Switching control
When a shift is performed, certain elements are released while others are brought into operation. Ideally, this process occurs simultaneously (synchronously) to avoid jerky shifting.
The duration of the switching process must remain within the specified time range.
In normal gear shift control, the increase and decrease of pressure in the shift elements is adjusted and determined for ideal conditions (for synchronous shifting). Since there is no way to influence control in the case of different degrees of wear of the switching elements in cases where the gearbox has worked for a very long time, it is possible that the increase and decrease of pressure will no longer occur synchronously.
The result of a premature reduction in pressure in the element being switched off is an undesirable increase in the turbine shaft speed, since the element being switched on cannot transmit the primary torque.
The result of the delayed pressure reduction in the element being switched off is an undesirable reduction in the turbine shaft speed, since both elements being switched on transmit torque. In this case, the torque is transmitted to the gearbox housing by means of an internal lock.
In both cases, there will be a jerking sensation when shifting.
In addition, wear in the shifting elements leads to an increase in the duration of the shifting procedure. Consequently, as the service life of the gearbox increases (increases mileage), the shifting becomes increasingly longer.
Switching control using ESSC
The 4F27E automatic transmission uses Electronic Shift Control (ESSC).
ESSC monitors shifting performance and is able to compensate for wear of shifting components throughout the life of the transmission.
This is made possible because the switching elements are operated by modulating valves.
The system controls the shift timing and gear shift synchronicity.
If the PCM detects a deviation from the stored values for the switching time and switching process timing, the pressure increase or decrease will be adjusted accordingly.
Throttle Position Sensor (TP)
The TP sensor is located on the throttle body. It provides the PCM with information about the throttle position. It also determines the speed at which the throttle is applied. The PCM uses these signals to perform the following functions, among others:
- determining the switching order,
- pressure control in the main line,
- control of the locking clutch of the torque converter,
- to operate the "Kickdown" function (gear shifting when the accelerator pedal is pressed).
If the TP signal is missing, the engine management uses the MAF and IAT sensor signals as substitutes. The pressure in the main line increases and hard shifting may occur.
Mass Air Flow (MAF) Sensor and Intake Air Temperature (IAT) Sensor
The MAF sensor is located between the air cleaner housing and the air intake hose that goes to the throttle body. The IAT sensor is integrated into the MAF sensor housing. The MAF sensor, along with the IAT sensor, provides the primary load signal to the PCM. The PCM uses these signals to perform the following functions, among others:
- switching control,
- pressure control in the main line.
If the MAF sensor fails, the TP sensor signal is used as a substitute.
Crankshaft position sensor (CPS)
The CKP sensor is located on the engine/gearbox flange. The CKP sensor is an inductive sensor that provides the PCM with information about the engine crankshaft speed and crankshaft position. The signal is used to perform, among other things, the following functions:
- control of the locking clutch of the torque converter,
- torque converter slip checks,
- pressure control in the main line.
There is no substitute signal for the CKP sensor. If the CKP sensor signal is missing, the engine stops.
Turbine Shaft Speed Sensor (TSS)
The TSS sensor is located in the gearbox housing above the gearbox input shaft. The TSS sensor is an inductive sensor that senses the rotational speed of the gearbox input shaft. The signal is used to perform the following functions:
- switching control,
- control of the locking clutch of the torque converter,
- checking torque converter slippage.
If the TSS sensor fails, the signal from the secondary shaft speed sensor (OSS) is used as a substitute.
Secondary Shaft Speed Sensor (OSS)
The OSS sensor is located in the gearbox housing above the rotor in the differential. The OSS sensor is an inductive sensor that uses the rotor in the differential to detect the vehicle's speed. The signal is used to perform the following functions, among others:
- determining the switching order,
- supplying an input signal about the vehicle speed to the PCM.
If the OSS sensor fails, the TSS sensor signal is used as a substitute.
Transmission Range (TR) Sensor
The TR sensor is located on the manual shaft on the gearbox housing.
When the manual shaft is moved using the manual selector lever cable, the engagement pin in the inner ring of the TR sensor moves through various positions. Signals are transmitted to the PCM, reverse lights and starter interlock relay.
NOTE: Correct operation of the TR sensor is only guaranteed when the manual selector lever cable is properly adjusted.
The TR sensor signals are used to perform the following functions:
- manual gear lever position recognition
- activation of the starter interlock relay
- turning on the reversing lights
There is no substitute signal for the TR sensor. If the electrical circuit is broken, the vehicle will not start.
Brake light switch
The brake pedal position (BPP) switch is located on the brake pedal bracket. It operates the brake lights and notifies the EEC V PCM that the brakes are applied. The signal from the brake light switch is used by the PCM to perform the following functions:
- release of the torque converter locking clutch when the brake pedal is pressed,
- disabling the manual gear shift lever lock when pressing the brake pedal in position "P".
There is no substitute signal for the BPP switch. If the electrical circuit of the BPP switch is broken, the manual gear selector lever cannot be moved out of the "P" position.
Transmission Fluid Temperature (TFT) Sensor
The TFT sensor is located on the internal wiring harness that goes to the oil pan solenoids. It is a resistor that measures the temperature of the transmission fluid. The transmission fluid temperature information is used by the PCM to perform the following functions:
- the torque converter clutch is not allowed to be engaged until the transmission fluid temperature reaches a certain temperature;
- in conditions of extremely low negative temperatures, engaging 4th gear is not permitted until normal operating temperature is reached;
- if the transmission fluid temperature exceeds the normal value, the preset shift curve is selected and the torque converter locking clutch is locked in the gearbox position "2", "3" or "4", and the gearbox indicator lamp comes on. There is no substitute signal for the TFT sensor.
Overdrive (O/D) switch
The O/D switch sends a signal to the PCM to select or inhibit 4th gear selection when the manual shift lever is in the "D" position. The O/D switch signal is used to perform the following functions:
- as an input signal for transmitting the driver's desire to the RCM,
- to display the driver's wishes using the O/D indicator lamp on the instrument panel.
There is no substitute signal for the O/D switch. If it is faulty, it is always possible to shift to 4th gear with the manual gear selector lever in the "D" position.
Manual gear shift lock solenoid
| Pos. | Spare Part No | Name |
| 1 | - | Electromagnet |
| 2 | - | Locking pin |
| 3 | - | Manual release mechanism |
When the ignition is switched on, the manual gearshift lever lock solenoid is activated by pressing the brake pedal (signal from the brake light switch). This causes the locking pin to retract, and thus the manual gearshift lever can be moved out of the "P" position.
Substitute function
If, due to malfunction, the signal from the brake is not received or is incorrect, manual release of the lock is possible.
To do this, remove the release mechanism cover and insert a suitable object (ignition key) into the hole until the manual gear selector lever can be moved out of the "P" position.
NOTE: If the "P" range is selected again, the manual gear selector lever will again be locked.
Air conditioning
If the PCM detects a kickdown signal (WOT, 95% throttle open), the air conditioning system is turned off for a maximum of 15 seconds.
Starter interlock relay
The relay prevents the engine from starting when the manual gear selector lever is in position "R", "D", "2" or "1".
The relay receives information about the position of the gear selector lever directly from the TR sensor.
Ignition Key Lock Electromagnet
The electromagnet is built into the ignition switch. When the gear selector lever is in the "P" position, the grounding circuit of the electromagnet is broken. The locking pin is not fixed in the ignition switch.
In all other positions of the manual gear selector lever, the solenoid ground circuit is closed and the locking pin is locked in the ignition switch.
When the manual gear selector lever is in a position other than "P", the key cannot be removed from the ignition switch.
O/D indicator light
The O/D indicator is a green indicator located on the instrument cluster.
It informs the driver that the gearbox control is blocking the shift to 4th gear.
Powertrain control lamp
The powertrain indicator light is an orange light located on the instrument panel.
When it is turned on, it informs the driver that the transmission control has switched to the emergency operating program or that the transmission fluid temperature is too high.
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