Mechanical Shift

Diagnostic Chart:

Wiring Diagram:

Circuit Description
The 4WD low circuit consists of a 4WD indicator assembly, a selector quadrant switch, a front axle switch and wiring. When the operator moves the selector quadrant switch to 4 LOW, the 4WD indicator lamp illuminates and circuit 1694 voltage to the VCM changes from B+ to zero. The front axle switch closes and completes circuit 1695 to circuit 150, illuminating the front axle lamp.

The front axle switch does not affect circuit 1 694 signal input to the VCM because the switch is also closed in the 4WD high position. The 4WD low switch signal corrects the transmission output speed signal to the VCM. This signal compensates for transfer case gear reduction. The VCM uses the transmission output speed signal to adjust shift points, line pressure and TCC scheduling. When the VCM detects a continuous open or short to ground in the 4WD low circuit, then DTC P1875 sets. DTC P1875 is a type A DTC.

Conditions for Setting the DTC

^ No Throttle Position (TP) DTCs P0121, P0122, or P0123.
^ No Vehicle Speed Sensor Assembly (VSS Assy.) DTC P0502.
^ No TCC Sol. Valve DTC P0740.
^ No TCC Stuck ON DTC P0742.
^ No 1-2 SS Valve DTC P0751 or P0753.
^ No 2-3 SS Valve DTC P0756 or P0758.
^ No Transmission Fluid Pressure Manual Valve Position Switch (TFP Val. Position Sw.) DTC P1810.
^ No TCC Pulse Width Modulation (PWM) Sol. Valve DTC P1860.
^ The engine speed is greater than 450 RPM for 8 seconds.
^ Not in fuel cutoff.
^ The gear range is D4.
^ The vehicle speed is greater than 11 km/h (7 mph).
^ The TP angle is 17-50%.
^ The engine torque is between 50-400 lb. ft.
^ The engine vacuum is between 0-105 kPa.
^ The shift solenoid performance counters are zero.
^ The Transmission Fluid Temperature (TFT) is 20-120°C (68-248°F).
^ All of the above conditions are met and either of the following fail conditions occurs:

- Four-wheel drive low is stuck ON for 5 seconds. The TCC slip speed is -3000 to -50 RPM. Four-wheel drive low range is not engaged. The speed ratio is 0.8-1.2. (The speed ratio is the engine speed divided by the transfer case output speed).
- Four-wheel drive low is stuck OFF for 10 seconds. The TCC is commanded ON. The TCC slip speed is 100-3000 RPM. Four-wheel drive low range is engaged. The speed ratio is 2.5-2.9.

Action Taken When the DTC Sets

^ The VCM commands a normal shift pattern (not a 4WD low shift pattern).
^ The VCM illuminates the Malfunction Indicator Lamp (MIL).

Conditions for Clearing the MIL/DTC

^ The VCM turns OFF the MIL after three consecutive ignition cycles without a failure reported.
^ A scan tool can clear the DTC from the VCM history. The VCM clears the DTC from the VCM history if the vehicle completes 40 warm-up cycles without a failure reported.
^ The VCM cancels the DTC default actions when the fault no longer exists and the ignition is OFF long enough in order to power down the VCM.

Diagnostic Aids

^ Inspect the wiring for poor electrical connections at the VCM. Inspect the wiring for poor electrical connections at the transmission 20-way connector. Look for the following conditions:

- A bent terminal
- A backed out terminal
- A damaged terminal
- Poor terminal tension
- A chafed wire
- A broken wire inside the insulation

^ When diagnosing for an intermittent short or open condition, massage the wiring harness while watching the test equipment for a change.

Test Description
The numbers below refer to the step numbers on the diagnostic chart.

3. This step tests the voltage input to the 4WD indicator assembly.
4. This step tests the voltage input to the selector quadrant switch through the 4WD indicator assembly.