Diagnostic Instructions

DTC Descriptors

DTC P0851:  Park/Neutral Position (PNP) Switch Circuit Low Voltage

DTC P0852: Park/Neutral Position (PNP) Switch Circuit High Voltage

Diagnostic Fault Information

Circuit	Short to Ground	Open/High Resistance	Short to Voltage	Signal Performance
Park/Neutral Signal	P0851	P0852	P0852	--

Typical Scan Tool Data

Park/Neutral Position Switch - Engine Control Module

Circuit	Short to Ground	Open/High Resistance	Short to Voltage
Operating Conditions: Engine running, normal operating temperature Parameter Normal Range: In-Gear, Park/Neutral
Park/Neutral Signal	Park/Neutral	In-Gear	In-Gear

Circuit/System Description

The internal mode switch (IMS), also known as the transmission manual shift shaft switch assembly, is a sliding contact switch attached to the manual shift shaft inside the transmission. The switch consists of 5 separate circuits, 4 of which indicate gear selector position to the transmission control module (TCM), and 1 that indicates Park/Neutral position to the engine control module (ECM). The ECM provides 12-volts to the Park/Neutral position (PNP) switch of the IMS. When the transmission in PARK or NEUTRAL, the switch closes and pulls the ECM voltage low, 0 volts. When the transmission is not in PARK or NEUTRAL, the ECM voltage is high, 12 volts. The ECM uses the PNP switch information in order to enable starter operation. The ECM park/neutral diagnostic monitors the park/neutral signal circuit to detect low voltage when the ECM expects to see high voltage and high voltage when the ECM expects to see low voltage.

Conditions for Running the DTC

P0851

P0852

Conditions for Setting the DTC

P0851

P0852

The ECM detects In Gear, the Park/Neutral switch signal equals 12 volts, High, when the IMS reports a Park/Neutral range for 3 seconds.

Action Taken When the DTC Sets

Conditions for Clearing the DTC

DTC P0851 and P0852 are type C DTCs.

Diagnostic Aids

It is possible for this DTC to set due to a stack-up or misalignment issue between the IMS and the park/neutral switch, where no electrical fault is present. Ensure the gear shift selector cable is properly adjusted and there are no service bulletins that address this concern before replacing components.

Reference Information

Schematic Reference

Automatic Transmission Controls Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Special Tools

Circuit/System Verification

If there are any other engine or transmission related DTCs set, diagnose those DTCs first. With a scan tool observe the Park/Neutral switch parameter in engine controls data list. Move the gear shift lever from PARK, to REVERSE and through all the ranges and observe the parameter as you move the shift lever. The parameter should display PARK/NEUTRAL when in PARK or NEUTRAL and In-Gear when in REVERSE, DRIVE, or DRIVE Low. It is especially important to closely observe the PARK and NEUTRAL to DRIVE transitional positions while moving the shift lever and observing for possible misalignment or slack in the shifter mechanism.

Circuit/System Testing

1. Ignition OFF, transmission in PARK, disconnect the automatic transmission 20-way inline connector.
2. With the ignition ON, engine OFF, using a DMM and the J 35616 GM terminal test kit, measure the voltage between the park/neutral signal circuit, terminal W, on the engine side of the 20-way connector and chassis ground. The voltage should be 11.8-12.2 volts.

⇒	If the voltage is less than the specified range, test the park/neutral signal circuit for an open, high resistance or short to ground. If the circuit tests normal, replace the ECM.

⇒	If the voltage is greater than the specified range, test the park/neutral signal circuit for a short to voltage. If the circuit tests normal, replace the ECM.

3. Using the J 35616 GM terminal test kit, measure the resistance between the ground circuit, terminal K, on the engine side of the automatic transmission 20-way inline connector and chassis ground. The resistance should be less than 10 ohms.

⇒	If the resistance is greater than the specified value, repair the open or high resistance in the ground circuit.

4. 4. Ignition OFF, install the J 44152 jumper harness (20 pins) on the transmission side of the automatic transmission 20-way inline connector. Using the J 35616 GM terminal test kit, measure the resistance between the park/neutral position signal circuit, pin W, and the ground circuit, pin K. The resistance should be less than 10 ohms. Repeat with transmission in NEUTRAL.

⇒	If the resistance is greater than the specified range, test the park/neutral signal circuit and the ground circuit for an open or high resistance. If an open or high resistance condition is found, replace the transmission wiring harness. If the circuits test normal, replace the IMS.

5. Using the J 35616 GM terminal test kit, measure the resistance between the park/neutral signal circuit, pin W, and the transmission case. The resistance should be greater than 10K ohms.

⇒	If the resistance is less than 10K ohms, test the park/neutral signal circuit for a short to the transmission case. If a short to the case is found, replace the transmission wiring harness. If the circuits test normal, replace the IMS.

6. With the transmission IN GEAR, using the J 35616 GM terminal test kit, measure the resistance between the park/neutral signal circuit and all other circuits of the J 44152 jumper harness (20 pins) including the ground circuit, pin K. The resistance should be greater than 10K ohms.

⇒	If the resistance is less than 10K ohms, replace the transmission wiring harness. If the circuits test normal, replace the IMS.

Component Testing

1. Ignition OFF, transmission in PARK, disconnect the automatic transmission 20-way inline connector.
2. IMS - Install the J 44152 jumper harness (20 pins) on the transmission side of the automatic transmission 20-way inline connector. Using the J 35616 GM terminal test kit, measure the resistance between the park/neutral position signal circuit, pin W, and the ground circuit, pin K. The resistance should be less than 100 ohms. Repeat with transmission in NEUTRAL.

⇒	If the resistance is less than the specified value, test the park/neutral signal circuit for an open, high resistance or short to ground. If the circuit tests normal, replace the ECM.

⇒	If the resistance is greater than the specified value, test the park/neutral signal circuit for a short to voltage. If the circuit tests normal, replace the ECM.

3. ECM - With the ignition ON and engine OFF, using a DMM and the J 35616 GM terminal test kit, measure the voltage between the park/neutral signal circuit, terminal W, on the engine side of the 20-way connector and chassis ground. The voltage should be 11.8-12.2 volts.

⇒	If the voltage is less than the specified range, test the park/neutral signal circuit for an open, high resistance, or short to ground. If the circuit tests normal, replace the ECM.

⇒	If the voltage is greater than the specified range, test the park/neutral signal circuit for a short to voltage. If the circuit tests normal, replace the ECM.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.