Code B127, Gear Selector Switch Circuit: Notes

The gear selector switch (or PRNDD21) is mounted on transaxle assembly. It is a multi-signal switch sending information relative to gear selector position to BCM and PCM. The PRNDD21 switch assembly also contains the neutral safety switch. The PRNDD21 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 occur:

• BCM PRNDD21 switch input indicates park/neutral, but PCM input does not.
• BCM PRNDD21 switch input does not indicate park/neutral, but PCM input does.
• Decoder and parity 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 PRNDD21 display 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 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.
   NOTE: Test numbers (5 to 9) refer to numbers on diagnostic chart (2 of 3).
5. Checks for short to ground in gear selector switch to BCM circuits.
6. Checks for open in circuits to BCM.
7. Circuit shorted to ground will result in a "0" for that circuit.
8. Checks for open PRNDD21 sensor ground.
9. Circuit with open will result in a "1" for that circuit.
   NOTE: Test numbers (10 and 11) refer to number on diagnostic chart (3 of 3).
10. The snap data value of BD41 display will be value that BCM recognized when code was set. This is used to determine whether this value is valid. If it is a valid value, PCM park/neutral input must be incorrect to set code.
11. If snap value of BD41 display is invalid, it should be determined which of digits are invalid. If all digits read "1", PRNDD21 ground is open. If only one digit is incorrect, that circuit, either in switch or harness, was shorted or open when code was set.