Circuit Description
The engine control module (ECM) provides 12-volts to the Park/Neutral 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 five separate circuits, four of which indicate gear selector position to the transmission control module (TCM), and one 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 Park/Neutral 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 TCM DTCs P0601, P0602, P0603, P0604 or P062F. |
| • | No IMS DTCs P1820, P1822, P1823, P1825, P1826 or P1915. |
| • | The ignition voltage is 9-18 volts. |
Conditions for Setting the DTC
The Park/Neutral switch circuit voltage is low, 0 volts, for .01 seconds while the TCM is reporting the gear selector is in Reverse or any forward range.
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 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 | Value(s) | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |||||
2 |
Important:
| -- | 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 |
Important: The automatic transmission wiring harness consists of 2 separate harnesses, the Automatic Transmission Wiring Harness Assembly and the Automatic Transmission Wiring Extension Harness Assembly. In order to avoid the unnecessary replacement of parts, determine which harness contains the fault and replace only that harness. Replace the automatic transmission wiring harness. Refer to Input Speed Sensor and Wiring Harness Removal in the 4T80-E section of the Transmission Unit Repair Manual. Did you complete the replacement? | -- | Go to Step 10 | -- | ||||
8 | Replace the transmission internal mode switch. Refer to Control Valve Lower Body Disassemble in the 4T80-E section of the Transmission Unit Repair Manual. Is the action complete? | -- | Go to Step 10 | -- | ||||
9 | Replace the ECM. Refer to Control Module References for replacement, set-up and programming. Is the action complete? | -- | Go to Step 10 | -- | ||||
10 | Perform the following procedure 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 |
