Circuit Description
The engine control module (ECM) provides 12-volts to the park/neutral position (PNP) switch of the manual shift shaft position switch. The manual shift shaft position switch is a sliding contact assembly that attaches to the manual shift shaft inside the transmission. The switch consists of 5 separate circuits, 4 indicate gear selector position to the transmission control module (TCM), and 1 indicates park/neutral position to the ECM. When the transmission is 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 switch in order to enable starter operation.
If the PNP switch and the transmission manual shift shaft position switch do not agree for a specified length of time, DTC P0851 sets. DTC P0851 is a type C DTC.
DTC Descriptor
This Diagnostic procedure supports the following DTC:
DTC P0851 Park/Neutral Position (PNP) Switch Circuit Low Voltage
Conditions for Running the DTC
| • | No communication DTC U0073, U0100, U0121 or U0140. |
| • | The ignition voltage is 9-18-volts. |
| • | The engine is running. |
Conditions for Setting the DTC
The PNP switch is open for 3.2 seconds while the TCM is reporting the gear selector is in PARK or NEUTRAL.
Action Taken When the DTC Sets
| • | The ECM does not illuminate the malfunction indicator lamp (MIL). |
| • | The ECM uses the transmission range data to enable engine cranking. |
| • | The ECM records the operating conditions when the Conditions for Setting the DTC are met. The ECM stores this information as Failure Records. |
| • | The ECM stores DTC P0851 in ECM history. |
Conditions for Clearing the MIL/DTC
| • | The ECM clears the driver information center (DIC) message when the condition no longer exists. |
| • | A scan tool can clear the DIC/DTC. |
| • | The ECM clears the DTC from ECM history if the vehicle completes 40 warm-up cycles without a non-emission related diagnostic fault occurring. |
| • | The ECM cancels the DTC default actions when the ignition switch is OFF long enough in order to power down the ECM. |
Step | Action | Yes | No | ||||
|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | |||||
2 |
Important:
Did you record TCM Failure Records for DTC U0073, U0100, U0121 or U0140? | Go to Step 3 | |||||
3 |
Does the PNP Switch display Park/Neutral? | Go to Step 4 | Go to Testing for Intermittent Conditions and Poor Connections | ||||
4 | Disconnect the automatic transmission inline 20-way connector. Does the PNP Switch display Park/Neutral? | Go to Step 6 | Go to Step 5 | ||||
5 | Test the PNP switch signal circuit for a short to ground between the automatic transmission inline 20-way connector and transmission internal mode switch. Refer to Testing for Short to Ground . Did you find the condition? | Go to Step 7 | Go to Step 8 | ||||
6 | Test the PNP switch signal circuit for a short to ground between the automatic transmission inline 20-way connector and the ECM. Refer to Testing for Short to Ground . Did you find and correct the condition? | Go to Step 10 | Go to Step 9 | ||||
7 | Replace the automatic transmission wiring harness. Refer to Wiring Harness Replacement . Did you complete the replacement? | Go to Step 10 | -- | ||||
8 | Replace the transmission internal mode switch. Refer to Manual Shift Detent Lever with Shaft Position Switch Assembly Removal . Did you complete the replacement? | Go to Step 10 | -- | ||||
9 | Replace the ECM. Refer to Control Module References for replacement, setup and programming. Did you complete the replacement? | Go to Step 10 | -- | ||||
10 | Perform the following procedure in order to verify the repair:
Has the test run and passed? | Go to Step 11 | Go to Step 2 | ||||
11 | With the scan tool, observe the stored information, capture info and DTC info. Does the scan tool display any DTCs that you have not diagnosed? | System OK |
