Hydraulic pressure variations

Hydraulic pressure variations

The pressures in the hydraulic system are regulated by the control module, in order to achieve the following 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 vehicle 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 km/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, depending 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 and 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 1st gear and reverse gear, which permit higher transmission ratios.

These jolts caused by the engagement of 1st 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 following features:

• engagement of 1st gear or reverse gear with a reduction in driving torque (garage function);
• 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);

with this function the control module sends a signal to the fuel injection-ignition control unit so as to reduce the driving torque, reducing the jolt caused by the engagement of 1st gear or reverse gear.