The brake transaxle shift interlock (BTSI) is a safety circuit that prevents the vehicle from accidentally shifting out of PARK when the ignition is in the RUN position. The steering column is equipped with a shift interlock actuator. This actuator, when energized, physically blocks the gear shift lever from moving out of PARK when the ignition is in the RUN position. When the driver presses the brake pedal, the shift interlock actuator de-energizes, allowing the driver to move the gear shift lever.
The transaxle internal mode switch (IMS) determines the position of the gear shift lever and provides this information to the powertrain control module (PCM) via the following 4 circuits:
| • | CKT 771 (BLK/WHT) |
| • | CKT 772 (YEL) |
| • | CKT 773 (GRY) |
| • | CKT 776 (WHT) |
When the gear shift lever is in PARK, the PCM sends a Class 2 message informing the body control module (SBM) of the position of the shift lever.
When the ignition switch is in the RUN position, the SBM energizes the shift interlock actuator control. This module output applies battery positive voltage (B+) from connector C2 terminal A6 to CKT 275 (LT GRN). This circuit supplies battery positive voltage (B+) through the normally-closed stoplamp/BTSI switch and CKT 1135 (DK GRN/WHT) to terminal A of the shift interlock actuator. The actuator, which is always grounded on terminal B by CKT 350 (BLK), energizes, locking the gear shift lever in the PARK position. Depressing the brake pedal opens the contacts of the stoplamp/BTSI switch, removing power from the shift interlock actuator. The actuator de-energizes, allowing the driver to move the gear shift lever out of PARK.
When the gear shift lever moves out of PARK, the transaxle internal mode switch (IMS) signals the PCM of the change via the 4 input circuits. The PCM provides this information to the SBM via a Class 2 message. The SBM in turn grounds the shift interlock actuator control. This module output de-energizes the shift interlock actuator. Once the vehicle is out of PARK, the shift interlock actuator remains de-energized when the brake pedal is released and the BTSI/stoplamp switch contacts return to their normally-closed state.
At all times the SBM internally monitors CKT 275 (LT GRN) for a short to ground or a short to battery positive voltage (B+). If the SBM detects one of these conditions, the SBM sets one of the following diagnostic trouble codes:
| • | DTC B0802 -- BTSI Short to Ground |
| • | DTC B0803 -- BTSI Short to Battery Positive Voltage (B+) |
If DTC B0802 or DTC B0803 is current in SBM memory, the SBM disables CKT 275 (LT GRN). Always begin diagnosis of the shift interlock actuator with an inspection of the SBM DTCs using a scan tool.
