Lock-up solenoid, SLU

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Fig 1: Identifying Lock-Up Solenoid, SLU

Courtesy of VOLVO CARS OF NORTH AMERICA.

The lock-up-solenoid, SLU is located in the gearbox control system, which is mounted on the front edge of the gearbox.

The lock-up-solenoid, SLU consists of an electric coil that controls a hydraulic valve. The solenoid is controlled, like all linear solenoids in the gearbox, by a pulsed (PWM) current with a frequency of 300Hz and grounded via the Transmission Control Module (TCM). The average value on the current signal that deploys the solenoid varies between 0.1 A and 1 A and the demand controls the deployment.

The solenoid controls the torque converter's Lock-up engagement. Engagement occurs by the solenoid pulsing, which gives a smooth engagement of the lock-up function. The solenoid allows the torque converter to work in one of three positions:

Open
Controlled slipping
Locked.

The hydraulic function of the solenoid is linear.

There is a diagnostic for the lock-up solenoid.