Transmission Diagnosis

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

Schematic:

Circuit Description with 4L80-E / 4L85-E Automatic Transmission
Vehicles equipped with an active transfer case have a four-wheel drive (4WD) low circuit consisting of a Powertrain Control Module (PCM), an active transfer case control module, and the circuit wiring. The active transfer case control module controls the 4WD low signal on circuit 1694. When the operator selects 4WD low, the active transfer case control module grounds circuit 1694, 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) signal. The PCM uses the transmission OSS signal to adjust shift points, line pressure and Torque Converter Clutch (TCC) scheduling.

Vehicles not equipped with an active transfer case, have a 4WD low circuit consisting of a PCM, transfer case switch, and the circuit wiring. The transfer case switch is used to control the 4WD low signal on circuit 1694. When the operator moves the 4WD selector lever to 4WD low, the transfer case switch closes, and the 4WD low signal voltage on circuit 1694 changes from B+ voltage to zero volts. The PCM then compensates for transfer case gear reduction in the transmission output shaft speed (OSS) sensor readings. The PCM uses the transmission OSS sensor signal in order to adjust shift points, line pressure, and TCC scheduling.

If the PCM detects a continuous open or short to ground in the 4WD low signal circuit, then DTC P1875 sets. DTC P1875 is a type B DTC.

CONDITIONS FOR RUNNING THE DTC
^ No ISS sensor DTCs P0716 or P0717.
^ No OSS sensor DTCs P0502 or P0503.
^ No TCC DTC P0742.
^ No 1-2 SS valve DTCs P0751, P0752, P1842 or P1843.
^ No 2-3 SS valve DTCs P0756, P0757, P1845 or P1847.
^ No TFP manual valve position switch DTC P1810, P1815, P1816, P1817 or P1818.
^ No TCC PWM solenoid valve DTC P1860.
^ The gear range is D4.
^ The APP angle is 9-99%.
^ The engine is running greater than 475 RPM for more than 7 seconds.
^ The vehicle speed is greater than 3.2 km/h (2.0 mph).
^ The engine torque is 95-675 N.m (70-500 lb ft).
^ System voltage is 8.0-18.0 volts.
^ The TFT is 20-130°C (68-248°F).

CONDITIONS FOR SETTING THE DTC
All of the above conditions for running the DTC are met and one of the following fail conditions exist:
^ Four wheel drive low circuit is closed (grounded) for greater than 5 seconds in any one gear, and the transfer case ratio is 0.95-1.05:1 for 2 occurrences.
^ Four wheel drive low circuit is open for 1.5 seconds in two different commanded gears, and the transfer case ratio is 2.65-2.75:1 for one occurrence.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM illuminates the Malfunction Indicator Lamp (MIL).
^ The PCM freezes shift adapts from being updated.
^ When stuck ON (closed), the PCM assumes a non 4WD low state for a transmission shift pattern.
^ When stuck OFF (open), the PCM assumes a 4WD low state for a transmission shift pattern.
^ The PCM stores DTC P1875 in PCM history.

CONDITIONS FOR CLEARING THE MIL/DTC
^ For Federal and California emissions, the PCM turns OFF the MIL during the third consecutive trip in which the diagnostic test runs and passes.
^ A scan tool clears the DTC from PCM history.
^ For Federal and California emissions, the PCM clears the DTC from PCM history if the vehicle completes 40 consecutive warm-up cycles without an emission related diagnostic fault occurring.
^ For vehicles equal to or greater than 15,000 lbs GVW, the PCM clears the DTC from PCM history if the vehicle completes 40 consecutive warm-up cycles without a non-emission related diagnostic fault occurring.
^ The PCM cancels the DTC default actions when the fault no longer exists and the ignition switch is OFF long enough in order to power down the PCM.

DIAGNOSTIC AIDS
Inspect the wiring for poor electrical connections at the PCM, the active transfer case control module (active transfer case vehicles), and at the transmission transfer case connector. Look for the following conditions:
- A backed out terminal
- A damaged terminal
- Reduced terminal tension
- A chafed wire
- A broken wire inside the insulation
- Moisture intrusion
- Corrosion
^ When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.
^ A faulty front axle actuator may cause a delayed, or a no upshift condition in 4WD low.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.
3. This step tests for an open in circuit 1694, a faulty transfer case control module (active transfer case vehicles) or a faulty transfer case switch (non-active transfer case vehicles).
4. This step tests for a short to ground in circuit 1694.