Why the ECU wants to know about the brake
Most people think of the brake switch as the thing that turns on the brake lights. For an Elite ECU, it's an input that other features depend on:
- Drive by wire safety: the ECU can treat brake plus throttle as a plausibility check.
- Cruise control: touching the brake has to cancel cruise, every time.
- Starter lockout: requiring the brake pedal before cranking, like a modern car.
- Datalogging: seeing exactly when the driver braked makes logs far easier to read, especially on track.
It's also a common condition for generic functions, such as a line lock or a two-step arming rule.
One switch or two?
ESP has room for two brake switches. Plenty of factory pedal boxes have two: one normally open and one normally closed, or one for the lights and one for cruise.
| Setting | Options | Our advice |
|---|---|---|
| Brake Pedal Switch 1 Input Mode | Digital switched input, or vehicle CAN | Use CAN if the car already broadcasts brake status and the Elite supports that vehicle; otherwise wire it |
| Brake Pedal Switch 2 Input Mode | Disabled, or digital switched input | Wire the second switch if you have one, especially on DBW or cruise builds, for redundancy |
Wiring it to an SPI
An SPI (synchronised pulsed input) is a natural home for an on/off switch. You'll set three things:
| Setting | What it means | How to choose |
|---|---|---|
| Edge Select | Which voltage change counts as "pressed" | Falling if pressing the pedal grounds the input. Rising if pressing the pedal feeds voltage to it. |
| Sensor Type | How the input interprets the signal | Hall Effect, for any switch |
| Pullup | An internal pullup so the input sits high when the switch is open | Enable it for a switch that only grounds the input, otherwise the "off" state just floats |
A switch-to-ground with the pullup enabled and a falling edge is the most common, and the easiest to get right.
Wiring it to an AVI
An AVI (analog voltage input) reads a voltage rather than an edge, so you tell it which voltages mean on and off. Two thresholds, with a gap between them, stop the reading flickering if the voltage wobbles.
- Switch On Voltage: cross this and the switch is considered on.
- Switch Off Voltage: cross back past this and it's considered off.
Whether "cross" means going above or below depends on which way your switch works. Leave margin away from the real voltages so noise and battery swings don't trigger it.
Example: switch pulls to ground
The input sits at 5V with the pedal up and drops to 0V when you press it. Settings of On at 0.5V and Off at 4.5V work: the ECU calls it on once the voltage falls under 0.5V, and off only once it climbs back over 4.5V. If your wiring is noisier, tighten those toward the middle, for example On at 1.0V and Off at 4.0V, which gives more margin from the real voltages.
Example: switch feeds 12V
This one idles around 2.5V with the pedal up and sees 12V when pressed. Here On at 4.5V and Off at 3.0V make sense: it switches on once the voltage rises past 4.5V and off once it falls back below 3.0V.
Before you set the thresholds, watch the raw input voltage in ESP with the pedal up and down. Base your numbers on what the car actually shows, not what you expect it to show.
Testing
- Go online and watch the brake switch channel while someone presses and releases the pedal several times.
- Check it follows every press, including a light touch, since that's what should cancel cruise.
- If you've wired two switches, confirm both change state together.
- Take a short drive with logging on and make sure there's no flicker over bumps.
Troubleshooting
| Symptom | Usually |
|---|---|
| Brake reads pressed all the time | Edge set the wrong way, or On/Off thresholds reversed |
| Switch state flickers randomly | Pullup disabled on a ground-only switch, or AVI thresholds too close to the real voltages |
| Cruise won't cancel with a light touch | Switch adjusted too far down the pedal travel |
| No response at all | Input not assigned, or the switch isn't getting power (check with a meter) |
Brake switch FAQ
Can I share the brake light switch with the ECU?
Often, yes, if you read the side that switches 12V and set the input up for that. Check the voltage first, and make sure the input you choose is set up for it.
Is a brake switch required for drive by wire?
It's strongly recommended. It adds a real safety layer on a DBW car.
Related: cruise control, clutch switch and drive by wire on the Elite.
