P1871

DTC P1871 Undefined Gear Ratio (Gas):

Performance Chart:

CIRCUIT DESCRIPTION
The Vehicle Control Module (VCM) calculates ratio based on the transmission input speed and output speed sensor readings. The VCM compares the known transmission ratio calibration values to the calculated ratio.

When the VCM 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) DTCs P0121, P0122, or P0123.
^ No Vehicle Speed Sensor (VSS) DTC P0502.
^ No Input Speed (ISS) DTCs P0716 or P0717.
^ No Transmission Fluid Pressure Valve Position Switch (TFP Val. Position Sw.) DTC P1810.
^ No Four Wheel Drive Lo DTC 1875.
^ The Four Wheel Drive Lo 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).
^ 10 seconds must have elapsed since the last gear range change.
^ The engine torque is 80 lb. ft. to the following:

- 375 lb. ft (470 N.m).

^ The engine is running for greater than 5 seconds and is not in fuel cut off.
^ 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

ACTION TAKEN WHEN THE DTC SETS

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

CONDITIONS FOR CLEARING THE DTC

1. 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.
2. 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.
^ First diagnose and clear any engine DTCs or TP Sensor codes that are present. Then inspect for any transmission DTCs that may have reset.
^ A false or incorrect VSS signal can set DTC P1871. Inspect for any after-market equipment.

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 P1870.
3. Step 3 tests for proper ratios in each commanded gear state.