Common Causes of Transmission Complaints
Non-Transmission Sensors Causing Transmission Complaints
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.
Sensors used to calculate the load can include the Manifold Air Pressure (MAP), Mass Airflow Sensor (MAF) and Throttle Position Sensor (TPS). If unmetered air is entering the cylinder through a leak, the engine load will be below the actual percentage causing 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 dirty air flow meter, blocked crank case ventilation system, or restrictive air filter 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 weak or shifting motor mounts, hot exhaust systems, and impact with road debris.
The grounds for solenoids and sensors have changed dramatically since the mid-1990s. Never assume the chassis ground is coming through the valve body or case. Before trying to diagnose a dead or open solenoid, study the wiring diagrams. Some transmissions can have multiple ground points for the module, sensors, and solenoids.