Why line pressure matters
Inside a typical automatic, the gear you're in is decided by which clutches and bands are applied, and they're applied by pressurized fluid from an engine-driven pump. Something has to regulate that pressure. Older designs use a relief valve, some use a variable pump, and electronically controlled transmissions use a solenoid run by a control unit. With a Nexus, that control unit can be your ECU.
Two things depend on getting it right:
- Holding power in gear. Too little pressure and the clutches slip under load, which builds heat and wears them out fast.
- Shift feel. More pressure gives a quicker, firmer shift. Less pressure lets the clutches slip a little as they engage, which feels smoother.
Factory calibrations usually aim for soft shifts at light load, for comfort, and quick shifts at high load, to limit slip and heat. We follow the same logic on most builds. The solenoid you're controlling may also be called a Power Control Solenoid or Variable Pressure Solenoid in factory manuals.
Choosing a control type
| Mode | You tune | Best for | Trade-off |
|---|---|---|---|
| Open Loop | Two duty cycle tables: one In Gear, one Shifting | Getting a car driving quickly | No correction for changes in battery voltage, fluid temperature or wear |
| Open Loop w/ Target | Target pressure tables (In Gear and Shifting), plus a duty table that turns target pressure into duty | Tuning in pressure terms without a sensor | Still no feedback; the duty table has to be accurate |
| Closed Loop | Target pressure and base duty tables for In Gear and Shifting, plus P, I and D gains | The most consistent results over time and between cars | Needs a line pressure sensor and takes the longest to tune well |
Open Loop w/ Target in practice
This mode separates what pressure you want from what duty gets you there. You fill in target pressure tables for in gear and for shifting, then build the duty cycle table with target pressure on one axis. For the other axis we normally use transmission fluid temperature, because the solenoid's resistance changes with temperature and so does the duty needed for the same pressure. It's a nice middle ground: shift quality is tuned in pressure units, but you don't need extra hardware.
Closed Loop in practice
Closed loop needs a line pressure sensor fitted, wired, enabled and calibrated. The ECU compares measured pressure against the target and uses a PID controller to trim the duty. Because it's measuring the result, it copes on its own with voltage swings, fluid temperature and age, wear, and part-to-part differences between transmissions. That last one matters if you want to move a calibration from one car to another.
You get a target pressure table and a base duty table for in gear (In Gear Target and In Gear Base Duty), and another pair for shifts (Shifting Target and Shifting Base Duty Cycle). A good base duty table means the PID only has to fix small errors, just like closed loop boost control.
Line pressure should follow engine torque: more torque needs more clamping force. Shift targets are usually lower than in-gear targets so the shift has a little controlled slip. Build the tables around torque or load, then refine shift feel from logs and a test drive.
Output frequency
The solenoid has to be driven at the frequency it was designed for. In order of preference:
- Import a Haltech base map for your transmission. It will already have the right value.
- Find the frequency used by the factory transmission controller in the service information.
- Use known values as a guide. For example, the GM 4L60 runs at 293 Hz and the Toyota A340 at 300 Hz.
P, I and D gains
The gains only apply in Closed Loop. Their values depend on the pressure units selected in NSP's Preferences, so if you change units, or copy numbers from a map set up in different units, check the gains again. Put simply, proportional reacts to how far off target you are, integral removes the error that's left over time, and derivative calms things down if pressure is moving fast. We start soft and only add gain until pressure follows target without oscillating.
Troubleshooting
| Symptom | Usually |
|---|---|
| Flare or slip during shifts | Shifting target or duty too low for the torque being made |
| Harsh, banging shifts at light throttle | Shifting pressure too high at low load |
| Shift feel changes as the fluid warms up | No fluid temperature compensation; see Temperature Correction |
| Pressure oscillates in closed loop | Gains too high, or a poor base duty table |
| No pressure change at all | Check the solenoid wiring, output assignment and frequency |
FAQ
Do I need a line pressure sensor?
Only for Closed Loop. Open Loop and Open Loop w/ Target both work without one.
Which mode should I start with?
We usually get a car driving in Open Loop w/ Target, then move to closed loop once a sensor is fitted and the basics are right.
