The EPC Electronic Pressure Control Solenoid
Today’s electronic transmissions use a control module to control transmission line pressure, shift timing and quality. It receives inputs from sensors like the MAF or MAP, VSS, and ECT and then compares them to information stored in its memory. It then pulses this solenoid on and off to raise or lower line pressure according to demand.
The EPC controls a pressure regulating spool valve and spring that holds or releases fluid pressure, typically to the transmission's sump. It’s a variable force solenoid that changes at a rate of around 32 Hz (32 times per second). This gives the electronic solenoid more precise control than a conventional hydraulic pressure control system. The EPC system is more efficient and assists the engine to stay within its power band by changing system pressures.
The system uses spring pressure to keep the valve closed providing maximum system pressure. As the module increases current to the solenoid's windings, the EPC plunger and spool valve are moved, opening the exhaust port and reducing line pressure.
On many of these systems high line pressure is the result of an unplugged or damaged circuit leading to the solenoid. This is because the control module pulses the solenoid longer with a high duty cycle to reduce line pressure (~70-80% on time) and shorter duty cycle to increase line pressure. The longer it's on the lower the pressure.
If the vehicle’s line pressure is above or below specifications, the EPC solenoid and its circuit can be tested and diagnosed with a scanner and an ohmmeter. Check with the vehicle manufacturer for the type of system used and any special testing procedures that must be performed. Always use a high impedance multimeter when testing sensitive computer circuits.