Late model Chrysler A-500/A-518 transmissions and the transaxles used in most Chrysler/Mitsubishi vehicles rely on electronics to control the shifting into overdrive gear. Electronics are also involved in the control of the converter clutch in all late-model Chrysler transmissions.

The A-500 and A-518 transmissions are fully automatic units with an overdrive attached to the rear of the transmission. The first through third gear shifts are controlled hydraulically through apply devices in the main body of the transmission.

Fourth gear operation is controlled by a manually operated overdrive switch on the instrument panel, center console, or shift lever. The overdrive switch is wired into a circuit with an overdrive solenoid, located on the valve body, and the single board engine controller (SBEC). The torque converter clutch lockup solenoid is also wired into this circuit. The overdrive switch prevents a shift into fourth gear when the switch if off. Therefore, the operation of the torque converter clutch is also inhibited when the overdrive switch is off.

The shift from third to fourth gears is electronically controlled and hydraulically activated by the SBEC. The SBEC controls the overdrive solenoid using information received from several sensors.

Chrysler A-604/41TE Controls The A-604/41TE transaxle is an electronically controlled 4-speed transaxle. The transaxle uses hydraulically operated clutches, which are controlled by the transaxle controller. The controller receives information from various inputs and controls a solenoid assembly through the electronic automatic transaxle (EATX) relay. The solenoid assembly consists of four solenoids that control hydraulic pressure to four of the five clutches in the transaxle and to the lockup clutch of the torque converter.

The controller has an adaptive learning characteristic that learns the release rate and application rate of various transaxle components during various operating conditions. Adaptive learning allows the controller to compensate for wear and other events that might occur and cause the normal shift programming to be inefficient.

The controller may receive information from two different sources: directly from a sensor, or through a twisted-pair bus circuit, which connects all of the vehicle computer systems. This modulated bi-directional bus system is called Chrysler collision detection (CCD) bus and allows the various computers in the vehicle to share information.

When the controller receives an ignition run from the ignition switch, it performs a series of circuit and relay checks. If a problem is found, the controller will provide voltage to the EATX relay, which causes its contacts to close. This sends voltage to the solenoid assembly.

Direct battery voltage is supplied to the controller. If the controller loses source voltage, the transaxle will enter into limp-in mode. The transaxle will also enter into limp-in if the controller senses a transmission failure. At this point, a fault code will be stored in the memory of the controller and the transaxle will remain in limp-in until the transaxle is repaired. While in limp-in, the transaxle will operate only in park, neutral, reverse, and second gears. The transaxle will not upshift or downshift. This allows the vehicle to be operated, although its efficiency and performance is hurt.

The solenoids are controlled by the controller, which sends voltage to the EATX relay. The EATX relay, in turn, send voltage to the solenoid assembly. The controller also completes the ground circuit of the solenoids when a particular solenoid should be activated. The controller also monitors the operation of the solenoids through inputs from low-reverse, 2-4 pressure, and overdrive switches. These switches are located in the solenoid assembly and inform the controller when a hydraulic circuit is open.

Inputs The direct inputs are those sensors that provide information to the controller and do not use the CCD bus circuit. The CCD bus inputs use the bus circuit to supply the controller and other computers with information.

Typical CCD bus inputs used by the transaxle controller are from an ambient or battery temperature sensor, brake switch, coolant temperature sensor (CTS), and manifold absolute pressure (MAP) sensor. Other information, such as engine and body identification, the SBEC's target idle speed, and speed control operation are not the result of monitoring by sensors, rather these have been calculated or determined by the SBEC and made available on the bus. The ambient or battery temperature sensor monitors intake air temperature. The SBEC uses the temperature of intake air and current flow to calculate the temperature of the battery. The SBEC uses this temperature calculation to estimate transaxle fluid temperature.

The brake switch is used to disengage the torque converter clutch when the brakes are applied. Its input has little to do with the up and down shifting of gears. The input from the CTS is critical to the operation of the transaxle. If the engine's coolant temperature is cold, the controller may delay upshifts to improve driveability. The controller may also lock the converter clutch in second or third gear if the coolant temperature rises. The MAP sensor keeps the controller informed of changes in engine load.

Although engine speed information is available at the bus, the controller receives this signal directly from the distributor pick-up coil or crank angle sensor. With the direct feed, any time delay at the bus circuit is avoided and the controller is aware of current engine speeds.

Other direct inputs to the controller include battery voltage, selector lever positions, throttle position, turbine shaft speed, output shaft speed, and information from the low-reverse, overdrive, and 2-4 pressure switches at the solenoid assembly. The output shaft and turbine shaft speed sensors are magnetic pick-up type sensors that generate an AC voltage.

The controller processes these inputs and selects the proper shift schedule for the transaxle. The controller will then control the power feed to the solenoid assembly through the EATX relay and the ground circuit of the solenoids to force upshifts and downshifts. The controller will also activate the backup lamp relay based on these inputs.

42LE Controls The 42LE four-speed transaxle, released in 1994 by Chrysler for use in the their New Yorker, Intrepid, LHS, and Vision models, also uses fully adaptive controls. Like the 41TE, this transaxle uses hydraulically applied clutches to shift gear but controls the hydraulics electornically.

The hydraulics of the transaxle provide for the control of torque converter fluid flow and oil cooler flow, the regulation of mainline pressure, and the movement of the manual shift valve. Fluid flow to the various apply devices is directly controlled by the solenoids. The basic hydraulic system also includes a logic-controlled solenoid switch valve, which locks out first gear when second, direct, or overdrive gears are engaged. This solenoid switch also redirects fluid to the torque converter lockup clutch. To regain access to first gear, a special sequence of solenoid commands from the control unit must be used to unlock and move the solenoid switch. This prevents the engagement of first gear when other gears are engaged.

The solenoids act directly on steel poppet and ball valves. Two of the solenoid valves are normally venting and the other two are normally applying. This combination allows for a fault mode of operation. If electrical power to the transaxle is lost, the transaxle will only provide second gear in all forward drive ranges.

There are three pressure switches that give input to the transaxle controller. They are all located within the solenoid assembly. One speed sensor reads input speed at the turbine shaft and another speed sensor reads output speed. The third sensor monitors the position of the manual shift valve.

Engine speed, throttle position, temperature, engine load, and other typical engine-related inputs are also used by the controller to determine the best shift points. Many of these inputs are available through multiplexing and are inputted from the common bus.

The adaptive learning takes place as the controller reads input and output speeds over 140 times per second. The controller responds to each new reading. This learning process allows the transaxle controller to make adjustments to its program so quality shifting always occurs.

The basic shift logic of the controller allows the releasing apply device to slip slightly during the engagement of the engaging apply device. Once the apply device has engaged and the next gear is driven, the releasing apply device is pulled totally away from its engaging member and the transmission is fully into its next gear. This allows for smooth shifting into all gears. The adaptive learning capability of the transaxle controller allows for this smooth shifting throughout the life of the transmission. The controller learns the characteristics of the transaxle and changes its programming accordingly.