P1871

DTC P1871 Undefined Gear Ratio (Diesel):

Performance Chart:

CIRCUIT DESCRIPTION
The PCM calculates a ratio based on the transmission input speed and output speed sensor readings. The PCM compares the known transmission ratio to the calculated ratio, for the selected gear range.

When the PCM detects an unknown ratio, then DTC P1871 sets. DTC P1871 is a type D DTC.

CONDITIONS FOR SETTING THE DTC
The following conditions are met for 10 seconds.

^ No Throttle Position (TP) Sensor DTCs P0121, P0122, P0123, P0220, P0221, P0222, P0223, P0225, P0226, P0227, P0228.
^ No Input Speed (ISS) Sensor DTCs P0716 or P0717.
^ No 0SS Sensor DTCs P0722 or P0723.
^ No Transmission Fluid Pressure Valve Position Switch (TFP Val. Position Sw.) DTC P1810.
^ No Four Wheel Drive Low DTC 1875.
^ The Four Wheel Drive Low fail counters are clear.
^ The Vehicle Speed is greater than 11 km/h (7 mph).
^ The throttle gear is greater than 25%.
^ The Transmission Fluid Temperature (TFT) is greater than 20°C (68°F).
^ 4.5 seconds must have elapsed since the last gear range change.
^ The engine speed is greater than 3650 RPM.
^ The engine torque is 80-450 lb ft.
^ The Gear Ratio is one of the following:

- Greater than 2.63 or less than 2.38
- Greater than 1.58 or less than 1.43
- Greater than 1.05 or less than 0.95
- Greater than 0.80 or less than 0.70
- Greater than 2.17 or less than 1.97

^ All conditions occur for 10 seconds.

ACTION TAKEN WHEN THE DTC SETS

^ The PCM commands maximum line pressure.
^ The PCM freezes shift adapts.

CONDITIONS FOR CLEARING THE MIL/DTC
1. The PCM turns off the Malfunction Indicator Lamp (MIL) after three consecutive ignition cycles without a failure reported.
2. A scan tool can clear the DTC from the PCM history. The PCM clears the DTC from the PCM history if the vehicle completes 40 warm-up cycles without a failure reported.
3. The PCM cancels the DTC default actions when the fault no longer exists and the ignition is OFF long enough in order to power down the PCM.

DIAGNOSTIC AIDS

^ Inspect the wiring for poor electrical connections at the PCM. 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.
^ First diagnose and clear any engine DTCs or Accelerator Pedal Position (APP) Sensor codes that are present. Then inspect for any transmission DTCs that may have reset.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic chart.

2. Step 2 tests the indicated range signal to the actual selected range. A faulty switch could set a TFP Val. Position Sw. DTC P1871.
3. Step 3 tests for proper ratios in each commanded gear state.