DTC 85 Undefined Gear Ratio L57 MFI
| Table 1: | 4L80-E Range Reference, modified for DTCs P1870 & P1871 |
| Table 2: | DTC 85 |
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 transmission control module (TCM) calculates gear ratio from the automatic transmission input shaft speed (A/T ISS) sensor and output shaft speed (OSS) sensor readings. The TCM compares the known transmission gear ratio to the calculated gear ratio for the selected gear range.
When the TCM detects an unknown transmission gear ratio, DTC 85 sets.
Conditions for Running the DTC
| • | No DTCs 21, 22, 24, 28, 71, 72 and 87. |
| • | TP is greater than 25%. |
| • | Not in P/N or 4th gear commanded gear. |
| • | Engine speed is greater than 300 RPM. |
| • | Vehicle speed is greater than 7 mph. |
Conditions for Setting the DTC
| • | Transmission fluid temperature (TFT) is greater than 24.5°C (76°F). |
| • | All conditions are met for 2 seconds. |
Action Taken When the DTC Sets
| • | The TCM does not illuminate the malfunction indicator lamp (MIL). |
| • | The TCM commands maximum line pressure. |
| • | The TCM inhibits TCC operation. |
| • | The TCM stores DTC 85 in TCM history. |
Conditions for Clearing the DTC
| • | A scan tool clears the DTC from TCM history. |
| • | The TCM clears the DTC from TCM history if the vehicle completes 40 consecutive key cycles without a diagnostic fault occurring. |
| • | The TCM 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 TCM. |
Diagnostic Aids
| • | DTC 85 sets when an unknown gear ratio is detected for any gear but 4th gear. |
| • | First, diagnose and clear any engine DTCs or TP sensor codes 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 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 manual valve position switch could set DTC 28.
-
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 Transmission Fluid Checking Procedure complete? | -- | Go to Transmission Fluid Check | |||||||||||||
Important:: Before clearing the DTCs, use the scan tool in order to record the Failure Records. Using the Clear Info function erases the Failure Records from the TCM. Refer to the Range Signal table. Does the Scan Tool TFP Switch A/B/C display match each selected gear range? | -- | |||||||||||||||
Are the gear ratios within the specified range for each gear? (Refer to the Gear Ratio table). | -- | Go to Diagnostic Aids | ||||||||||||||
5 |
Refer to Line Pressure Check . Is the line pressure within specifications for each selected gear range? | -- | ||||||||||||||
6 |
Refer to Automatic Transmission Fluid and Filter Replacement . Was excessive contamination found? | -- | Go to Unit Repair | |||||||||||||
7 | Inspect the transmission for the following problems:
Was a condition found? | -- | Go to Unit Repair | |||||||||||||
8 | Is the system line pressure low, only in the specific gear which indicated an incorrect gear ratio? | -- | Go to Symptom Diagnosis Low Line Pressure | |||||||||||||
9 | Inspect for fluid pressure loss in the following areas:
Was a condition found? | -- | Go to Unit Repair | |||||||||||||
10 | Perform the following procedure in order to verify the repair:
Are the gear ratios within the specified range for each of the selected gears? | 1st: 2.38-2.63 2nd: 1.43-1.58 3rd: 0.95-1.05 Rev: 1.97-2.17 | System OK |
