Fluid Coupling
An automatic transmission or transaxle relies on a hydrodynamic fluid coupling known as the Torque Converter to transfer engine torque to the transmission input shaft without a mechanical foot clutch.
Internal Components of the Torque Converter
The torque converter housing is bolted directly to the engine flexplate (drive plate) and rotates at engine speed. Inside the sealed steel assembly are four primary components:
- Impeller (Pump): The outer rear housing containing curved radial fins. As the engine rotates, centrifugal force throws automatic transmission fluid (ATF) outward toward the turbine.
- Turbine: Splined directly to the transmission input shaft. The high-velocity fluid thrown from the impeller strikes the turbine blades, causing the turbine—and input shaft—to rotate.
- Stator: Positioned between the impeller and turbine. Contains a one-way overrunning clutch (sprag or roller clutch). When fluid leaves the turbine, it travels in a direction that opposes impeller rotation. The stator redirects this returning fluid back into the impeller in the direction of engine rotation, creating torque multiplication (up to 2.1:1 ratio) during acceleration from a stop. As vehicle speed matches engine speed (coupling phase), the stator overruns and freewheels.
- Torque Converter Clutch (TCC): A hydraulically applied internal friction disc that mechanically locks the turbine to the converter cover at highway speeds. TCC engagement eliminates fluid slip, improving fuel economy and lowering ATF temperatures.
Flexplate and Torque Converter Counter Specialist Knowledge
The Flexplate connects the engine crankshaft to the torque converter. It is a thin stamped-steel disk with a welded outer ring gear for starter engagement. Parts specialists must check flexplates for stress cracks around the crankshaft bolt circle, elongated torque converter mounting holes, or ring gear tooth damage. When supplying a replacement torque converter, specialists must advise technicians to verify converter hub depth and fully seat the converter drive notches into the transmission fluid pump gears prior to bellhousing bolt installation to prevent catastrophic pump destruction.
Planetary Gear Sets, Friction Elements & Hydraulic Actuation
Automatic transmissions achieve multiple forward gear ratios and reverse using compound Planetary Gear Sets controlled by friction and holding devices.
Planetary Gear Mechanics
A simple planetary gear set consists of three primary elements:
- Sun Gear: The central gear.
- Planet Pinion Gears & Carrier: Multiple small gears mesh with the sun gear, supported by a rotating carrier frame.
- Ring Gear (Annulus): The outer internal-toothed ring encircling the pinions.
By holding one element stationary (via a brake band or clutch pack) and driving another element, the third element provides output at reduced speed (underdrive), increased speed (overdrive), or reverse direction.
Holding and Driving Devices
- Multi-Plate Wet Clutch Packs: Alternating friction discs (steel plates faced with paper/graphite friction material) and steel separator plates housed inside a drum. Hydraulic pressure applied against a ring piston compresses the pack, locking two rotating shafts together.
- Brake Bands: Flexible steel bands lined with friction material wrapped around the outside of a clutch drum. A hydraulic servo piston tightens the band to hold the drum stationary against the transmission case.
- One-Way (Sprag/Roller) Clutches: Mechanical holding devices that hold a gear element in one direction of rotation while allowing freewheeling in the opposite direction, enabling smooth gear transitions without timing hydraulic releases.
Hydraulic Controls, Valve Body & Solenoids
The hydraulic system is the brain and muscle of the automatic transmission.
Transmission Fluid Pump & Pressure Regulation
Driven directly by the torque converter hub, the internal gear or vane pump generates line pressure (ranging from 60 PSI at idle up to 300+ PSI under heavy load). Line pressure is regulated by a pressure regulator valve controlled by a Line Pressure Control Solenoid (Electronic Pressure Control - EPC).
Valve Body Assembly & Solenoid Controls
The Valve Body contains a labyrinth of cast hydraulic passages, spring-loaded spool valves, check balls, and separator plates.
- Shift Solenoids: Electro-hydraulic actuators operated by the Transmission Control Module (TCM). They open or close hydraulic circuits to direct fluid to specific clutch packs or band servos. Solenoids operate as simple ON/OFF valves or Pulse-Width Modulated (PWM) duty-cycle valves for smooth engagement.
- Sensors: The TCM monitors Input Shaft Speed (ISS) and Output Shaft Speed (OSS) sensors to calculate gear ratio slip and shift timing. The Transmission Fluid Temperature (TFT) sensor alters shift strategies during cold warm-up or high-heat conditions.
| ATF Specification | Base Fluid / Type | Primary Application Scope | Key Chemical Features |
|---|---|---|---|
| Dexron VI | Full Synthetic / Low Viscosity | GM 6-speed/8-speed/10-speed automatics | High shear stability, extended drain intervals |
| Mercon LV / ULEV | Synthetic Low / Ultra-Low Viscosity | Ford 6R80, 10R80 automatics | Low temperature fluidity, anti-shudder friction |
| ATF+4 | Synthetic Group III | Chrysler / Dodge / Jeep automatics | High shear stability, specific anti-shudder additives |
| Multi-Vehicle Synthetic | Synthetic Blend / Full Synthetic | Broad aftermarket replacement | Balanced friction coefficient for diverse clutches |
| CVT Fluid (NS-2 / NS-3) | Non-DHP Specialized Synthetic | Push-belt Continuously Variable Transmissions | High steel-on-steel friction, anti-foaming agents |
| DCT Fluid (DCTF) | Synthetic Dual-Clutch Fluid | Wet/Dry Dual-Clutch Transmissions | High thermal stability, synchronizer protection |
Maintenance, Filtration & Fluid Requirements
Automatic Transmission Fluid (ATF) performs four vital functions: power transmission in the torque converter, hydraulic control, friction modulation for clutches, and component lubrication/cooling.
Fluid Compatibility & Filter Replacement
Using non-specified ATF alters friction coefficient characteristics, leading to clutch slippage, shift shudder, or seal degradation. Modern low-viscosity fluids (e.g., Dexron VI, Mercon LV) cannot be substituted with older high-viscosity formulations like Dexron III/Mercon.
- Filters & Gaskets: Transmission service kits include a pan gasket (reusable steel-core elastomeric or high-grade rubber) and a suction filter (felt medium or fine mesh screen). Parts specialists must remind technicians to clean magnet pads inside the oil pan to capture ferrous wear particles and to inspect cooler lines for restriction. When replacing a failed transmission, flushing the auxiliary fluid cooler with approved solvent or installing a new cooler is essential to prevent trapped metal debris from destroying the replacement unit.
In a torque converter assembly, which component is mounted on a one-way overrunning clutch and redirects fluid returning from the turbine back into the impeller to multiply torque?
A customer is purchasing an automatic transmission replacement filter and pan gasket kit. What critical advice should a parts specialist provide regarding transmission fluid cooler maintenance during a major transmission replacement?
Which element of a simple planetary gear set must be held stationary by a brake band or clutch pack while driving the sun gear to achieve a speed reduction (underdrive) forward gear ratio?
An automatic transmission exhibits erratic shifting and an illuminated check engine light. Scan tool diagnostics show code P0730 (Incorrect Gear Ratio). Which transmission sensor measures the rotational speed of the input shaft for slip calculations?