DTC P1871 Undefined Gear Ratio Diesel Only
| Table 1: | 4L80-E Range Reference, modified for DTCs P1870 & P1871 |
| Table 2: | P1871 (97 C/K & G600 PCM) |
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 | -- | |
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Circuit Description
The Powertrain Control Module (PCM) calculates a ratio based on the Automatic Transmission Input Shaft Speed (A/T ISS) Sensor and Output Shaft Speed (OSS) Sensor readings. The PCM compares the known transmission ratio to the calculated ratio, for the selected gear range.
If 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 A/T ISS Sensor DTCs P0716 or P0717. |
| • | No OSS Sensor DTCs P0722 or P0723. |
| • | No TFP Val. Position Sw. DTC P1810. |
| • | The Vehicle Speed is greater than 11 km/h (7 mph) |
| • | The APP Angle 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 speed is greater than 475 RPM for at least 7 seconds. |
| • | The engine speed is less than 3750 RPM. |
| • | The engine torque is 100-565 N·m (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
- 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.
- 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 problems: |
| - | 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, massage the wiring harness while watching the test equipment for a change. |
| • | First diagnose and clear any engine DTCs that are present. Then inspect for any transmission DTCs that may have reset. |
| • | Sticking or contaminated shift valves may cause an undefined gear ratio. |
| • | Refer to the Shift Solenoid Valve State and Gear Ratio table. |
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 TFP Val. Position Sw. could set DTC P1871.
-
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? | -- | ||||||||||||||
2 | Perform the transmission fluid checking procedure. Refer to Transmission Fluid Check . Is the procedure complete? | -- | Go to Transmission Fluid Check | |||||||||||||
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 A/B/C display match each selected gear range? (Refer to the Range Signal table). | -- | Refer to DTC P1810 Pressure Switch Assembly Malfunction , in this Section. | ||||||||||||||
Are the gear ratios within the parameters for each specified range? | -- | Refer to Diagnostic Aids | ||||||||||||||
5 |
Refer to the Line Pressure Check . Is the line pressure within specifications for each selected gear range? | -- | ||||||||||||||
6 |
Refer to Changing the Fluid and Filter, in On-Vehicle Service. Did you find excessive contamination? | -- | Go to Transmission Overhaul , in Unit Repair | |||||||||||||
7 | Inspect the transmission for the following problems:
Did you find a problem? | -- | Go to Transmission Overhaul , in Unit Repair | |||||||||||||
8 | Is the system line pressure low only in the specific gear which indicated an incorrect gear ratio? | -- | Go to System Diagnosis Table; Low Line Pressure | |||||||||||||
9 | Inspect for fluid pressure loss in the following areas:
Did you find a problem? | -- | Go to Transmission Overhaul , in Unit Repair | |||||||||||||
10 | In order to verify your repair, perform the following procedure:
Has the test run and passed? | -- | System OK |
