Automatic Transmission EPC Solenoid
Today’s electronic transmissions use control modules to control the transmission pressure, shift timing, and quality. The module uses inputs from sensors like the MAF or MAP, VSS, and ECT and then compares them to a shift schedule stored in its memory.
The module is either integrated in to the PCM or a standalone TCM. The EPC or (electronic pressure control) is a PWM (pulse width modulated) solenoid. It typically replaces the throttle valve or vacuum modulator in hydraulic operated transmissions.
The TCM uses this solenoid to control mainline pressure through out the transmission. The EPC contains a spool valve and a spring. 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 the conventional hydraulic pressure control system.
The solenoid uses spring pressure to keep closed providing maximum system pressure. As the module increases current to the solenoids windings the EPC plunger/spool valve is moved opening the exhaust port and reducing system pressure.
If the vehicles line pressure is below specifications the EPC solenoid needs to be tested. Connect a pressure gauge to the transmissions EPC port and then start the engine. Then check the reading against manufacturer specifications. If the pressure is within specifications there may be an internal leak downstream to the EPC.
