BCM Code B127: PRND321 Sensor: Notes

The PRND321 switch is a multi-signal switch sending information relative to gear selector position to BCM and ECM. Switch is mounted on transaxle assembly. The switch assembly also contains neutral safety switch, back-up light switch, and provides park input to automatic door lock module (if equipped). The switch uses 4 discrete circuits to pull BCM terminal voltages low in various combinations to indicate each gear selection. Voltage level of each circuit is represented in BCM parameter BD41 as "0" for low, "1" for high. The 4 digits displayed represent decoder "A", "B", "C" and PARITY inputs in sequence.

The PRND321 switch also provides park/neutral input to ECM in a similar manner using one discrete input. This can be seen using ECM input EI74 where HI or LO represent voltage level of input terminal.

Code B127 sets when BCM PRND321 input indicates park/neutral but ECM input does not, BCM PRND321 input does not indicate park/neutral but ECM input does, or decoder and parity inputs do not agree with possible combinations of CORRECT VALUES chart. If Code B127 is set, CCDIC will display the GEAR SELECT PROBLEM warning and Instrument Panel Cluster (IPC) will flash PRND321 display.

NOTE: Test numbers refer to test numbers on diagnostic chart.

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

Test Number 2) Check BCM inputs only when decoder "B" and "C" inputs are pulled low. This will complete voltage check of all 4 BCM inputs.

Test Number 3) Check for a PRND321 switch problem in other gear ranges.

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

Test Number 5) Checks for short to ground in PRND321 to BCM circuit(s).

Test Number 6) Checks for open in PRND321 to BCM circuit(s).

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

Test Number 8) Checks for open PRND321 sensor ground.

Test Number 9) Open circuit will result in a "1" display for the circuit.

Test Number 10) The snap data value of BD41 will be what the BCM saw when the code was set. If it was a valid value, then the ECM Park/Neutral must be wrong to cause the code to set.

Test Number 11) If the snap data value of BD41 is invalid, it has to be determined which of the digits is invalid. If all digits are "1", the PRNDL ground circuit is open. If only one digit is incorrect, That circuit was open or shorted when the code was set.