What drive by wire actually does
There is no cable between the pedal and the throttle. An accelerator pedal position (APP) sensor reports how far you pressed, the Nexus decides how far the throttle should open, and it drives the electric motor in the throttle body to get there. Because the ECU is in the middle, you can shape the pedal response, let the ECU handle idle air directly, and use the throttle for lift-off anti-lag strategies.
Built-in safety, and why it needs two of everything
A throttle that opens by itself is dangerous, so DBW systems copy the OEM approach of redundancy:
- The pedal has two position sensors.
- The throttle body has two throttle position sensors.
The ECU constantly cross-checks each pair. If the readings disagree, a sensor drops out, or the throttle isn't where it was told to be, the Nexus stops driving the motor. The throttle then springs back to its rest position, which is usually slightly cracked open. How far it rests is set by the throttle body's mechanical design, not by the ECU. This is why a DBW car with a tripped fault can usually still idle.
All those checks create a lot of diagnostic channels. That's normal. When something goes wrong, those channels are your best friend, and our guide on diagnosing DBW issues shows how to read them.
Setting it up in NSP
- Wire the pedal and throttle body following DBW wiring.
- Set the number of throttles (below).
- Run the DBW calibration for pedal and throttle.
- Set the TPS limit conditions.
- Shape the pedal to throttle curve.
- Adjust damping for feel: open rate, close rate and lift rate.
Settings
| Setting | What it does | Our advice |
|---|---|---|
| Number of Throttles | How many separately controlled throttle bodies you run. A Nexus R5 handles up to two | If two bodies share one pair of motor outputs in parallel, choose 1 |
| Brake Pedal Check | Uses a configured brake pedal switch and unlocks Max Braking TPS | Worth enabling on any street car |
| Max Braking TPS | Caps throttle while the brake is pressed. It overrides every other throttle request | Low enough that the brakes always win. It also stops brake-torque burnouts |
| Vehicle Speed Check | Uses a configured speed sensor and unlocks Max Stopped TPS | Needs a working vehicle speed input first |
| Max Stopped TPS | Caps throttle when the car isn't moving | Handy on customer or valet cars to prevent free-revving abuse |
The brake check relies on a properly configured brake pedal switch. Test it before trusting it.
Shaping pedal feel: the APP to TPS graph
This graph decides how much throttle you get for a given amount of pedal. You drag two control points to bend the curve. A straight line gives one-to-one response. Pull the curve down at the start and the first part of pedal travel becomes gentle, which makes a big-throttle engine easier to drive in traffic or on a wet track. Push it up and the car feels sharper off the line, at the cost of fine control. To cap total opening, look at Max TPS.
Worked example
A customer has a big-throttle LS on a Bosch 82mm throttle body with a Bosch pedal, and complains it is jumpy around town. We bend the APP to TPS curve so the first third of pedal travel opens the throttle more gradually, then let it catch up to full opening at the end of travel. Wide open throttle is unchanged, but parking-lot manners improve a lot.
Troubleshooting
| Symptom | Usually |
|---|---|
| Throttle sits slightly open and won't respond | A fault check tripped and the motor was shut off. Check sensor agreement in diagnostics |
| Throttle won't open with foot on brake | Max Braking TPS working as intended, or a brake switch stuck on |
| Limited revs while parked | Max Stopped TPS doing its job |
| Faults after changing throttle bodies | Recalibration needed |
FAQ
Can I use one pedal to run two throttle bodies?
Yes. Either control them individually, or run them in parallel from one pair of outputs and set Number of Throttles to 1.
Why does my car still idle when DBW faults?
The throttle body's rest position lets a small amount of air in. That is mechanical and intentional, so the engine can keep running while you get off the road.
