Line Pressure Control
Most automatic transmissions use pressurised transmission fluid to control which clutches and bands are engaged at any one time in order to select the desired gear. The fluid is typically pressurised by a pump which is turned by the engine, with a mechanism to control the pressure to the optimal pressure for the current conditions. Some transmissions accomplish this with a pressure relief valve, and some transmissions use a variable geometry pump. The pressure in an electronically controlled transmission is controlled by an electronic control unit (can be Transmission Control Unit, Powertrain Control Unit, Engine Control Unit) to provide the ideal pressure for the current operating conditions. The line pressure has to be high enough to prevent any unwanted slippage of clutches/bands when in gear, and also impacts the speed of a gear shift. Higher line pressure results in a faster and harsher shift, lower line pressure results in a smoother shift, as the internal clutches slip more during engagement of the new gear. Typical road cars optimise for smooth shift when at low load for the comfort of the driver and passengers, and fast shifts at high load to prevent excessive wear and heat.
Some road cars have a Line Pressure sensor to provide the electronic control unit with information about the current line pressure. The control unit typically uses this pressure reading as feedback for a PID controller in order to get the pressure closer to the desired pressure. This allows the control unit to automatically handle changes in required duty cycle due to things such as fluid temperature, fluid age, mechanical component manufacturing tolerances, mechanical wear, and variations in battery voltage. Most aftermarket applications benefit in the same way, but some applications prefer an open-loop system to give the tuner more direct control.
Other common names for the Line Pressure Control Solenoid include: Power Control Solenoid, Variable Pressure Solenoid.
Line Pressure Control Type
Open Loop - This is the simplest mode to choose, which can get the car tuned faster, but isn't as good at handling much variation in conditions (e.g. changes in battery voltage).
In this mode, you get a Duty Cycle table for when In Gear and another for while Shifting gears.
Open Loop w/ Target - This mode allows the tuner to control two target line pressure tables (one while In Gear and the other while Shifting) to generate a target pressure. This target pressure should then be used as one of the axes on the Duty Cycle table. The other axis of the duty cycle table will typically be Transmission Fluid Temperature, as the resistance of the solenoid varies with temperature. An example of the Duty Cycle table is shown below.
Closed Loop - This mode requires a Line Pressure Sensor to be installed, wired into the ECU, enabled and calibrated.
This mode takes longer to tune appropriately, but gets the most repeatable results, as it can more easily compensate for things like battery voltage changes, temperature changes, component wear, and variation between transmission components if you import a tune from one vehicle to another. But it is much harder to tune properly.
Closed Loop mode utilises PID (Proportional, Integral, Derivative) control with feedback from a pressure sensor in order to give more accurate control of the line pressure over a wide range of operating conditions.
There are various videos on freely available on YouTube which can help understand PID controllers and how to tune them. e.g. https://www.youtube.com/watch?v=dfC8csOFQxY
When in this mode, the tuner will have Proportional Gain, Integral Gain and Derivative Gain settings, as well as Target Pressure tables and Base Duty Cycle tables.
There is a Target Pressure and Base Duty Cycle table for when the transmission is currently in gear, and another set for when a gear shift is occurring. It is common to use lower line pressures during a shift. Line pressures typically need to be higher when the engine generates more torque and lower when the engine generates less torque.
Ideally import a Haltech base map for your transmission to get an ideal value for this setting.
If that's not available, try to find out from vehicle manuals what frequency the factory transmission controller uses. If you can't, the following examples give a guideline.
GM 4L60: 293 Hz
Toyota A340: 300Hz
P, I and D Gains
These are only used in Closed Loop mode.
Values in these fields depend on your pressure units selected in Preferences.