Power Locks: Description and Operation

When a door lock switch is activated in the power door lock system, all of the doors lock or unlock in unison. Each lock can also be operated manually from the locking knob. The locks are operated by reversible motors that receive voltage from the door lock relay. This relay turns on the motors by applying voltage to one terminal and ground to the other terminal.

Moving either door lock switch to the LOCK position completes the circuit to the "lock" coil in the door lock relay. A set of relay contacts closes to apply voltage to the door lock motors, which are grounded through the relay contacts for the "unlock" relay. The motor in each door runs to operate the door locks. When the door lock switch is released the "lock" relay contacts open to ground the motors; the motors turn off.

A similar action occurs with the "unlock" relay when it is energized by moving either door lock switch to the UNLOCK position. Voltage polarity to the motors reverses causing the motors to run in the opposite direction and unlock the doors.

The door lock switches are usually closed for just a moment. If they are held closed, an electronic circuit breaker in each motor opens to protect against damage. The circuit breakers have positive temperature coefficients (PTC) and reset automatically when voltage is removed from the motor.