Powertrain Controls - ECM/PCM

Code 31 Diagnostic Chart:

Gear Select Switch:

CIRCUIT DESCRIPTION:
The PRNDL switch is mounted on the transaxle assembly. It is a multi-signal switch sending information relative to gear selector position to the PCM. The PRNDL assembly also contains the neutral safety (start) switch. The PRNDL switch uses 4 discrete circuits to pull 4 PCM voltages low in various combinations to indicate each gear range. The voltage level of each of the circuits is represented as LO = Grounded, HI = Open Circuit. The 4 digits displayed represent decoder A, B, C and parity inputs in sequence.

Code 31 sets when:
^ Ignition is "ON" and engine is "OFF"
^ Vehicle speed is 0 mph
^ PRNDL indicates a gear other than park or neutral
^ All conditions are met for 0.13 seconds on 3 consecutive start-ups

OR

^ The PRNDL status indicators (P,A,B,C) indicate an incorrect combination of LO and HI for more than 10 seconds.

When a Code 31 is present, the PCM assumes the D4 position until a correct PABC switch combination is detected. This can mean that some gear select combinations may not be possible until the problem is repaired.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1 & 2 Check for proper operation of the PRNDL.

3. This step checks for an open or grounded circuit. When changing from "Park" to Drive (D4) the condition of all four switches should change (P & A go from low to high, B & C go from high to low.) Any switch that does not change condition is part of an open or grounded circuit.

DIAGNOSTIC AIDS:
An intermittent may be causes by poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check for:

Poor connection or damaged harness. Inspect for PCM harness connectors for backed out terminal, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.

Intermittent test. Monitor a scan tool while moving related connectors and wiring harness. If the failure is induced, the scan data will change from LO to HI or HI to LO. This may also help isolate the location of the malfunction.