DTC P1871 Undefined Gear Ratio 5.7L and 7.4L Gas
| Table 1: | 4L80-E Range Reference, modified for DTCs P1870 & P1871 |
| Table 2: | P1871 |
Range | Gear | 1-2 Sol | 2-3 Sol | Fourth Clutch | Overrun Clutch | O/D Roller Clutch | FWD Clutch | Direct Clutch | Front Band | Interm. Sprag Clutch | Interm Clutch | Lo Roller Clutch | Rear Band |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Overdrive | 1st | On | Off | -- | -- | H | A | -- | -- | * | -- | H | -- |
2nd | Off | Off | -- | -- | H | A | -- | -- | H | A | O | -- | |
3rd | Off | On | -- | -- | H | A | A | -- | O | A | O | -- | |
4th | On | On | A | -- | O | A | A | -- | O | A | O | -- | |
| |||||||||||||
Circuit Description
The 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 TP DTCs P0121, P0122, or P123. |
| • | No VSS DTC P502. |
| • | No ISS DTCs P0716 or P0717. |
| • | No 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 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) (5.7L) |
| - | 500 lb ft (627 N·m) (7.4L) |
| • | 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
- 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. |
| • | 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 table.
-
This step tests the indicated range signal to the actual selected range. A faulty switch could set a TFP Val. Position Sw. DTC P1870.
-
This step tests for proper ratios in each commanded gear state.
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||
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 TFP Switch display match each selected gear range? | -- | Refer to DTC P1810 Pressure Switch Assembly Malfunction Diagnosis, Section 7 | ||||||||
Are the gear ratios within the parameters for each specified range? | -- | Refer to Diagnostic Aids | Refer to Symptom Diagnosis tables in this section | |||||||
4 | In order to verify your repair, perform the following procedure:
Has the test run and passed? | -- | System OK |
