Chart C-1A Park/Neutral Switch Diagnosis

Chart C-1A, Park/Neutral Switch Diagnosis:

Park/Neutral Switch Circuit Diagram:

CIRCUIT DESCRIPTION
The ECM supplies a 12 volt reference signal to the Park/Neutral (P/N) switch, through a current limiting resistor, and then monitors the voltage in the circuit (circuit 434). When the gear selector is in the park or neutral position, the switch is closed, grounding the circuit. This causes a drop in the circuit voltage. Low voltage (less than 1 volt) in circuit 434 indicates the transmission is park or neutral. The PCM uses this information for the following:
^ Idle Air Control valve operation
^ Vehicle Speed Sensor diagnosis
^ EGR control

NOTE: If circuit 434 indicates Park/Neutral while transmission is in a drive range (circuit 434 voltage low - switch grounded), the EGR system will not operate. This may cause detonation (engine "PING") under load. If circuit 434 voltage remains high (circuit open) while transmission is in Park/Neutral, a drop in idle speed may be noticed when transmission is placed in a drive range.

TEST DESCRIPTION
Numbers below refer to circled numbers on diagnostic chart.
1. Checks for a closed switch to ground in park position. Different makes of "SCAN tools display P/N differently. Refer to specific manufacturers instructions for correct display on tool being used.
2. Determines if switch indicates a change of state when a drive range is selected.
3. Determines if drive range indication while in Park/Neutral is due to faulty or misadjusted P/N switch. To check for intermittent fault or misadjusted switch, wiggle shifter while observing "SCAN" tool display. Be sure "SCAN" tool indicates drive whenever transmission is in drive range.
4. Bridging circuit 434 to ground should cause ECM to indicate Park/Neutral. If not, there is an open in circuit 434, poor ground connection in circuit 803, poor connections at the ECM, or the ECM is faulty.