The ZF 4HP 16 automatic transmission utilizes a line pressure control system during upshifts to compensate for normal wear of clutch fiber plates, seals, springs, etc. By adjusting the line pressure, the transmission control module (TCM) can maintain acceptable transmission shift times. This process is known as adaptive learning, or shift adapts, and is similar to the closed loop fuel control system used for the engine.

In order for the TCM to perform a shift adapt, it must first identify if an upshift is acceptable to analyze. For example, upshifts that occur during cycling of the A/C compressor or under extreme throttle changes could cause the TCM to incorrectly adjust line pressure. When an upshift is initiated, a number of contingencies, such as throttle position, transmission temperature, and vehicle speed, are checked in order to determine if the actual shift time is valid to compare to a calibrated desired shift time. If all the contingencies are met during the entire shift, then the shift is considered valid and the adapt function may be utilized if necessary.

Once an adaptable shift is identified, the TCM compares the actual shift time to the desired shift time and calculates the difference between them. This difference is known as the shift error. The actual shift time is determined from the time that the TCM commands the shift to the start of the engine RPM drop initiated by the shift. If the actual shift time is longer than the calibrated desired shift time, a soft feel or slow engagement, then the TCM decreases current to the pressure control (PC) solenoid in order to increase line pressure for the next, same, upshift under identical conditions. If the actual shift time is shorter than the calibrated desired shift time, a firm engagement, then the TCM increases current to the PC solenoid in order to decrease line pressure for the next, same, upshift under identical conditions.

The purpose of the adapt function is to automatically compensate the shift quality for the various vehicle shift control systems. It is a continuous process that will help to maintain optimal shift quality throughout the life of the vehicle.

Transmission adaptive pressure (TAP) information is displayed and may be reset using a scan tool. The adapt function is a feature of the TCM that either adds or subtracts line pressure from a calibrated base line pressure in order to compensate for normal transmission wear.

Updating TAP information is a learning function of the TCM designed to maintain acceptable shift times. It is not recommended that TAP information be reset unless one of the following repairs has been made:

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.