What a longitudinal G sensor measures
"Longitudinal" means along the length of the car. This sensor reports how hard the vehicle is being pushed forward under acceleration, and how hard it's being pulled back under braking. Its partner, the lateral G sensor, measures side-to-side force in corners.
Why add one to an Elite? Logged acceleration data tells you a lot that engine channels can't: how hard a launch really pulled, whether a gear change lost drive, how consistent braking zones are, and how changes to the tune or the tires show up on the road. It's a cheap way to put a number on "it feels faster."
Setting it up in ESP
The Longitudinal G Sensor function has three tabs, plus one related table elsewhere in ESP. Work through them in this order.
1. Wiring tab: pick an input
The sensor can go on either an analog voltage input (AVI) or a synchronized pulsed input (SPI).
| Input type | When to use it | Settings to check |
|---|---|---|
| AVI | The preferred choice. A G sensor outputs a voltage, and that's what AVIs are built to read. | Pull up: switches in a 1k resistor to 5V |
| SPI | Only when you've run out of AVIs | Sensor Type: set to Hall Effect for any analog signal. Pull up: a weaker 10k resistor to 5V. |
For the pullup, think about how the sensor is powered. Pullups exist to give two-wire sensors, like thermistor temperature sensors, a reference voltage. A G sensor that has its own 5V supply and drives its own output generally doesn't want one, so leave it disabled unless the sensor maker says otherwise. The same goes if the sensor is shared with another ECU, such as a factory ECU in a piggyback setup, because that ECU will usually already provide any pullup that's needed.
2. Calibration tab: tell the ECU what the voltage means
The ECU only sees a voltage. The calibration turns that voltage into G. ESP ships with a set of ready-made calibration files. Click the yellow Open icon to load one. If your sensor isn't on the list, build your own from the manufacturer's data sheet (voltage at a known G, usually at zero and at the ends of the range), then save it with the blue Disk icon beside Open so you can reuse it.
Out of the box, this input is set to the calibration for the Haltech 150 psi pressure sensor, which has nothing to do with G force. Always load or create the right calibration, or your readings will be meaningless.
Once it's calibrated, park the car on level ground and check the reading. It should sit very close to zero. A steady offset usually means the sensor isn't mounted level, or the calibration's zero point is off.
3. Diagnostics tab: catch wiring faults
These two optional limits watch the raw sensor voltage for readings that can only mean a wiring problem. Both are disabled by default.
| Setting | Fault code | What it catches | Our advice |
|---|---|---|---|
| Raw Min | C1038 | Voltage below your limit, typical of a short to ground or an open circuit | Set it just below the lowest voltage the sensor can produce in normal use |
| Raw Max | C1039 | Voltage above your limit, typical of a short to supply or a broken ground | Set it just above the highest voltage the sensor can produce in normal use |
If you're running the Engine Protection function, you can give either fault a DTC severity level. For a data-only sensor like this one we'd usually keep it to a warning rather than anything that limits the engine.
4. G-Force Filter Scale
You'll find this table under ECU Navigator, then Sensor Properties, then G-Force Filter Scale. It sets how much filtering is applied to the G signal, and it's shared by the lateral and longitudinal sensors when both are set up. Filtering smooths out chassis vibration and engine shake so the reading reflects how the car is moving, not how the bracket is rattling.
Too little filtering leaves a noisy, jagged trace. Too much makes the reading lag behind real events, which matters if you use it for anything more than logging. We start with moderate filtering, log a few runs, and adjust until the trace is clean but still shows the sharp step of a launch or a hard brake application.
Mounting tips
- Mount the sensor solidly to the chassis, not to a flexible panel, and as close to the car's center of gravity as practical.
- Line it up with the car's centerline. A sensor mounted at an angle picks up some cornering force as fake acceleration.
- Keep it level, and away from heat and strong vibration sources.
Troubleshooting
| Symptom | Usually |
|---|---|
| Reading makes no sense at all | Default pressure sensor calibration still loaded |
| Not zero when parked on level ground | Sensor not mounted level, or calibration offset wrong |
| Very noisy trace | Not enough filtering, or a soft mounting point |
| Reading pinned high or low | Pullup enabled on a powered sensor, or a wiring fault. Set Raw Min and Raw Max to flag it. |
Longitudinal G FAQ
Can I use a G sensor from another brand?
Yes, as long as it outputs a voltage and you have its data sheet to build a calibration.
AVI or SPI for a G sensor?
AVI whenever one is free. SPI is the fallback.
Does the G sensor change how the engine runs?
Not on its own. It's an input you can log and use in other functions. Need help picking a sensor or deciding where to mount it? Get in touch, or browse the rest of our Haltech knowledge base.
