Why log brake pressure?
A brake light switch only tells you on or off. A pressure sensor tells you how hard, how quickly and how consistently the driver is braking, which is some of the most useful data you can log on a track car. With front and rear sensors you can also see brake bias as it happens. Beyond logging, the Nexus can use brake pressure to generate braking channels that drive outputs, like brake lights or hazard lights under heavy braking.
Brake pressure sensors are powered analog sensors with three wires: 5V supply, signal and signal ground. The Haltech 2000 psi brake/nitrous pressure sensor is a good fit, and its calibration can be loaded straight into NSP.
Main settings: switch and hard braking
The main node sets up two derived channels from the pressure reading, plus the sensor calibration when the sensor is on an analog input.
| Setting | What it does | Our advice |
|---|---|---|
| Switch Threshold | Pressure at which the Brake Pressure Front/Rear Switch State channel turns On | Low enough to catch light braking, but above any residual pressure with your foot off the pedal |
| Hard Braking Threshold | Pressure the ECU treats as hard braking, used for things like flashing brake lights or hazards | Base it on logged emergency stops, not a guess |
| Hysteresis | A dead band around each threshold so the channel doesn't flicker | Enough to stop chatter near the threshold |
How hysteresis works
Say the Switch Threshold is 1000 kPa (about 145 psi) and Hysteresis is 100 kPa (about 15 psi). The switch channel turns on when pressure passes 1000 kPa, but it doesn't turn off again until pressure drops below 900 kPa (about 130 psi). Without that gap, a driver holding the pedal right around the threshold would make the channel, and any light driven by it, flicker on and off.
Calibration
The calibration node turns sensor voltage into pressure. You can enter points by hand, or use Load from File to load the calibration for a Haltech-supplied sensor. Save to File exports your calibration so you can reuse it on another car. If you're using a third-party sensor, get the voltage-to-pressure data from its maker, and check it by seeing that the ECU reads zero with the brakes released.
Display and diagnostics
- Warning Minimum and Warning Maximum set when number displays, gauges and bars showing brake pressure go into their low or high warning state.
- The diagnostics node lets you turn on high and low voltage DTCs for the sensor. Each has its own severity, which links to an Engine Protection level. A signal stuck at the top or bottom of the voltage range almost always means a broken wire or failed sensor, so it's worth turning these on if brake pressure drives any outputs.
Wiring
The wiring node is where you assign the input. Input Type offers two choices:
- Analogue - Voltage: the normal choice. Wire the sensor to any spare AVI, and we recommend turning the AVI's pull-up off for this sensor.
- CAN Input: only for cars whose factory Vehicle CAN already broadcasts brake pressure, where the ECU can read it from the network instead of a separate sensor.
Fit the sensor with brake-rated fittings, bleed the system thoroughly afterwards, and keep the sensor body away from exhaust heat. For front and rear sensors, plumb them into separate circuits so the logs actually show front and rear pressure, not two readings of the same line.
Troubleshooting
| Symptom | Usually |
|---|---|
| Pressure reads above zero with the pedal released | Wrong calibration, or the pull-up left on |
| Brake lights flicker at light pedal | Hysteresis too small, or Switch Threshold too close to normal light braking |
| Reading pinned at maximum | Signal wire shorted to 5V, or a missing sensor ground |
| No reading at all | Input not assigned, or no 5V at the sensor |
Using brake pressure to run lights? See Brake Lights and Brake Lights Wiring. For a simple on/off signal, the brake pedal switch input may be all you need. Using this sensor on an Elite? See the Elite version.
