With 4L60-E Automatic Transmission

4WD Low Switch Check (With Mechanical Shift) Diagnostic Chart:

4WD Low Switch Check:

CIRCUIT DESCRIPTION
The 4WD low circuit consists of a 4WD indicator assembly, a selector switch assembly, front axle switch and wiring. When the operator moves the transfer case select lever to 4WD low, the selector switch completes CKT 39 to CKT 150. When the circuit is complete the 4WD low indicator lamp will illuminate, CKT 1694 to the PCM will change from B+ to zero. The front axle switch will close and complete CKT 1695 to CKT 150. The front axle switch will modify antilock brakes and illuminate the front axle lamp. The front axle switch does not effect CKT 1694 signal input to the PCM because the switch is also closed in the 4WD high position. The 4WD low switch signal is used to correct the transmission output speed signal from the Vehicle Speed Signal (VSS) Buffer Module to the Powertrain Control Module (PCM) to compensate for transfer case gear reduction.

ACTION TAKEN (PCM will default to)
No DTC(s) will be set; however, if CKT 1694 is shorted to B+, the transmission will have late or no upshifts in 4WD low. If CKT 1694 is shorted to ground or open, the transmission will have an early shift pattern.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic charts.
1. This test checks for mechanical adjustment and ignition feed.
2. This test checks the integrity of the 4WD low circuit.
3. This test checks the selector switch.

DIAGNOSTIC AIDS
For further information, refer to Transmission and Drivetrain. For further information on the 4WD indicator refer to the appropriate wiring diagrams.