The brake transmission shift interlock (BTSI) solenoid requires the ignition to be in the run or start position and the brake pedal to be applied before the transmission may be shifted from the park position.

With the ignition in the RUN or START position and the automatic transmission in the park position, battery voltage flows from the TURN/BU fuse 16, through CKT 139 (PNK) to the park/neutral position (P) switch. The PNP switch is closed in park which allows battery voltage to flow through CKT 275 (LT GRN) to the stoplamp switch. When the automatic transmission is in the park position, the stoplamp switch allows battery voltage to flow through CKT 1135 (DK GRN/WHT) to energize the BTSI solenoid. The BTSI solenoid is grounded through CKT 150 (BLK) to ground G200. With the BTSI solenoid energized, it will lock the automatic transmission in the park position.

When the brake is applied, the stoplamp switch will interrupt battery voltage flow and deenergize the BTSI solenoid. With the BTSI solenoid deenergized, the automatic transmission may then be placed in another gear which will cause the contacts of the PNP switch to open and interrupt battery voltage to the stoplamp switch.