Code B127, Gear Selector Switch Circuit: Notes

Fig 1: Code B127 - Wiring Schematic (Toronado & Trofeo)

Courtesy of GENERAL MOTORS CORP.

The gear selector switch (or PRND  D21) is mounted on transaxle assembly. It is a multi-signal switch sending information relative to gear selector position to BCM and PCM. The PRND  D21 switch assembly also contains the neutral safety switch. The PRND  D21 switch uses 4 discrete circuits to pull 4 BCM voltages low in various combinations to indicate each gear range. The voltage level of each of circuits is represented in BD41 display as "0" equals grounded and "1" equals not grounded. The 4 digits displayed represent decoder "A", "B", "C" and parity inputs in sequence.

Switch status can be seen by monitoring BD41 or ED31 data parameter, where switch status is displayed as a 4-digit binary number. Code B127 will set if following conditions:

• BCM PRND  D21 switch data input indicates park/neutral, but PCM input does not
• BCM PRND  D21 switch data input does not indicate park/neutral, but PCM input does
• Decoder and parity data inputs do not agree with possible combinations per BD41 display values found in DATA DISPLAY in introduction portion of this article.

If Code B127 is set, Driver Information Center (DIC) will display warning message GEAR SELECT FAULT and IPC PRND  D21 display will flash.

1. Checks BCM data inputs when only decoder "A" and parity circuit are pulled low.
2. Checks BCM data inputs when only decoder "B" and "C" inputs are pulled low. This will complete HI/LO 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 PCM and BCM inputs.