Circuit Description
The 4WD low circuit consists of a transfer case switch, a front axle switch, and a front axle actuator. When 4WD low is engaged, circuit 1694 voltage to the VCM changes from ignition voltage to zero. 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 TP DTCs P0121, P0122, or P0123 |
| • | No VSS Assy. DTC P0502 |
| • | No TCC Stuck ON DTC P0742 |
| • | No 1-2 SS Valve DTC P0753 |
| • | No 2-3 SS Valve DTC P0758 |
| • | No TFP Val. Position Sw. DTC P1810 |
| • | No TCC PWM Sol. Valve DTC P1860 |
| • | No TCC Sol. Valve DTC P1864 |
| • | The engine speed is greater than 400 RPM for 8 seconds. |
| • | Not in fuel cutoff |
| • | The gear range is D4. |
| • | The TP angle is 17-50%. |
| • | The 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 soft landing to 3rd gear. |
| • | The VCM commands maximum line pressure. |
| • | The VCM inhibits TCC engagement. |
| • | The VCM freezes shift adapts from being updated. |
| • | 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 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) Powertrain On Board Diagnostic (OBD) System Check (5.0L and 5.7L) | |||||||||||||
2 |
Important: Before clearing the DTCs, use the scan tool in order to record the Freeze Frame and Failure Records for reference. The Clear Info function will erase the data. Does the scan tool 4WD Low display No when you select 4HI, then Yes when you select 4LO? | -- | Go to Diagnostic Aids | |||||||||||||
Does the scan tool 4WD Low indicate Yes? | -- | |||||||||||||||
Does 4WD Low indicate Yes when circuit 1694 is grounded? | -- | |||||||||||||||
5 | Replace the transfer case selector switch. Refer to Transfer Case Selector Switch Replacement, Section 7. Is the replacement complete? | -- | -- | |||||||||||||
6 |
Refer to Electrical Diagnosis, Section 8. Did you find a problem? | -- | ||||||||||||||
7 |
Refer to Electrical Diagnosis, Section 8. Did you find a problem? | -- | ||||||||||||||
8 | Replace the VCM. Refer to Section 6: VCM Replacement/Programming (4.3L) VCM Replacement/Programming (5.0L and 5.7L) Is the replacement complete? | -- | -- | |||||||||||||
9 | In order to verify your repair, perform the following procedure:
Has the test run and passed? | -- | System OK |
