For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptors

DTC P0850: Park/Neutral Position (PNP) Switch Circuit
DTC P0851: Park/Neutral Position (PNP) Switch Circuit Low Voltage
DTC P0852: Park/Neutral Position (PNP) Switch Circuit High Voltage

### Diagnostic Fault Information

The Diagnostic Fault Information table identifies the circuit or circuits that correspond to the DTC and identifies the DTC associated with the circuit fault.

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Park/Neutral Signal | P0850/P0851 | P0850/P0852 | P0850/P0852 | --

### Typical Scan Tool Data

The Typical Scan Tool Data table identifies the circuit or circuits that correspond to the DTC failure. The table gives the scan tool values which are available for viewing and how they would appear with the fault condition present. By viewing the scan tool data, you should able to identify the type of fault as well as which circuit is affected.

#### PNP Switch

Circuit | Normal Range | Short to Ground | Open/High Resistance | Short to Voltage
--- | --- | --- | --- | ---
Operating Conditions: Engine running, normal operating temperature, range selector in Park. Parameter Normal Range: 12 Volts = In-Gear, 0 Volts = Park/Neutral
Park/Neutral Signal | Park/Neutral | Park/Neutral | In-Gear | In-Gear

### Circuit/System Description

The automatic transmission manual shift shaft position assembly, also known as an internal mode switch (IMS), is a sliding contact switch that mounts to the lower control valve body. The switch has a sliding rail that connects to the transmissions’ manual valve. The slide rail follows the position of the manual valve to provide manual valve location to the transmission control module (TCM) and engine control module (ECM). Internal to the switch are metal contact fingers that attach to a slide rail. The fingers make contact with segmented stainless steel strips that run the length of the switch. Contact between the fingers and the stainless steel strips will cause the TCM signal voltage to be Low, 0 volts. When there is no contact between a finger and a strip, the TCM signal voltage will remain High, 12 volts. As a result of the different High and Low signal combinations, the TCM and ECM can decode the transmission manual valve position.

For diagnostic purposes, the High and Low signals can be displayed on the scan tool. There are four signal circuits used to identify the manual valve position and are identified as parameter A, B, C and P (parity). The switch also has two additional circuits which are D, a common ground circuit, and S, a Park/Neutral start circuit for the ECM. The P/N signal circuit goes from the switch directly to the ECM, using the transmission 16-way connector as a pass through only. The P/N circuit does not go into the TCM, only the ECM, where it can be viewed on the scan tool through the ECM. The TCM and ECM use the decoded information for engine controls, starter enable, as well as determining the transmission shift patterns.

### Conditions for Running the DTC

#### P0850, P0851, or P0852

• | Ignition voltage is between 8-18 volts.

• | Engine Speed is greater than 1000 RPM.

• | TP is equal to or greater than 8 percent.

• | Engine torque is equal to or greater than 75 N·m (55 lb ft).

• | Vehicle speed is equal to or greater than 10 km/h (6 mph).

### Conditions for Setting the DTC

#### P0850 or P0851

The ECM detects the Park/Neutral switch signal equals 0 volts, Low, when the IMS reports a Drive range.

#### P0852

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

### Action Taken When the DTC Sets

• | DTC P0850, P0851, and P0852 are Type C DTCs.

• | The ECM uses IMS Range for engine start-up.

### Conditions for Clearing the DTC

DTC P0850, P0851, and P0852 are Type C DTCs.

### Diagnostic Aids

• | Disconnecting the 16-way connector from the transmission will disable TCM and ECM communications.

• | A loose or unlocked transmission 16-way connector can cause DTC P0852 to set as well as a no start condition.

• | If the engine will not start, look for an open in the PNP signal circuit.

⇒ | If an open is found, go to Circuit/System Testing.

• | The IMS is part of a 6 wire harness, which connects electrically to the control solenoid (w/body and TCM) valve assembly. Inspect the IMS, harness, connector, and control solenoid (w/body and TCM) valve assembly electrical pins for metallic debris. Use a terminal test kit for any test that requires probing the control solenoid (w/body and TCM) valve assembly harness connector or a component harness connector.

• | Use the scan tool IMS parameters to compare them to the ECM P/N parameter. When the transmission is in park or neutral, the ECM P/N switch parameter will read park/neutral. When the transmission is in a drive range, the P/N will read In Gear. Be alert for scan tool data parameters which frequently toggle or vary while testing. Switch positions which are transitional, display invalid or are open are not normal IMS positions when shifter detents are engaged.

• | With the engine running, compare the PNP switch state to the range selector positions. Move the shifter through all gear ranges. If the PNP switch parameter always reads Park/Neutral in all positions, test the signal circuit from the ECM to the transmission for a short to ground.

⇒ | If a short is not found, go to component testing to evaluate the manual shift shaft switch assembly and the control solenoid (w/body and TCM) valve assembly.

⇒ | Due to the properties of the stainless steel contact strips inside the switch, it is difficult to obtain meaningful resistance readings. For this reason, measuring the switch resistance may not help to conclude a faulty switch. However, the absence of all resistance where there should be would indicate a circuit failure.

• | If the PNP switch parameter always reads In Gear in all gear ranges, test the signal circuit pin 3 of the transmission 16-way connector to pin F of the control solenoid (w/body and TCM) valve assembly side for an open.

⇒ | If no open is found, replace the manual shift shaft switch assembly.

• | Note -- In order to view the Park/Neutral data parameters with the engine off, it may be necessary to place the transmission shifter in gear and then move the start switch to the start position. This will allow the parameter to operate without the engine running.

### Reference Information

#### Schematic Reference

• | Automatic Transmission Controls Schematics

• | Engine Controls Schematics

#### Connector End View Reference

Component Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Description and Operation

Electronic Component Description

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

If there are any other engine or transmission related DTCs set, diagnose those DTCs first. With a scan tool observe the PNP Switch parameter in Engine Data list and move the gear shift lever from Park, to Reverse and through all the Drive 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 ranges. 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. Refer to Transmission Internal Mode Switch Logic.

### Circuit/System Testing

1. Ignition OFF, transmission in Park or Neutral, disconnect the transmission 16-way connector.
2. With the ignition On, test for 12 volts on pin 3 on the engine side of the transmission 16-way connector.

⇒ | If not as specified, test the signal circuit for an open or short to ground. If the circuit tests normal, continue with testing.

⇒ | If no open or short to ground is found, replace the ECM.

3. Ignition OFF, transmission in Park, connect the transmission 16-way connector. With the ignition On, note any change with the scan tool Park/Neutral status.

⇒ | If no status change was noted, test the signal circuit for a short to voltage.

⇒ | If no short voltage is found and Park/Neutral status changed, test the manual shift shaft position switch for a short to ground.

⇒ | If no short to ground is found, continue testing.

4. Remove the transmission bottom fluid pan. Disconnect the manual shift shaft position switch connector from the control solenoid (w/body and TCM) valve assembly. Do not disconnect the transmission 16-way connector.

⇒ | If this action has no effect on the scan tool parameter, replace the control solenoid (w/body and TCM) valve assembly.

⇒ | If the Park/Neutral status changed, replace the manual shift shaft position switch.

### Component Testing

Important: The manual shift shaft position switch contacts are made of stainless steel. This material makes it difficult to obtain stable ohm readings to determine the switch resistance. Therefore, the diagnosis should eliminate all other components first, leaving the switch as the last possible cause.

1. Ignition OFF, transmission in Park or Neutral, disconnect the transmission 16-way connector and the manual shift shaft position switch connector from the control solenoid (w/body and TCM) valve assembly.
2. Test for infinite resistance between the control solenoid (w/body and TCM) valve assembly 16-way connector pin 3 and the manual shift shaft position switch connector pin F on the control solenoid (w/body and TCM) valve assembly side with the switch in the Open position -- Reverse or Drive.

⇒ | If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the control solenoid (w/body and TCM) valve assembly.

⇒ | Perform the Control Solenoid Valve and Transmission Control Module Assembly Inspection before replacing the control solenoid (w/body and TCM) valve assembly.

3. Test for infinite resistance between the IMS connector pin F and the IMS case ground with the switch in the Open position -- Reverse or Drive.

⇒ | If not the specified value, replace the IMS.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

If the IMS switch has been replaced, inspect the valve body detent spring to be sure it is centered over the detent lever. This prevents the detent spring from hitting the IMS switch.

• | Control Module References for control solenoid (w/body and TCM) valve assembly or ECM replacement, setup, and programming.

• | Manual Shift Shaft Position Switch Replacement for IMS replacement.