The electronic pressure control (EPC) transmission switch controls the EPC unit, which regulates the hydraulic pressure in your automatic transmission. This regulation allows the transmission to change its resistance levels when the engine begins to work harder. With greater pressure, the engine has to work harder to shift the gears, which prevents it from running up against its redline. Conversely, when the engine is under load, the pressure is reduced by the EPC so the engine works less on having to moving the transmission gears and can use its energy to move the load.
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Within your automatic transmission there is a clutch (or two clutches in the case of a dual clutch transmission) that engages the gears (the number of gears will depend on your make and model of car). Hydraulic pressure pushes the clutch forward and back. The pressure is adjusted via a hydraulic pump and changing the spring rate on a solenoid.
The EPC solenoid (a electrically moved diaphragm) bleeds off pressure from the transmission, thus increasing the amount of work the hydraulic pump has to do to push or pull the clutch. This bleeding reduces excessive internal hydraulic pressure in order to prevent damage inside the transmission. While it may seem contradictory to intentionally increase the work an engine has to do, the goal is to keep the engine in its powerband. A powerband is a range of revolutions per minute (RPM) in which an engine performs best. By making the engine work more to pump the hydraulics, the EPC solenoid causes the engine to artificially stay within its powerband.
The EPC transmission switch is normally not user accessible. Only mechanics or professional drivers should modify the EPC switch settings. The switches are modified to increase or decrease the resistance of the solenoid so that it opens at different settings. The EPC monitors the airflow coming into the engine, the engine RPMs and throttle position.
On older cars the EPC switch can be modified with a screwdriver, but many older cars simply do not allow this switch to be changed. It is very easy to damage your car by changing these settings, either by having too much pressure and blowing out the walls of the transmission or by having too little and causing the clutch to get stuck in gear. On newer cars the EPC switch is software-based, meaning you need specific diagnostic equipment to interface with the car's computer.
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