The four wheel drive (4WD) low circuit consists of the powertrain control module (PCM), a transfer case control module and the circuit wiring. The transfer case control module controls the 4WD low signal on the 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 PCM then compensates for transfer case gear reduction in the transmission output shaft speed (OSS) sensor signal. The PCM uses the transmission OSS sensor signal to adjust shift points, line pressure and torque converter clutch (TCC) scheduling.
When the PCM detects a continuous open, short to ground or short to power in the 4WD low circuit, then DTC P2771 sets. DTC P2771 is a type B DTC.
DTC P2771 sets if one of the following conditions occurs:
The numbers below refer to the step numbers on the diagnostic table.
This step tests for a short to ground in the 4WD low signal circuit.
This step tests for an open in the 4WD low signal circuit or a faulty transfer case control module.
Did you perform the Diagnostic System Check - Engine Controls?
Go to Step 2
Go to Diagnostic System Check - Engine Controls in Engine Controls - 4.8L, 5.3L and 6.0L
Important: Before clearing the DTC, use the scan tool in order to record the Freeze Frame and Failure Records. Using the Clear Info function erases the Freeze Frame and Failure Records from the PCM.
Important: It may be necessary to drive the vehicle slightly forward to engage 4LO on the transfer case.
Does the scan tool 4WD Low display Disabled when you select 4HI, then Enabled when you select 4LO?
Go to Intermittent Conditions in Engine Controls - 4.8L, 5.3L and 6.0L
Go to Step 3
Does the scan tool 4WD Low indicate Enabled?
Go to Step 6
Go to Step 4
Does the scan tool 4WD Low indicate Enabled when the 4WD low signal circuit, is grounded?
Go to Step 5
Go to Step 7
Replace the transfer case control module.
Did you complete the replacement?
Go to Step 9
Test the 4WD low signal circuit for a short to ground.
Did you find and correct the condition?
Go to Step 8
Test the 4WD low signal circuit for an open.
Replace the powertrain control module (PCM).
Perform the following procedure in order to verify the repair:
Has the test run and passed?
Go to Step 10
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?
Go to Diagnostic Trouble Code (DTC) List in Engine Controls - 4.8L, 5.3L and 6.0L
When the PCM detects a continuous open, short to ground or short to power in the 4WD low circuit, then DTC P2771 sets. So make sure the gray/black wire is not shorting out anywhere.