Transmission System Overview

Transmission System Overview

The ZF 6HP26 automatic transmission has been developed for vehicles with an engine torque of up to 600 Newton-meters (Nm). This transmission uses planetary gears with hydraulic-electronic control. The Transmission Control Module (TCM) and the main control valve body unit forms a composite element that is installed as a single unit inside the automatic transmission.

The 6HP26 has the following features:
- Six forward speeds.
- A torque converter with an integral Torque Converter Clutch (TCC).
- Electronic shift and pressure controls.
- A single planetary gear set.
- A double planetary gear set.
- Two fixed multi-disc brakes.
- Three multi-plate clutches.

All hydraulic functions are directed by electronic solenoids to control:
- Engagement feel.
- Shift feel.
- Shift scheduling.
- Modulated TCC applications.
- Engine braking utilizing the coast clutch.

Engine power reaches the transmission by a torque converter with integral TCC. The 6 forward gears and 1 reverse gear are obtained from a single planetary set followed by a double planetary set also known as lepelletier-type gear sets, these gear sets make it possible to obtain 6 forward gears.

The 6HP26 automatic transmission is a six speed electronically controlled transmission comprising the basic elements of a TCM and main control valve body unit, a torque converter, one solenoid valve and six pressure regulators. Gearshift selection is achieved by the control of automatic transmission fluid flow to operate various internal clutches. The TCM operates the electrical components and provides for the control of gearshift selection shift pressure that increases refinement and torque converter slip control.

In the event of a system fault the TCM also provides for Failure Mode Effect Management (FMEM) to maintain maximum functional operation of the transmission with a minimum reduction in driver, passenger or vehicle safety. In the event of a total loss of control or electrical power the basic transmission functions park (P), reverse (R), neutral (N) and drive (D) are retained. Also third or fifth gear is retained by the hydraulic system, the gear retained is dependant upon the gear selected at time of the failure.

The transmission also contains Input Shaft Speed (ISS) and Output Shaft Speed (OSS) sensors, an internal transmission range (P, R, N, D) selector shaft position sensor, and a transmission fluid temperature sensor. The TCM also requires information from the gearshift selector to determine when the driver has initiated manual gear selection. The TCM communicates with other electronic control modules by the Controller Area Network (CAN).

The TCM also provides for legislated transmission diagnostics, which meet the requirements of CARB OBDII legislation, monitoring all components, which may effect vehicle emissions. Additional diagnostic functions are also supported to ensure fast repairs of all failures in the service environment.

Upshifts
Transmission upshifting is controlled by the TCM. The TCM receives inputs from various engine or vehicle sensors and driver demands to control shift scheduling, shift feel and TCC operation.

Downshifts
Under certain conditions the transmission will downshift automatically to a lower gear range (without moving the gearshift lever). There are three categories of automatic downshifts, coastdown, torque demand and forced or kickdown shifts.

Coastdown
The coastdown downshift occurs when the vehicle is coasting down to a stop.

Torque Demand
The torque demand downshift occurs (automatically) during part throttle acceleration when the demand for torque is greater than the engine can provide at that gear ratio. If applied, the transmission will disengage the TCC to provide added acceleration.

Kickdown
For maximum acceleration, the driver can force a downshift by pressing the accelerator pedal to the floor. A forced downshift into a lower gear is possible below calibrated speeds. Specifications for downshift speeds are subject to variations due to tire size, engine, and transmission calibration requirements.

Range Selection
Depending on the vehicle options selected the transmission range selector may have different range positions. The standard range selector has eight positions: P, R, N, D, 5, 4, 3 and 2.