The 4L80-E transmission uses a line pressure control system that has the ability to adapt line pressure to compensate for normal wear of the following parts:
| • | The clutch fiber plates |
| • | The springs and seals |
| • | The apply bands |
This adaptive feature is similar to the fuel and idle control systems, where the PCM has the ability to learn and adjust for monitored system changes.
The PCM maintains information for the following transmission adaptive systems:
1-2, 2-3, 3-4 Upshift Adapts -- The PCM monitors the automatic transmission input (shaft) speed sensor (AT ISS) and the output speed sensor (OSS), to determine when the transmission has started, and completed an upshift. The PCM looks at the time from the beginning, until the completion of the upshift. If the time of the upshift was longer than a calibrated value, then the PCM adjusts the current to the transmission pressure control (PC) solenoid to increase line pressure for the next, same, upshift under identical conditions. If the time of the upshift was shorter than a calibrated value, then the PCM adjusts the current, to the transmission PC solenoid, to decrease line pressure for the next, same, upshift under identical conditions.
Clearing Transmission Adaptive Pressure (TAP)
Transmission adaptive pressure (TAP) information is displayed and may be reset using a scan tool. The adapt function is a feature of the PCM that either adds or subtracts line pressure from a calibrated base line pressure in order to compensate for normal transmission wear. The TAP information is divided into 13 units, called cells. The cells are numbered 4 through 16. Each cell represents a given torque range. TAP cell 4 is the lowest adaptable torque range and TAP cell 16 is the highest adaptable torque range. It is normal for TAP cell values to display zero or negative numbers. This indicates that the PCM has adjusted line pressure at or below the calibrated base line pressure.
Updating TAP information is a learning function of the PCM designed to maintain acceptable shift times. It is not recommended that TAP information be reset unless one of the following repairs has been made:
| • | Transmission overhaul or replacement |
| • | Repair or replacement of an apply or release component (clutch, band, piston, servo) |
| • | Repair or replacement of a component or assembly which directly affects line pressure |
Resetting the TAP values using a scan tool will erase all learned values in all cells. As a result, the PCM will need to relearn TAP values. Transmission performance may be affected as new TAPs are learned. Learning can only take place when the PCM has determined that an acceptable shift has occurred. The PCM must also relearn TAP values if it is replaced.
