What the ECU does with speed
A good speed signal is one of the most useful inputs you can give a Nexus. Beyond logging, it lets you tune fuel and ignition by speed, run traction control, work out which gear you're in, trim boost and idle, and drive features like a pit lane speed limiter.
NSP accepts up to four wheel speed sensors plus one drive train sensor (a driveshaft or transmission output sensor). Each one can be hard-wired to the ECU or, on supported vehicles, read from the factory CAN bus.
Which sensors traction control needs
Traction control works by comparing something that is driven with something that isn't. The rules come down to that idea:
| Drivetrain | Minimum sensors |
|---|---|
| Rear wheel drive | One rear wheel sensor or the drive train sensor, plus at least one front wheel sensor |
| Front wheel drive | One front wheel sensor or the drive train sensor, plus at least one rear wheel sensor |
| All wheel drive | Any two of: a front wheel, a rear wheel, or the drive train sensor |
More sensors give the system a better picture, especially in corners where inside and outside wheels travel at different speeds.
Setting it up in NSP
- Wiring: enable each sensor you have and choose whether it's wired or on CAN. Pick the Main Source, the one sensor every speed-dependent function will use as "vehicle speed".
- Connections: for wired sensors, choose Digital Frequency and assign an SPI input to each. Set the sensor type and edge (details below).
- Calibration: enter a calibration speed and calibrate each sensor while driving at exactly that speed.
- Derived channels, diagnostics and filtering: decide how driven and undriven speeds are calculated, and turn on fault detection.
Wiring options
| Setting | What it does | Our advice |
|---|---|---|
| Input | Which ECU pin reads the sensor | Use an SPI (synchronized pulsed input), as NSP expects |
| Sensor Type | Hall effect (digital square wave) or reluctor (AC signal) | Match the sensor. Two-wire sensors are usually reluctors, three-wire usually Hall |
| Edge Select | Which edge of each pulse the ECU counts | Falling, unless the sensor's documentation says otherwise |
| Pull Up | Adds a pull-up for Hall sensors that need one | Turn it on if the sensor or harness doesn't provide one |
| Arming Voltage (reluctor on SPI) | The voltage the signal must reach before the ECU starts watching for the active edge | Keep it above the noise floor at low speed but low enough to catch a slow-rolling wheel |
| Glitch Filter (SPI) | How long, in microseconds, a pulse must hold before it counts. Anything shorter is thrown away | Raise it slowly if you see spikes, but not so far that real pulses at top speed get rejected |
Calibrating the sensors
NSP's calibration is simple once you understand it. You enter a Calibration Speed, drive at exactly that speed, and press Calibrate for each enabled sensor. For wired sensors, the ECU counts pulses and stores a Pulses/Mile (or Pulses/km) figure. If you already know the right number, you can type it in instead.
For CAN sensors there are no pulses to count, so the ECU measures the percentage difference between what the sensor reports and the calibration speed. That's how it compensates for a change in tire rolling diameter.
Use GPS, not the old speedometer, as your reference, and pick a road where you can hold the speed steady on a flat, straight section. A cruise-control hold makes it easier. Calibrate with the tire pressure and tire size you'll race or drive on.
Driveshaft RPM sensor
These settings appear once a drive train sensor is enabled in Wiring.
- Number of Teeth: how many pulses per driveshaft turn. With a magnet collar, it equals the magnet count, commonly 2, 8 or 12. The Haltech split collar range uses 8 magnets.
- Max Derivative: the fastest change in driveshaft rpm the ECU will accept. It filters out noise spikes that no real driveshaft could produce.
- Warning Maximum: above this rpm, driveshaft gauges in the software flash red.
Derived channels
NSP builds a Driven Wheel Speed and an Undriven Wheel Speed channel from your sensors. For each, you choose whether it uses the average of that axle's wheels or the fastest one. Averaging is smoother on a road course. Taking the fastest driven wheel reacts sooner to a single wheel breaking loose, which is often what a drag or launch strategy wants.
Diagnostics
Each enabled sensor can raise a trouble code, and you can give it a DTC severity so it triggers an engine protection level. What counts as a fault depends on how many sensors you have:
- One or two sensors: a code sets when speed reads 0 even though the engine looks like it is coasting in gear: throttle below the User Demand Minimum, MAP below -60 kPa, and 2000 to 5000 rpm, all held for more than 10 seconds.
- More than two sensors: a code sets when one sensor differs from the average of all of them by more than 15 km/h (about 9 mph), checked below 4000 rpm and under 20% throttle.
There's also a Vehicle Speed Filter node for smoothing the final channel if your signal is noisy.
Troubleshooting
| Symptom | Usually |
|---|---|
| No speed reading at all | Wrong input assigned, sensor type wrong, or a Hall sensor missing its pull-up |
| Speed reads but is off by a fixed percentage | Calibration wasn't done at a true, steady speed. Recalibrate against GPS |
| Random spikes | Electrical noise. Check shielding, then use the glitch filter or arming voltage |
| Reluctor reads at speed but drops out slowly | Arming voltage set too high for the low-speed signal |
| Wheel speed DTC under normal driving | One sensor is miscalibrated or has a damaged tone ring |
FAQ
Can I use my car's ABS sensors?
Yes. On supported vehicles, NSP can read them over CAN. Otherwise, many can be hard-wired to SPI inputs. Call us if you're unsure which applies to your car.
Which sensor should be the Main Source?
Usually an undriven wheel, because it tells the truth about road speed even when the driven wheels spin. On a car without front sensors, the drive train sensor is the usual choice.
Do I need speed for launch control?
Many launch control strategies use speed to know when the car has left the line, so it's worth setting up before you tune a launch.
