Diagnostic Instructions
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°C (-4 to +266°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. |
Reference Information
Schematic Reference
Automatic Transmission Controls Schematics
Connector End View Reference
Component Connector End Views
Description and Operation
Transmission General Information
Electrical Information Reference
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference
Transmission Control Module Scan Tool Information
Special Tools
J 35616
GM-Approved Terminal Test Kit
Circuit/System Verification
Important: It may be necessary to drive the vehicle slightly forward to engage 4LO on the transfer case.
Verify the scan tool displays Disabled when you select 4HI and Enabled when you select 4LO.
Circuit/System Testing
- Remove connector from the transfer case control module. With ignition ON and engine OFF, observe with the scan tool the 4WD Low parameter.
⇒ | If the scan tool indicates Enabled, test the 4WD Low signal circuit for a short to ground. |
- Using the
J 35616
GM-approved terminal test kit, install a fused jumper between the 4WD low signal circuit and ground at the transfer case control module connector.
⇒ | If the scan tool 4WD Low parameter indicates Enabled when grounded, replace the transfer case control module. |
- Test the 4WD Low signal circuit for an open.
⇒ | Fix the open in the 4WD Low circuit. |
- Replace the ECM.
Component Testing
Verify the 4WD Low parameter on scan tool while cycling the 4WD Low switch.
Repair Instructions
Perform the
Diagnostic Repair Verification
after completing the repair.
Control Module References
for replacement, setup, and programming