Most transmission control modules use inputs from other sensors on the vehicle. If a vehicle can’t accurately calculate the load on the engine, it will adjust the line pressure and slippage to the inaccurate calculation. This can damage the transmission.
Sensors used to calculate the load can include the Mass Airflow Sensor (MAF), Throttle Position Sensor (TPS) and Manifold Air Pressure (MAP). If unmetered air is entering the cylinder through a leak, the engine load will be below the actual percentage. This can cause the module to use different shift points and line pressures. This could cause the transmission to delay shifts, overheat and possibly burn the fluid.
Maintenance items like a restrictive air filter, dirty air flow meter or blocked crank case ventilation system can change the calculated engine load to the point where it can influence shift points and shift quality.
Wiring Harness Problems
The wiring harness and connectors on most transmissions operate in a unique environment. Normal automatic transmission fluid’s conductivity is very low. Hybrids usually have a specification for a fluid that is non-conductive. In most cases, the fluid will not damage or short the connections; the detergents and chemicals will cause the degradation of the materials in the wiring harness that might be outside of the case.
Also, check for any damaged wires and connections that could be damaged by impact with road debris, weak or shifting motor mounts and hot exhaust systems.
Since the mid 1990s, the grounds for solenoids and sensors have changed dramatically. Never assume the chassis ground is coming through the case or valve body. Study the wiring diagrams before trying to diagnose a dead or open solenoid. Some transmissions can have multiple ground points for the solenoids, module and sensors.