Circuit Description
The Four Wheel Drive (4WD) low circuit consists of the Vehicle Control Module (VCM), a transfer case switch, and the circuit wiring. The transfer case switch controls the 4WD low signal on circuit 1694. When the operator selects 4WD low, the transfer case switch closes and the 4WD low signal voltage on circuit 1694 changes from ignition voltage to zero volts. The VCM then compensates for transfer case gear reduction in the transmission Output Shaft Speed (OSS) signal. The VCM uses the transmission OSS signal to adjust shift points, line pressure and Torque Converter Clutch (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 B DTC.
Conditions for Setting the DTC
DTC P1875 sets if the following conditions are met two consecutive times:
| • | No TP Sensor DTCs P0122, or P0123 |
| • | No VSS Assy. DTCs P0502 or P0503 |
| • | 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 TFP Val. Position Sw. DTC P1810 |
| • | No TCC PWM Sol. Valve DTC P1860 |
| • | The engine speed is greater than 450 RPM for 5 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 ft. lbs. |
| • | The engine vacuum is between 0-105 kPa. |
| • | The shift solenoid performance counters are zero. |
| • | The TFT is 20-130°C (68-266°F). |
| • | All of the above conditions are met and either of the following fail conditions occurs two consecutive times: |
| - | 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 illuminates the Malfunction Indicator Lamp (MIL). |
| • | The VCM commands a normal shift pattern (not a 4WD low shift pattern). |
| • | DTC P1875 stores in VCM history. |
Conditions for Clearing the MIL/DTC
| • | The VCM turns OFF the MIL after three consecutive trips 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 at the VCM, the transfer case switch connector and all other circuit connecting points 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 |
| • | Moisture intrusion |
| • | Corrosion |
| • | 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 table.
-
This step tests for a short to ground in circuit 1694.
-
This step tests for an open in circuit 1694 or a faulty transfer case switch.
Step | Action | Value(s) | Yes | No | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Powertrain On Board Diagnostic (OBD) System Check (4.3L) or Powertrain On Board Diagnostic (OBD) System Check (5.0L, 5.7L) in Engine Controls | |||||||||||||
2 |
Important: Before clearing the DTC(s), use the Scan Tool in order to record the Freeze Frame and Failure Records for reference. Using the Clear Info function will erase the stored Freeze Frame and Failure Records from the VCM. 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 Diagnostic Aids | |||||||||||||
Does the Scan Tool 4WD Low indicate Enabled? | -- | |||||||||||||||
Does 4WD Low indicate Enabled when circuit 1694 (GRY/BLK) is grounded? | -- | |||||||||||||||
5 | Replace the transfer case selector switch. Refer to Transfer Case Switch Replacement. Is the replacement complete? | -- | -- | |||||||||||||
6 |
Refer to General Electrical Diagnosis in Wiring Systems. Refer to Wiring Repairs in Wiring Systems. Did you find a short to ground condition? | -- | ||||||||||||||
7 |
Refer to General Electrical Diagnosis in Wiring Systems. Refer to Wiring Repairs in Wiring Systems. Did you find an open condition? | -- | ||||||||||||||
8 | Replace the VCM. Refer to VCM Replacement/Programming (4.3L) or VCM Replacement/Programming (5.0L, 5.7L) in Engine Controls. Is the replacement complete? | -- | -- | |||||||||||||
9 | In order to verify your repair, perform the following procedure:
Has the test run and passed? | -- | System OK |
