Automatic Transmission Description and Operation

AUTOMATIC TRANSMISSION DESCRIPTION AND OPERATION

Automatic Transmission Identification Tags

I. D. Tag Located on Transmission Case










All vehicles are equipped with a vehicle certification label, located on the driver side door lock post. See the code in the space marked TR. For model, service ID level or build date information, see the transmission service ID tag located on the transmission case.

Automatic Transmission Range Selection

The transmission has six range positions: P, R, N, (D), 2 and 1.






Park

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.

Reverse

In the REVERSE position:

- the vehicle may be operated in a rearward direction, at a reduced gear ratio.
- engine braking will occur.

Neutral

In the NEUTRAL position:

- there is no powerflow through the transmission.
- the output shaft is not held and is free to turn.
- the engine may be started.

Overdrive

Overdrive is the normal position for most forward driving.
The OVERDRIVE position provides:

- automatic shifts.
- application and release of the torque converter clutch (TCC).
- maximum fuel economy during normal operation.
- automatic shift 1-4 with O/D cancelled (5R44E/5R55E)
- engine braking with coast clutch applied in 1-4, R. (5R44E/5R55E)

Second Position 3rd Gear (5R44E/5R55E)

The 2nd position provides:

- second gear start and hold.
- the torque converter clutch may apply and release.
- improved traction on slippery roads.
- engine braking.

Manual Low Position

If this position is selected at normal road speeds, the transmission shifts into second gear, then into first when the vehicle reaches a speed below approximately 30 km/h (48 mph). This position provides:

- first gear operation only.
- engine braking for descending steep grades.

Automatic Transmission Shift Patterns

Upshifts

Transmission upshifting is controlled by the powertrain control module (PCM). The PCM receives inputs from various engine or vehicle sensors and driver demands to control shift scheduling, shift feel and torque converter clutch (TCC) operation.
The PCM has an adaptive learn strategy to electronically control the transmission which automatically adjusts the shift feel. When the battery has been disconnected, or a new battery installed certain transmission operating parameters may be lost. The PCM must relearn these parameters. During this learning process you may experience slightly firm shifts, delayed, or early shifts. This operation is considered normal and does not affect the function of the transmission. Normal operation returns once these parameters are stored by the PCM.

Downshifts

Under certain conditions the transmission downshifts automatically to a lower gear range (without moving the gearshift lever). There are 3 categories of automatic downshifts:

- coastdown
- torque demand
- 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, engine and transmission calibration requirements.