Transmission Control Systems: Description and Operation

Emergency programs in the event of a fault

An emergency program is activated to deal with the fault when the transmission control module (TCM) detects a transmission fault (permanent fault). The control system then implements corrective action to protect the transmission, while leaving the car in the best possible driveable condition. Minor malfunctions do not activate an emergency program.
There are different programs depending on the type of fault:
- Emergency mode I
- Emergency mode II
- Emergency mode III
- Limp-home mode
Emergency mode I is activated in the event of minor faults and the Limp-home mode is activated for the most serious faults. If the malfunction is intermittent, the transmission control module (TCM) returns to normal operation the next time the ignition is switched on.

Emergency mode I
- The warning lamp in the combined instrument panel lights (S80), flashes (S/V/C70) for certain diagnostic trouble codes (DTCs).
- The transmission shifts in all gears but transmits no signal to the lock-up solenoid. This means that lock-up is not available.

Emergency mode II
- Remedy as for Emergency mode I.
- No reduction of line pressure when moving the gear selector between positions P-R, N-R and N-D. This results in harsh shifting.
- No torque limiting request from the engine control module (ECM) during gear shifting.
- The warning lamp in the combined instrument panel lights / flashes.

Emergency mode III
- Remedy as for Emergency modes I and II.
- No control of line pressure solenoid SLT. Constant maximum system pressure. This results in harsh shifting and harsher gear engagement in positions P-R, N-R and N-D.

Limp-home mode
- The transmission control module (TCM) interrupts the activation of all solenoids. This results in the transmission not shifting at all. The transmission operates only in 3rd gear in positions 3 and L, 4th gear in position D and reverse in position R. Shifting can only be carried out manually between 3rd and 4th gear and reverse gear.
- No control of line pressure solenoid STH. Constant maximum system pressure. This results in harsh shifting and harsher gear engagement in positions P-R, N-R and N-D.
- The warning lamp in the combined instrument panel flashes.

Note! When starting and driving, the gear selector should first be moved to position L to minimize stress on the transmission.