Code B127, Gear Selector Switch Circuit: Notes

The gear selector switch is mounted on the transaxle assembly. It is a multi-signal switch sending information relative to gear selector position to the BCM and ECM. The switch assembly also contains the neutral safety switch and back-up light switch, and provides park input to the automatic door lock module (if equipped). The switch uses 4 discrete circuits to pull 4 BCM terminal voltages low in various combinations to indicate each gear range. The voltage level of each of the circuits is represented in BCM data BD41. "0" = low, "1" = high. The 4 digits displayed represent decoder A, B, C and parity inputs in sequence.

The switch also provides park/neutral inputs to the ECM in a similar manner using one discrete input. This can be seen using ECM input EI74, where "HI" or "LO" represent the voltage level of input terminal. Code B127 will set if the following conditions occur:

• BCM gear selector switch input indicates P/N but ECM input does not.
• BCM gear selector switch input does not indicate P/N but ECM input does.
• Decoder and parity inputs do not agree with possible combinations.

If Code B127 is set, the DIC will display the warning message "GEAR SELECT PROBLEM" and IPC gear position display (PRND21) will flash.

1. Checks BCM inputs when only decoder A and parity circuit are pulled low.
2. Checks BCM inputs when only decoder B and C inputs are pulled low. This will complete high/low check of all 4 BCM inputs.
3. This step is to check gear selector switch problem in other gear ranges.
4. Checks to see if code is due to discrepancy between ECM and BCM inputs.
5. Checks for short to ground in gear selector switch to BCM circuits.
6. Checks for open in gear selector switch to BCM circuits.
7. Circuit shorted to ground will result in a "0" for the circuit.
8. Checks for open gear selector sensor ground.
9. Circuit with open will result in a "1" for the circuit.