Hydraulic Pressure Variations; Increase In Line Pressure When Cold; Controlling Gear Changes; Engagement Of 1St Gear Or Reverse Gear (Garage Function) - Fiat Bravo Service Manual

Bravo-Brava

Hydraulic pressure variations

The pressures in the hydraulic system are regulated by the control module, in order to achieve the f o l -
lowing functions:

- increase in line pressure when cold;

- reduction in line pressure in the N-R change;
- modulation of driving pressure.
Increase in line pressure when cold
With the selector lever at position N, when the transmission oil temperature is below 10°C and the vehi-
cle speed is below 7 km/h, the control module sends a signal for a period of 5 seconds to the solenoid
STH, to increase the line pressure, so enabling the N-D and N-R engagement time to be reduced.
This function is excluded in the following cases:
- transmission oil temperature over 20°C;
- vehicle speed over 7 k m / h ;
- selector lever position other than N.
Reduction of line pressure in N-R change
When the selector lever is moved from position N to position R, the control module sends a signal to the
solenoide STH to control the line pressure.
This makes the line pressure fall, so as to reduce the jolt caused by the engagement of reverse gear.
Modulation of driving pressure
The driving pressure of the hydraulic valves (throttle pressure) is increased by the control module, de-
pending on the aperture of the throttle valve and the vehicle speed.
Control of gear change times
Comparing the ratios between the input and output speeds, provided by the main shaft rpm sensor
vehicle speed sensor respectively, with the memorized values of the transmission ratios of the different
gears, the control module assesses the time used for the gear changes.

Controlling gear changes

In an automatic transmission, the engagement of gears causes jolts resulting from the sudden variation
in transmission ratios; these are more pronounced when engaging 1 st gear and reverse gear, w h i c h per-
mit higher transmission ratios.
These jolts caused by the engagement of 1 st gear and reverse gear are inconveniently perceptible for the
occupants and also cause stresses on the gearbox gears.
To reduce these stresses and improve driving comfort, the AISIN automatic transmission uses the f o l -
lowing features:
- engagement of 1st gear or reverse gear with a reduction in driving torque (garage f u n c t i o n ) ;
- N-D change with pitch control (squat);
- N-R change with reduction in line pressure.

Engagement of 1st gear or reverse gear (garage function)

When the selector lever is moved as follows:
- from P or N to R;
- from N to D ( 3 - 2 - 1 ) ;
w i t h this function the control module sends a signal to the fuel injection-ignition control unit so as to re-
duce the driving torque, reducing the jolt caused by the engagement of 1st gear or reverse gear.
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Components and operation
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