For 1990-2009 cars

The brake transaxle shift interlock (BTSI) system consists of the body control module (BCM), powertrain control module (PCM), BTSI solenoid, stop lamp switch, and PRNDL selector switch. The BTSI system prevents the driver from moving the shift lever out of PARK unless the brake pedal is depressed while in RUN or CRANK.

The BCM controls the output to the BTSI solenoid by grounding the solenoid which energizes it, preventing shifting out of PARK. When the BCM receives input from the PCM over the Class II link that the transaxle is in PARK and the brake pedal is depressed, it will stop grounding the BTSI solenoid or de-energize it which will allow shifting. After the transaxle is moved out of the PARK position, the BCM shall monitor the status of the PRNDL serial data message sent by the PCM and shall continue to de-energize the solenoid until the transaxle has been returned to the PARK position.

In the event that the serial data messages are never received (approximately 5 seconds) by the BCM from the PCM, the BCM shall de-energize the solenoid to allow shifting. The transaxle can be shifted from PARK by placing the ignition switch in the ACC position. This eliminates power to the solenoid.