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 ECM. The PRNDD21 switch assembly also contains 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.

The PRNDD21 switch also provides Park/Neutral inputs to ECM in a similar manner using one discrete input. This can be seen using EI74, where HI or LO represent voltage level of input terminal. Code B127 will set if following conditions occur:

PRNDD21 switch input indicates Park/Neutral, but ECM input does not.
PRNDD21 switch input does not indicate Park/Neutral, but ECM input does.
Decoder and parity inputs do not agree with possible combinations per BD41 display value chart found in BCM DATA PARAMETERS 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.

NOTE: Test numbers refer to numbers on diagnostic chart.

Test Number 1) Checks BCM inputs when only decoder A and parity circuit are pulled low.

Test Number 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.

Test Number 3) This step is to check gear selector switch problem in other gear ranges.

Test Number 4) Checks to see if code is due to discrepancy between ECM and BCM inputs.

Test Number 5) Checks for short to ground in gear selector switch to BCM circuits.

Test Number 6) Checks for open in circuits to BCM.

Test Number 7) Circuit shorted to ground will result in a "0" for that circuit.

Test Number 8) Checks for open PRNDD21 sensor ground.

Test Number 9) Circuit with open will result in a "1" for that circuit.

Test Number 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, ECM Park/Neutral input must be incorrect to set code.

Test Number 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.