Computer Transmission Shift Adaptive Relearning
Many transmissions produced today are adaptive, or programmable. On these transmissions, the timing of the release and application of elements (clutch packs and bands) is controlled by the transmission control module (a microprocessor). As the transmission shifts gears, one element is released as another is applied. If too much time occurs between the release of one element and the application of the next, a ‘rev up’ of the engine will occur during the shift. If too little time occurs between the release of one element and the application of the next, a ‘bind up’ of the transmission will occur.
The processor adjusts the timing values as the vehicle is driven, seeking to achieve the ideal shift parameters. This adjustment of timing values, known as ‘learning’ or ‘adapting’ occurs over a period of time while driving. The transmission controller never reaches a perfect adaptation value, because changes trigger constant adaptation. These changes include the driving habits of the operator, changes in driving conditions, and wear within the transmission. When a complete overhaul of a transmission is performed, it is necessary to go through a ‘re-learning’ procedure conducted by a transmission technician.
Adaptives and Fail Safe on Mercedes Benz
Mercedes Benz transmissions have a totally unique method of controlling the shift valves and solenoids, and specialized equipment is required to relearn the shift adaptives. If a fault code is set the computer may put the transmission into fail safe, which means that the signal that energizes the solenoids is usually cut. When a vehicle goes into fail safe, it usually puts the vehicle into either 2nd or 3rd gear, and prevents it from shifting. It will still go forward and reverse. On a Mercedes, however, when the solenoid power is cut off, the transmission stays in its current gear, regardless of which gear that is. It will remain in that gear even if the engine is shut off and restarted. This can be of benefit if the transmission goes into fail safe while on a highway many miles from a service facility, as it will be easier to maintain highway speeds in your current gear. However, if it goes into fail safe in a lower gear, it will make driving normal speeds difficult and could cause damage if driven for an extended period of time in that state.
Lexus and Toyota Transmission Shift Learning Process
In some cases, the learning process of the transmission controller is slow to compensate for a more drastic change in the shift characteristics. This can occur during a fluid change if characteristics of the new fluid such as viscosity and lubricity vary greatly from the old fluid. We have seen this occur in the newer Toyota transmissions that come in the Camry, Highlander and Solara. The Lexus models include the ES300 RX300 and RX330. These transmissions are very sensitive and may operate undesirably after a transmission fluid change. The failure of the controller to properly adapt may cause gear slipping or binding during a shift. If the transmission fluid needs replacement, it should only be replaced using genuine Toyota fluid and only 3 quarts at a time, allowing a couple days of driving between changes. If major work is done on these transmissions, it may be necessary to reprogram the transmission controller using specialized scanning tools and software.
Transmission Adaptive Parameter Fault Codes
Over time, some transmissions may reach the limit of their adaptive parameters. While usually unobserved by the driver, the transmission control module may determine that an error has occurred, and produce an error code. This is quite common on the Buick Lesabre and Regal, the Chevy Impala, Lumina, and Monte Carlo, and the Pontiac Bonneville and Grand Prix models. The BMW series vehicles with the ZF5HP24 and ZF5HP30 transmissions are also susceptible to these fault codes, because the controller can no longer compensate for wear within hydraulic circuits or clutch packs.
On these vehicles and many others, this problem is caused by the calibration springs within the transmission. Springs under constant tension fatigue over time and lose a portion of their value, just like a worn out mattress. Replacing the springs can be done without removing the transmission, and usually restores proper function. The spring kit for BMW transmissions is made by ZF of Germany, and is called ‘Life 2’. Other vehicles can be upgraded using spring kits such as the one for the GM 4T65-E transmission made by TransGo.