Shift Interlock: Description and Operation

BTSI AND KEY INTERLOCK SYSTEM
The Brake/Transaxle Shift Interlock (BTSI) is a safety device which prevents the vehicle from being shifted out of PARK unless the driver is pressing the brake pedal. The system uses a BTSI solenoid which engages the shift lever and prevents it from being moved out of PARK unless the brake pedal is pressed. The control circuitry for the BTSI system is contained within the key interlock and BTSI Unit. B+ is applied to the BTSI solenoid through fuse F5. The key interlock and BTSI unit contains an input from the transmission range switch and a brake pedal input from the stoplamp switch. When the brake pedal is pressed and the transaxle is in PARK, the solenoid circuit is completed to ground. This energizes the BTSI solenoid and causes it to disengage the shift lever.
The key interlock system prevents the ignition key from being turned to LOCK unless the automatic transaxle is in PARK. The system consists of a key interlock solenoid (mounted to the ignition switch assembly) and control circuitry contained within the key interlock and BTSI unit. In the energized state (any transaxle position except PARK), the solenoid pin protrudes into the ignition cylinder housing, preventing the ignition key from being turned to LOCK. In the non-energized position (when the transaxle is in PARK), the solenoid pin retracts, allowing the ignition key to be turned to LOCK.
The PARK position is determined by the transmission range switch. Because the transmission range switch completes a circuit to ground when the transaxle is in PARK, a method is needed to de-energize the solenoid at that time. That is the function of the key interlock and BTSI unit. This is a solid-state module which acts as a relay. When the transmission range circuit is hot (transaxle is in PARK), the key interlock and BTSI unit de-energizes the solenoid and allows the solenoid pin to retract. When the transmission range circuit is open (any transaxle position except PARK), the key interlock and BTSI unit energizes the solenoid from a separate B+ circuit and causes the solenoid pin to protrude into the ignition cylinder housing.