Chapter 21 Allison Electronic Transmissions
Objectives (1 of 5) • Identify the three generations of Allison automatic transmissions. • Describe the operating principles of Allison partial authority transmissions. • Describe the modular design used in the WT transmission. • Outline how the WT transmission uses full authority management electronics to effect shifting and communicate with other vehicle electronic systems.
Objectives (2 of 5) • List some of the service and repair advantages of the modular construction of the WT transmission. • Group the WT modules into input, gearbox, and output categories. • Describe how the WT electronic control module masters the operation of the transmission.
Objectives (3 of 5) • Define the terms pulse width modulation, primary modulation, and secondary modulation. • Outline how the base WT transmission uses three interconnected planetary gearsets to stage gearing to provide six forward ranges, reverse, and neutral. • Identify the overdrive ranges. • Describe the WT integral driveline retarder components and operating principle.
Objectives (4 of 5) • Outline the function of the dropbox option in one WT model. • Describe the role of, and the components within, the electrohydraulic control module. • Outline the essential components in the WT electronic circuit and classify them as input circuit, processing, and output circuit components. • Describe how SAE J1939-compatible hardware and software allow WT to share componentry and data with other on-board electronic systems to optimize vehicle performance.
Objectives (5 of 5) • Describe how the WT clutches are controlled. • Outline the torque routes through the WT transmission in each range selected. • Describe how diagnostic codes are logged in the Allison WT ECU and the manner in which they are displayed. • Interpret some of the WT diagnostic codes. • Perform some basic diagnostic troubleshooting using the Allison recommended tooling.
Shop Talk • The term full authority is borrowed from the way we categorize engines. • In the early days of engine management electronics, the term partial authority was used to describe a hydromechanical system that was adapted for electronic management. • This description fits an Allison CEC transmission perfectly. A full authority engine was one that was designed to be managed by electronics. • Again, if we use these terms to categorize transmissions, full authority perfectly describes the WT transmission. It is capable of more comprehensively monitoring and controlling all transmission functions.
CEC Transmissions • The CEC has more in common with a non-computerized Allison transmission than with the newer WT electronic transmission. • The main difference is an electrohydraulic valve body that controls the hydraulic circuits using solenoids that replace the shift signal valves.
CEC Electronic Control Unit • The main power and ground inputs should be dedicated. • A minimum of 10V is required to operate the unit. • 16V continuous and 19V intermittent are the maximum voltages to which the unit should ever be subjected. • The ECU requires continuous power to retain nonvolatile RAM in which diagnostic codes and TPS calibration data are retained. Nonvolatile RAM is lost if the battery is disconnected from the unit. • The ECU is a sealed component and is not serviceable in the field. • PROM removal/replacement is the only field service permitted on Allison ECUs.
PROM Chips • Allison programmable read-only memory (PROM) chips log data that permit CEC systems to: • Be programmed for a variety of vehicle and equipment options • Have flexibility of operating characteristics • The PROM is located inside the ECU and can be accessed through a cover in the ECU housing. • PROM data is written magnetically to the chip in the same way that audio data is recorded to a cassette tape. This is the only means of logging programmed data to the CEC ECU.
Output Speed Sensor • The device has a magnetic sensor triggered by a pulse/reluctor wheel on the transmission output shaft. • Output shaft rotation drives the pulse wheel through a magnetic field, generating an AC voltage signal. • As the unit rotates at higher speeds, both the frequency and signal voltage increase. • The pulse wheel uses a 16-tooth gear for on-highway applications. • This 16-tooth wheel has become standard in all highway truck tailshaft speed sensors.
Throttle Position Sensor (TPS) (1 of 4) • With a hydromechanical engine • A throttle position sensor (TPS) is a simple potentiometer device actuated by a pull cable. • With an electronic engine • Usually only one TPS is required to signal both the Allison ECU and the engine ECM. • An interface is required. • When an Allison CEC is coupled to electronic engines, an interface module usually is required. • The function of an interface module is simply to allow the engine electronics to “talk” to the transmission electronics so that input signals such as that of the TPS can be shared.
Temperature Sensors • Temperature inputs to the ECU could result in: • Blocking of all shifts when the transmission is 225°F or below. This protects the transmission from damage that could be caused by heating up too quickly. • Limiting of transmission shifting to neutral, first range, and reverse (N, 1st, R) when temperature is between 225°F and 125°F. • When temperature exceeds 270°F, the “hot” light is energized (if equipped), a trouble code is logged (temperature code # 24, sub-code hot #23), and top gear is inhibited. Emergency vehicles such as fire trucks are usually programmed not to inhibit top gear in these circumstances.
Switches • Forward pressure switch–This switch signals the ECU when the transmission is in forward ranges. One of two types is used and it is plumbed into the clutch apply circuit. • Reverse pressure switch–This switch signals the ECU when the transmission is in reverse gear. One of two types is used, and it is plumbed into the clutch apply circuit. • Oil pressure switch–This switch signals the ECU when low fluid pressure or level exists.
Fluidic Oil Pressure Sensor • A jet of oil is directed at a sensor. • When the oil level is normal but is cold, the jet is blocked by a bi-metallic strip. • When the oil level is normal and the temperature is also normal, the jet is dispersed and doesn’t strike the sensor. • When the oil level is low and the temperature is normal, the jet is allowed to strike the sensor.
Solenoids • Non-latching solenoids have a plunger that is spring loaded to the “off” position. Non-latching solenoids, therefore, permit hydraulic flow only when electrically energized. • Latching solenoids must be momentarily energized to be switched, after which the plunger remains in position until energized again with reverse polarity. When a latching circuit solenoid is in the open position, it will permit hydraulic flow through its circuit until it receives an ECU switch command to close it.
Overview of CEC Valve Body • Latching solenoids replace the shift signal valves. • The neutral range valve is controlled by one latching and one non-latching solenoid. • The forward-reverse valve is controlled by one latching solenoid. • The trimmer regulator valve is controlled by a non-latching solenoid.
Solenoid Designations • Each solenoid receives a constant flow of main pressure. • It also is given a letter designation. • Solenoids A, B, C, and D are latching solenoids that replace the conventional shift signal valves. • Solenoids are all directly switched by the ECU.
Shop Talk • An Allison CEC transmission, although electronically controlled, is still a hydromechanical device. • As such, the importance of correct transmission fluid level cannot be emphasized enough. • If the fluid level is low, the converter and clutches will not receive enough fluid. If the fluid level is high, the fluid will aerate and the transmission will overheat.
WT Modular Construction • Input modules • Torque converter module • Converter housing module • Front support/charging pump module • Gearbox modules • Rotating clutch module • Converter housing module • P1 planetary module • P2 planetary module • Main shaft module • Output module • Rear cover module • P3 module • Output retarder or transfer gear module • Electronic control unit
Gearbox Modules • The gearbox modules combined contain five clutch assemblies and three planetary gearsets. • All WT transmissions have the hardware for six forward ratios, neutral, and reverse. • The MD and HD series transmissions are available with either close-ratio or wide-ratio gearing. • Chassis application will determine which is used. • All B series transmissions are built with close-ratio gearing. Close- or wide-ratio gearing is determined by the physical characteristics of the planetary gearsets used in the assembly of the transmission. • The upper two ranges of WT model transmissions have overdrive gear ratios.
Rotating Clutch Module (1 of 3) • The rotating clutch module is splined to the turbine shaft and, therefore, rotates with the turbine. • This module contains the turbine shaft assembly, the rotating clutch hub assembly, the C1 and C2 clutch assemblies, the rotating clutch drum, and the sun gear drive hub assembly for the P1 planetary gearset. • The P1 sun gear supplies constant rotational input to the P1 planetary set. • The C1 and C2 clutches consist of pistons, return spring assemblies, a drive hub, and clutch reaction and friction plates. • The reaction plates in both C1 and C2 clutches are splined to rotate with the rotating clutch hub assembly.
Rotating Clutch Module (2 of 3) • The C1 clutch is applied by hydraulic fluid acting on the C1 piston: This action forces the piston and the clutch reaction plates against the clutch friction plates. • Because the clutch friction plates are splined to the C1 drive hub, this will rotate providing turning force or torque at turbine speed to the main shaft assembly. • The means used to apply the C2 clutch is identical to that used to apply the C1 clutch. • Because the C2 reaction plates are splined to the C2 drive hub, when the C2 piston is hydraulically actuated, the C2 clutch transmits torque to the P2 planetary assembly.
Rotating Clutch Module (3 of 3) • A balance piston is used to enhance control of off-going and on-coming rotating clutches. The balance piston is the third piston in the rotating clutch assembly and is located between the C1 spring assembly and the C1 pressure plate. • The balance piston entraps fluid (lubrication pressure) between itself and the C1 piston, thereby balancing piston movement against exhaust backfill pressure behind the C1 piston.
P1 Planetary Gearset Module • The P1 is the first of the three planetary gearsets in the transmission. The three sub-components of the gearset are arranged as follows: • Sun gear. The sun gear is driven by the rotating clutch drum. • Planetary carrier. Its pinions mesh internally with the P1 sun gear and externally with the P1 ring gear. The ring gear of the P2 carrier is splined to the P1 planetary carrier. • Ring gear. The ring gear is housed in the C3 clutch assembly. • Rotation of the P1 planetary gears is controlled by the application of the C3 or C4 clutches.
P2 Planetary Gearset Module • The P2 planetary gearset is the second in the WT transmission. Its three sub-components are arranged as follows: • Sun gear. The sun gear is splined to the transmission mainshaft. • Planetary carrier. Its pinions are meshed with the sun gear internally and with the ring gear externally. • Ring gear. The ring gear is spline-coupled to the P1 carrier. • P2 planetary gearset rotation is controlled by the action of the P1 planetary gearset and by the application of either the C4 or C5 clutches. Input rotation to the P2 planetary carrier can either be provided by the P2 ring gear or by the P2 sun gear, which is splined to the transmission mainshaft.
Output Retarder Module • The action of the retarder can be compared to that of a torque converter operating in reverse. • When the retarder is activated, it is charged with transmission fluid stored in the external accumulator. • When the retarder is not in use, transmission fluid is drained from the retarder housing. • For the retarder to be applied, the following is required: • The vehicle dash-mounted Retarder Enable switch must be on. • The vehicle must be moving. • The TPS signal must indicate that throttle travel is close to zero. • The ABS system must not be activated.
Retarder Controls • WT retarders are fitted with a variety of apply devices. • Hand lever • Foot pedal • Pressure switch • Auto apply—auto full-on • Auto apply—auto percent-on • Combination
Transfer Gear/Dropbox • It consists of an output adapter housing, transmission output shaft adapter, transfer case, transfer case charging oil pump, C6 clutch assembly, and a transfer case scavenging pump (auxiliary pump) mounted at the right side PTO provision. • The addition of the dropbox adds a forward gear to the WT transmission and provides for full-time, all-wheel drive. • The transfer case is mounted to the adapter housing and is under-slung below the main transmission to provide for forward and rear drive yokes in line with the transmission. • The transmission output shaft adapter splines to the P3 carrier hub.
WTEC Electronics • Electronic management of the WT transmission is similar to the CEC, with some refinements. • There is an increased ability to interact with other on-board electronic systems and better programmability to suit specific vehicle applications. • The system is networked to the chassis data bus by a J1939 connection. • The latest generation of WTEC has the ability to broadcast range inhibit, check transmission light data, and send range status to the data bus for display onto the appropriate dash displays.
Input Circuit Components • Shift selector(s) • Throttle position sensor • Engine speed sensor • Turbine speed sensor • Output speed sensor • C3 pressure switch • Sump temperature sensor • Coolant temperature • Vehicle interface module (VIM)
Input Signals • The following is a partial list of the input signals the WT ECU may be programmed to process: • Secondary shift schedule • PTO enable • Shift selector transition • Engine brake and pre-select request • Fire truck pump mode
WT Output Circuit Components • The following are WT ECU switched output circuit components: • A–E clutch solenoids • Lockup clutch solenoid F • Forward latch solenoid G • Vehicle interface module (VIM) • PTO functions
Output Signals • The following is a partial list of the output signals that WT electronics may be programmed with to supply other chassis electronic management systems. • Engine brake enable • Sump/retarder temperature indicator • Range indicator • Output speed indicator • PTO enable • Engine overspeed indicator • Two-speed axle enable • Lockup indicator • Service indicator • Shift-in-progress indicator • Retarder indicator • Neutral indicator for PTO
Input Circuit • Input from the operator is sent to the ECU by means of the shift selector. • Other input signals come from sensors located in the transmission itself and from the vehicle interface module (VIM). • The VIM handshakes data exchange (both ways) between WT electronics and other vehicle electronic management systems using SAE J1939 communication protocols and hardware. • Temperature sensors, shaft speed sensors, and pressure switches all monitor conditions within the transmission that are required to compute an outcome. • Command input signals are usually operator-generated and often require the ECU to make a change in outcome.
Shift Selectors • A lever and two types of push-button shift selector are available. • The full function non-strip type push-button selector allows the operator to select any range programmed to WTEC. • Using these buttons does not override the automatic shifting function of the transmission. • The operator cannot select a shift schedule that could result in damage to either the transmission or drivetrain. • When the D range position is selected, WTEC will manage shift schedules using all the ranges programmed to WTEC. • Selection of any of the numeric ranges will confine shifting within the ranges below the selected range.
Vehicle Interface Module (VIM) • The VIM contains two 10-amp fuses and between two and six relays. • WT electronics will operate on either 12- or 24-volt. • One of the 10-amp fuses protects the main power connection to the ECU. The other protects the feed from the ECU to ignition circuit. • One relay is used to output a signal to the reverse warning circuit and another is used for the neutral start circuit. • As many as four other relays may be used for special function outputs.
Throttle Position Sensor (TPS) • A dedicated TPS is only required when a WT transmission is used with a hydromechanical engine. • In cases in which an electronically managed engine is used, WT electronics simply shares the TPS signal with the engine by means of the data bus. • The TPS should be of the potentiometer type. • The potentiometer is a three-wire variable resistor that converts position to a voltage value by sliding contacts across a resistive strip. • The TPS used with WT and CEC transmissions is a common component.
Output Circuit • The output circuit is responsible for effecting the outcomes of the processing cycle of the ECM. • In the Allison WT, the results of ECM processing are converted into actions by an electro-hydraulic control module. • This is mounted at the base of the transmission, and the channel plate forms its bottom enclosure. • Solenoids are used to convert electrical signals into the hydromechanical outcomes required to enable shifting.
Solenoids • Two types of solenoids are used in the WT electrohydraulic control module. • Both are somewhat similar in construction but differ in their switching characteristics. • Both types receive control main pressure from a supply port and may either route that pressure to exit through the solenoid regulator valve or to an exhaust port, depending on the switch status.
Normally Closed (NC) Solenoids • Closed until energized: An NC solenoid remains closed until energized by an ECM electrical signal. • Pressure is exhausted: When the solenoid is not energized, main pressure fluid is blocked by the seated check ball, routing it directly to the exhaust passage. • Pressure is applied: When energized, the check ball unseats, blocking the exhaust port and permitting main pressure fluid to exit to the solenoid regulator valve.
Normally Open (NO) Solenoids • Open until energized: An NO solenoid remains open until energized by an ECU electrical signal. • Pressure is applied: When the solenoid is not energized, the check ball is unseated, allowing main pressure fluid to flow into the solenoid through the supply passage and be routed to the solenoid regulator valve. • Pressure is exhausted: When the normally open solenoid is energized, the check ball is forced onto its seat. This blocks the passage to the regulator valve and routes main pressure to the exhaust.
Pulse-Width Modulation (PWM) • WT solenoids are controlled by pulse width modulated (PWM) signals. • Primary modulation controls the amount of on time. • The primary modulation signal to WT solenoids is delivered at a frequency of 63 Hz. • The percentage of on time during each 1⁄63 second is referred to as the solenoid duty cycle. • A 100% duty cycle would represent the maximum signal. • A 0% duty cycle would indicate no signal.
The WTEC Diagnostic Tools • The Allison electronic service tools (EST) required to read and reprogram WTEC electronics • MPSI ProLink digital diagnostic reader (DDR) • The Allison digital optimized connection (DOC) software used with a personal digital assistant (PDA)
The primary components of the hydraulic system Transmission fluid Charging pump Three integral filters Electrohydraulic control module Breather C3 pressure switch There are seven sub-circuits that make up the hydraulic circuit. Main pressure circuit Control main circuit Torque converter circuit Cooler/lubrication circuit Clutch apply circuit Exhaust circuit Exhaust backfill circuit The WT Hydraulic Circuit
Hydraulic Circuit OperationDuring an Electrical Failure • An interruption of electrical power will result in the solenoid regulator valves locking in their NO or NC states. • To minimize the impact of an electrical failure, the WT transmission defaults to totally hydraulic operation. • The C1 and C2 latch valves are used to accomplish this default mode of operation. • When an electrical failure occurs, the latch valves engage specific clutches based on the range the transmission was in when the failure occurred. • In case of an electrical failure, the latch valves and the two NO solenoids, A and B, default to “limp home” mode by reverting to total hydraulic operation.
Torque Paths and Range Management • In each range, the gearing is staged through the transmission. • The stages are sequenced numerically, but the stages of gearing do not necessarily correlate with the numbering of the planetary gearsets. • For instance, first range gearing is single stage and takes place in the P3 planetary gearset. • As many as three stages of gearing may be used, depending on the range selected.