What it's for
The neutral switch tells the ECU whether the transmission is in Park or Neutral. Its main job is safety: the ECU won't let the engine start in any other gear position. On some vehicles the same signal also carries information about gear selection and reverse light status, which the ECU can monitor.
Choosing the input mode
Under Options, Neutral Switch Input Mode gives you two choices:
- Digital - Switched: a physical switch wired to the ECU. Any free AVI or SPI can take it.
- CAN: the state comes from the vehicle's CAN bus, common on newer cars where the transmission or body module already knows the gear.
Wiring it to an AVI
An AVI switches on and off at two voltage thresholds that you set. Leave a healthy gap between them so a slightly noisy signal can't flip the state back and forth.
- Switch On Voltage: where the ECU decides the switch is on.
- Switch Off Voltage: where it decides the switch is off.
For example, a switch that pulls a 5V input to ground in neutral could use 1.0V as on and 4.0V as off. The input has to drop below 1.0V to register neutral, and climb back above 4.0V before the ECU believes you've left it.
Wiring it to an SPI
- Edge Select: Falling for a switch that grounds the input, Rising for one that feeds it voltage.
- Sensor Type: Hall Effect for any switch.
- Pull Up: turn it on when the circuit needs the input held high while the switch is open.
Glitch filter
Using an SPI also enables a glitch filter table. It sets a time in microseconds that a change of state has to last before the ECU accepts it. Anything shorter gets thrown away as noise. If a neutral switch shares a noisy loom or you see brief false readings in a log, this is the setting to look at.
Once it's configured, put the car in gear with the parking brake on and confirm it won't crank. Then check it starts in Neutral (and Park, on an automatic).
For related setups, see gear position, clutch switch and starter motor. On an Elite ECU, see our Elite neutral switch article.
