ZF 6HP26 Operation
Transmission Description
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 units form 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 converter lock up clutch.
^ 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 torque converter clutch (TCC) applications.
^ engine braking utilizing the coast clutch.
Engine power reaches the transmission by a torque converter with integral converter lock up clutch. 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. Gear selection is achieved by the control of Automatic Transmission Fluid (ATF) flow to operate various internal clutches. The TCM operates the electrical components and provides for the control of gear selection shift pressure which 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, Reverse, Neutral and Drive are retained. Also 3rd or 5th gear is retained by the hydraulic system, the gear retained is dependent upon the gear selected at time of the failure.
The transmission also contains turbine and output shaft speed sensors, an internal P, R, N, D selector shaft position sensor, and a transmission fluid temperature sensor. The TCM also requires information from the J-Gate 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 OBD II 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 torque converter clutch (TCC) operation.
The TCM has an adaptive learn strategy to electronically control the transmission which will automatically adjust the shift feel.
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 and 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.
J-Gate Range Selection
"P"
In the PARK position:
^ there is no power flow through the transmission.
^ the parking pawl locks the output shaft to the case.
^ the engine may be started.
^ the ignition key may be removed.
"R"
In the REVERSE position:
^ the vehicle may be operated in a rearward direction, at a reduced gear ratio.
^ backup lamps are illuminated.
"N"
In the NEUTRAL position:
^ there is no power flow through the transmission.
^ the output shaft is not held and is free to turn.
^ the engine may be started.
"D"
DRIVE is the normal position for most forward driving.
The D position provides:
^ automatic shift 1-6 and 6-1.
^ apply and release of the torque converter clutch.
^ maximum fuel economy during normal operation.
^ engine braking in 6th gear.
"5"
The 5 position provides:
^ automatic shift 1-5 and 5-1.
^ apply and release of the torque converter clutch.
^ engine braking in 5th gear.
"4"
The 4 position provides:
^ automatic shift 1-4 and 4-1.
^ apply and release of the torque converter clutch.
^ engine braking in 4th gear.
"3"
The 3 position provides:
^ automatic shift 1-3 and 3-1.
^ engine braking in 3rd gear.
"2"
The 2 position provides:
^ automatic shift 1-2 and 2-1.
^ engine braking in 2nd gear.
"S"
The sport mode switch:
^ allows the driver to select or de-select the automatic transmission sport mode.
^ allows the automatic transmission to operate normally when the sport mode is selected, but under acceleration the gear shift points are extended to make full use of the engine's power reserves.
^ allows the driver to drive the vehicle in the ' D' position with the full automatic transmission shift or manually shift gears in the 'second, third, fourth and fifth' positions.
^ is illuminated when Sport mode is selected.
^ communicates with the TCM through the CAN network to show the sport mode switch status.
Torque converter
The torque converter is a three element unit containing a single plate lock up clutch. The lock up clutch can be controlled and engaged in any gear 1 to 6. The clutch is applied by removing transmission fluid pressure from one side of the plate. The torque converter transmits and multiplies torque. The torque converter is a three-element device:
^ impeller assembly
^ turbine assembly
^ reactor assembly
The standard torque converter components operate as follows:
^ The impeller, which is driven by the engine, imparts a circular flow to the transmission fluid in the converter.
^ This transmission fluid strikes the turbine wheel, which causes the flow to change its direction.
^ The transmission fluid flows out of the turbine wheel close to the hub and strikes the stator, where its direction is changed again to a direction suitable for re-entering the impeller.
^ The change in direction at the stator generates a torque reaction that increases the torque reaching the turbine.
^ The ratio between turbine and impeller torque is referred to as torque multiplication or conversion.
^ The greater the difference in speeds of rotation at the impeller and turbine, the greater the increase in torque-, The maximum increase is obtained when the turbine wheel is stationary. As turbine wheel speed increases, the amount of torque multiplication gradually drops.
^ When the turbine wheel is rotating at about 85% of the impeller speed, torque conversion reverts to 1, that is to say torque at the turbine wheel is no higher than the torque at the impeller.
^ The stator, which is prevented from rotating backwards by a freewheel and the shaft in the transmission housing, runs freely in the transmission fluid flow and overruns the freewheel. From this point on, the converter acts only as a fluid coupling. During the torque conversion process, the stator ceases to rotate and bears against the housing by the freewheel.
Torque converter
Torque Converter Lock-up Clutch
The torque converter lock-up clutch is a device that eliminates slip in the torque converter and therefore helps to keep fuel consumption to a minimum.
The torque converter lock-up clutch is engaged and released by the control system. During the actuating phase, a slight difference is selected between the impeller and turbine wheels.
Pressure at the torque converter lock-up clutch piston is determined by an electronic pressure control valve.
The torque converter lock-up clutch can be controlled and engaged in any gear from 1 to 6. When decoupling takes place the actuating clutch A in the transmission is dependent on load and output speed.
When the torque converter lock-up clutch is released, transmission fluid pressures behind the lock-up clutch piston turbine area are equalized. The direction of flow is through the turbine shaft and the area behind the piston into the turbine area.
To engage the torque converter lock-up clutch the direction of transmission fluid flow is changed and reversed by a valve in the hydraulic control unit. At the same time the space behind the torque converter lock-up clutch piston is vented.
Oil pressure extends from the turbine area to the torque converter lock-up clutch piston and presses it against the cover outer shell of the torque converter. This locks the turbine wheel by way of the lined disc between the piston and the cover and enables the drive to pass with limited slip to the planetary gear train in normal operating conditions.
Geartrain
Power is transmitted from the torque converter to the planetary gearsets through the input shaft. Clutches are used to hold and drive certain combinations of gearsets. This results in six forward ratios and one reverse ratio, which are transmitted to the output shaft and differential.
Gear Ratio
Single Planetary Gearset
The single planetary gear overdrive carrier is driven by the input shaft.
The single planetary gear set consists of:
^ 1 sunwheel
^ 4 planetary gears meshing with the sunwheel
^ 1 planetary gear carrier
^ 1 ring gear
Single Planetary Gearset
Double Planetary Gearset
The double planetary gearset is splined to the output shaft.
The double planetary gear set consists of:
^ 2 sunwheels of different sizes
^ 3 short planetary gears meshing with the sunwheels
^ 3 long planetary gears meshing with the sunwheels
^ 1 planetary gear carrier
^ 1 ring gear
Double Planetary Gearset
Apply Components
Shift Elements
The other shift elements in addition to the torque converter lock-up clutch are:
^ three rotating multi-plate clutches A, B and E.
^ two fixed multi-disc brakes o C and D.
All gear shifts from 1st to 6th or from 6th to 1st are power-on overlapping shifts, that is to say during the shift one of the clutches must continue to transmit the drive at lower main pressure until the other clutch is able to accept the input torque.
The shift elements, clutches or brakes are engaged hydraulically. The transmission fluid pressure is built up between the cylinder and the piston, this presses the clutch plates together.
When the transmission fluid pressure drops, the cup spring pressing against the piston moves it back to its original position.
The purpose of these shift elements is to perform in-load shifts with no interruption to traction.
Multi-plate clutches A, B and E supply power from the engine to the planetary gear train-, multi-disc brakes o C and D bear against the transmission housing in order to achieve a torque reaction effect.
Multi Plate Clutch
Clutch E is equalized in terms of dynamic pressure, that is to say its piston is exposed to the transmission fluid flow on both sides, in order to prevent pressure build up in the clutch as the speed increases. This equalization process is achieved by a baffle plate and pressure-free transmission fluid supply by a lubricating passage, through which the space between piston and baffle plate is filled with transmission fluid.
The advantages of this dynamic pressure equalization are:
^ reliable clutch engagement and release in all speed ranges.
^ improved shift refinement.
Multi Plate Clutch
Shift overlap control
When overlap gearshift takes place, freewheels (one-way clutches) are not used but are replaced by suitable actuation of the relevant clutches. This both enables weight and space to be saved.
The electronic-hydraulic shift action is obtained by means of various valves in the transmission control module (TCM) and main control valve body, actuated by pressure regulators. They engage or disengage the relevant clutches or brakes at the correct moments.
Output is always by the ring gear of the second, downstream planetary gear set.
Hydraulic System
Fluid Pump
The fluid pump is of a "half-moon" pattern and delivers approximately 16 square cm of transmission fluid per revolution.
It is located between the torque converter and the transmission housing.
The torque converter is supported in the fluid pump by a needle roller bearing. The fluid pump is driven directly from the engine by the torque converter shell and supplies transmission fluid to the transmission and the hydraulic control unit.
The fluid pump draws in transmission fluid through a filter and delivers it at high pressure to the main pressure valve in TCM and main control valve body unit. This valve adjusts the pressure and returns excess transmission fluid to the fluid pan.
Fluid Pump
Fluid Pan, Gasket and Filter
The transmission fluid pan, gasket and filter is a one piece assembly, all transmission fluid is drawn from the transmission fluid pan by the fluid pump and passes through the fluid filter.
Transmission Control Module (TCM) and Main Control Valve Body
Electrostatic Discharge (ESD)
CAUTION: When working with the transmission control module (TCM) and main control valve body, all suitable safety precautions must be taken to protect the component against electrostatic discharge (ESD). Failure to follow these instructions may result in component damage.
Make sure all possible safety precautions are taken to protect the TCM and main control valve body unit against ESD.
Personal Wrist-Band Earthing
Earthing (grounding) by means of a wrist band or strap is the most reliable method of diverting electrostatic charges away from working personnel, and should therefore be used wherever possible, particularly if the person concerned is working while seated. The wrist band earthing (grounding) device consists of a bracelet closely attached to the wrist and a spiral earthing (grounding) cable connecting it to the earthing (grounding) contact point. This system must include a quick release service so that the wrist can be released in the event of danger.
Shoes and Foot Earthing Straps
Electrically conductive shoes should be worn by persons who mainly work standing up or either standing or sifting in ESD protection zones, particularly if wrist band earthing (grounding) is impracticable. The standard calls for ESD shoes to record values between 0 and 35 Mega-ohms (MOhm) resistance. However, for antistatic working shoes resistance values between 0.1 and 1000 MOhm are called for, and a through-conducting resistance for protective shoes of 0.1 to 100 MOhm. A lower limit value of not less than 0.1 MOhm must be maintained on account of the contact voltage risk. For this reason the minimum value has been set contrary to the standard at the higher figure of 0.75 MOhm.
Transmission Control Module (TCM) and Main Control Valve Body
The transmission control module (TCM) and main control valve body is a combination of hydraulic and electronic control units. Both these modules are installed in the transmission, in the fluid pan.
This technical principle has the following advantages:
^ Minimum tolerances (TCM) is mated to solenoids)
^ Better coordination of gear shifts
^ Increased refinement
^ Optimized shift quality
^ Good reliability, since the number of plug connections and interfaces is reduced.
Transmission Control Module (TCM) and Main Control Valve Body