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 of which indicate gear selector position to the transmission control module (TCM), and 1 that 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.
This Diagnostic procedure supports the following DTC:
DTC P0851 Park/Neutral Position (PNP) Switch Circuit Low Voltage
• | No communication DTCs U0073, U0100, U0121 or U0140. |
• | The ignition voltage is 9-18-volts. |
• | The engine is running. |
The PNP switch is open for 3.2 seconds while the TCM is reporting the gear selector is in PARK or NEUTRAL.
• | 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. |
• | 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 | Values | 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 in the Transmission Unit Repair Manual. 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 in the Transmission Unit Repair Manual. 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 |