What the reverse switch does
The reverse switch input simply lets the ECU know the transmission is in reverse. On its own it doesn't change anything. Other functions use it, for example logic that needs to behave differently when backing up, or a reverse lockout. Most transmissions already have a switch for the reverse lights, and that's often the easiest signal to share.
You'll find it under the Functions section of Main Setup in ESP. The Wiring tab is where you choose the input and describe the signal.
Choosing an input
The Reverse Switch Input can be one of two input types:
- SPI or DPI (Synchronised or Digital Pulsed Input): reads the switch as a clean on/off. Best when the switch goes to ground or to 12V and nothing else.
- AVI (Analog Voltage Input): reads the actual voltage and lets you decide what counts as on and off. Best when the signal doesn't swing all the way to 0V or 12V, or when you've run out of digital inputs.
Setting up an SPI or DPI
| Setting | What it does | Our advice |
|---|---|---|
| Edge Select | Which direction of voltage change means "switch on" | Falling if the switch grounds the wire in reverse. Rising if it supplies voltage in reverse |
| Sensor Type | The kind of signal source | Always Hall Effect for a switch input |
| Pullup | Holds the input at a voltage when the switch is open | Turn it on for a switch that only grounds, so the input isn't floating when the switch is off |
A ground-switching reverse switch with the pullup on and a Falling edge is the most common setup we see.
Setting up an AVI
With an AVI you give ESP two thresholds: a Switch On Voltage and a Switch Off Voltage. The switch is considered on once the voltage crosses the on threshold, and it stays on until the voltage crosses back past the off threshold.
Leave a gap between the two. Real signals wander with charging voltage, loads switching and ground offsets. If both thresholds sit close together, or right at the voltage the switch actually produces, a little noise makes the input flicker on and off. The gap between them is hysteresis, and it's what keeps the reading steady.
Example: switch pulls a 5V signal to ground
Say the input sits at 5V with the switch open and drops to 0V when reverse is engaged. Pick an on threshold just above zero and an off threshold just under 5V. Haltech's example uses 0.5V on and 4.5V off: once the signal falls under 0.5V the ECU sees reverse, and it won't drop out until the signal climbs back over 4.5V. If your signal is noisier, move the thresholds further in. The help text also mentions 1.0V on and 4.0V off as another valid pair for the same kind of switch, with more margin from the actual 0V and 5V levels.
Example: switch lifts a 2.5V signal to 12V
Now the input idles at about 2.5V and jumps to battery voltage in reverse. Here on sits above off. Haltech's suggested values are 4.5V on and 3.0V off: above 4.5V is reverse, and the input only returns to off once it's back under 3.0V. Both thresholds clear the 2.5V resting level and sit far below 12V, so small swings on either side don't matter.
How to pick your own numbers
- Measure the input voltage with the ECU connected, in reverse and out of reverse, engine running.
- Put the on threshold a comfortable distance from the "on" reading, toward the "off" reading.
- Put the off threshold a comfortable distance from the "off" reading, toward the "on" reading.
- Check the switch state in ESP while shifting in and out of reverse a few times.
Always watch the reverse switch channel in ESP while you shift. If it flickers at idle or with headlights on, widen the gap between thresholds or fix the ground.
Troubleshooting
| Symptom | Usually |
|---|---|
| Always shows reverse | Edge selected backwards (SPI/DPI), or on/off thresholds swapped (AVI) |
| Never shows reverse | Pullup off on a ground-switching switch, or wrong input selected |
| Flickers on and off | Thresholds too close together or too close to the real signal voltage |
| Works until lights are on | Ground offset. Improve the switch ground, then widen hysteresis |
FAQ
Can I share the reverse light switch?
Usually, yes. If the switch supplies 12V to the reverse lights, set a Rising edge on a digital input, or use an AVI with on above off.
Is this setup the same for other switches?
The same ideas apply to the neutral switch, clutch switch and brake pedal switch. On a Nexus, see reverse switch.
