A/T - Intermittent DTC 87/High Ratio Error
File In Section: 7 - TransmissionBulletin No.: 57-71-15
Date: September, 1995
INFORMATION
Subject:
Intermittent DTC 87-High Ratio Error
Models:
1991 Chevrolet and GMC Truck R/V Models
1991-95 Chevrolet and GMC Truck C/K, P Models, G Vans
with 4L80E (MT1) Automatic Transmission
The following information will assist in the diagnosis of a Diagnostic Trouble Code (DTC) 87-High Ratio Error when it is setting intermittently and the normal diagnostic procedures are not leading to a resolution or the condition cannot be duplicated.
DTC 87 will set when ALL of the following conditions are met:
^ NO DTC(s) 21, 22, 24, 28, 71, 72, and 74 are present,
^ Throttle Position (TP) sensor is GREATER THAN 25%,
^ The transmission is not in Park, Reverse or Neutral,
^ Engine speed is GREATER THAN 300 RPM,
^ Vehicle speed is GREATER THAN 7 MPH,
^ Transmission fluid temperature is GREATER THAN 20°C (68°F),
^ Transmission gear ratio is numerically GREATER THAN 1.42 when 3rd or 4th gear is commanded.
ALL OF THESE CONDITIONS MUST BE PRESENT FOR 2 SECONDS.
Diagnostic Aids
1. DTC 87 indicates that a MECHANICAL malfunction exists.
2. When DTC 87 sets and is a "current" code, the PCM/TCM will immediately default the transmission to 2nd GEAR ONLY. The PCM/TCM will resume normal shifting patterns when DTC 87 is cleared or becomes "history".
3. Inspect the 2-3 SHIFT VALVE and the 3-4 SHIFT VALVE for sticking.
4. Inspect the CONTROL VALVE BODY FLUID PASSAGES, SPACER PLATE AND GASKETS for debris.
5. Inspect the TRANSMISSION CASE FLUID PASSAGES for debris.
6. Inspect the 1-2 and 2-3 SHIFT SOLENOIDS for a clogged filter.
7. The PCM/TCM has to command an upshift to 3rd or 4th gear with the transmission remaining mechanically in 2nd gear to set DTC 87. This indicates that electrical components, such as the SHIFT SOLENOIDS, WIRING, TOSS, TP SENSOR, ETC., are functioning properly.
8. DTC 81 will set when the 2-3 Shift Solenoid ELECTRICAL CIRCUIT has failed.
9. DTC 82 will set when the 1-2 Shift Solenoid ELECTRICAL CIRCUIT has failed.
10. DTC 85 will set when the gear ratio is NOT WITHIN a known gear ratio range (slipping components).
Road Testing Aids
1. Use a TECH 1 to ensure a MINIMUM 25% TP SENSOR POSITION during the 2-3 and 3-4 shift commands.
2. Duplicate the code setting condition by road testing the vehicle when the TRANSMISSION FLUID TEMPERATURE is cool, 20°C (68°F) and again when the the TRANSMISSION FLUID TEMPERATURE is at normal operating temperature, 82-93°C (180-200°F).
3. Verify line pressure using a mechanical gauge during the 2-3 shift and the 3-4 shift.
4. With the vehicle at a stop, brakes applied and selector lever in O/D, use the TECH 1 output test to command 3rd gear. Moderately accelerate the vehicle and observe the engine RPM. The engine RPM will raise quickly if the transmission has a hydraulic failure. If the vehicle accelerates slowly and smoothly, the 3rd gear hydraulic circuit is most likely not leaking.
5. Repeat the procedure in Step 4 using the TECH 1 output test to command 4th gear. The engine RPM will raise quickly if the transmission has a hydraulic failure. If the vehicle accelerates slowly and smoothly, the 4th gear hydraulic circuit is most likely not leaking.