* Perform the Diagnostic System Check - Vehicle (See: Testing and Inspection/Initial Inspection and Diagnostic Overview/Diagnostic System
Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (See: Testing and Inspection/Initial Inspection and Diagnostic Overview/Strategy Based Diagnosis) for an
overview of the diagnostic approach.
* Diagnostic Procedure Instructions (See: Testing and Inspection/Initial Inspection and Diagnostic Overview/Diagnostic Procedure
Instructions)provides an overview of each diagnostic category.
DTC Descriptor
DTC P2771
- Four Wheel Drive (4WD) Low Switch Circuit
Circuit/System Description
The four wheel drive (4WD) low circuit consists of the engine control module (ECM), a transfer case control module and the circuit wiring. The
transfer case control module controls the 4WD low signal circuit. When the operator selects 4WD low, the transfer case control module grounds the
signal circuit, and the 4WD low signal voltage on the circuit changes from ignition voltage to zero volts. The ECM then compensates for transfer
case gear reduction in the transmission output shaft speed (OSS) sensor signal. The ECM uses the transmission OSS sensor signal to adjust shift
points, line pressure and torque converter clutch (TCC) scheduling.
Conditions for Running the DTC
* No throttle position (TP) sensor DTCs P0122, or P0123.
* No OSS assembly DTCs P0722 or P0723.
* No cam or crank sensor DTCs.
* The system voltage is 9-18 volts.
* The engine speed is 1,000-5,500 RPM for at least 30 seconds.
* The vehicle speed is at least 40 km/h (25 mph).
* The TP angle is 5-40 percent.
* The gear range is not PARK, NEUTRAL or REVERSE.
* The TFT is -20 to +130锟斤拷C (-4 to +266锟斤拷F).
Conditions for Setting the DTC
Condition 1
* The 4WD low switch is not in 4WD low, open/12 volts.
* The Transfer Case Ratio is 2.64-2.9 for 2 seconds.
* Both conditions must be met twice.
Condition 2
* The 4WD low switch is in 4WD low.
* The Transfer Case Ratio is 0.8-1.75 for 2 seconds.
Action Taken When the DTC Sets
* DTC P2771 is a Type B DTC.
* The ECM commands a normal shift pattern, not a 4WD low shift pattern.
Conditions for Clearing the DTC
DTC P2771 is a Type B DTC.
Diagnostic Aids
A malfunctioning speed sensor could cause DTC P2771 to set.
* Verify that the input speed sensor (ISS) and the OSS are functioning properly.
* In 3rd gear at approximately 64 km/h (40 mph), command TCC ON. If functioning properly the OSS will track ISS and the engine RPM. If
not refer to DTC P0716 for the ISS and DTC P0723 for the OSS.
* If the speed sensors pass testing, refer to Testing for Intermittent Conditions and Poor Connections (See: Testing and Inspection/Component
Tests and General Diagnostics/General Electrical Diagnostic Procedures/Circuit Testing/Testing for Intermittent Conditions and Poor
Connections).
Reference Information
Schematic Reference
Automatic Transmission Controls Schematics (See: Transmission and Drivetrain/Automatic Transmission/Transaxle/Diagrams)
Connector End View Reference
