What the LPG function does
Many dual-fuel vehicles, especially in fleet and off-road use, run LPG (liquefied petroleum gas, or propane) through a separate gas system alongside the normal gasoline injection. The two must never fuel the engine at the same time. The Elite's LPG function watches a switch input, and when the LPG switch is on, the ECU turns off its injectors and the fuel pump. Turn the switch off and gasoline injection and the pump come back.
The LPG system itself, meaning its converter, gas injectors or mixer and its own controller, is separate from the Haltech. This function only handles the handover on the gasoline side.
How it's set up
Add LPG from the Functions list in ESP Main Setup. There's one thing to configure: the Switch Input on the Wiring tab. Either an SPI (Synchronised Pulsed Input) or an AVI (Analogue Voltage Input) will work, and the settings depend on which you choose.
Using an SPI
| Setting | What it does | Our advice |
|---|---|---|
| Edge Select | Tells the ECU what "on" looks like | Falling if the switch connects the input to ground when LPG is selected. Rising if it supplies voltage |
| Sensor Type | How the input reads the signal | Hall Effect for any switch |
| Pullup | Holds the input at a voltage when the switch is open | On for a switch that only grounds the input, so it doesn't float when off |
Using an AVI
An AVI measures voltage, so you define on and off with two thresholds:
- Switch On Voltage: the level at which the ECU considers the LPG switch on.
- Switch Off Voltage: the level at which it considers the switch off again.
Don't set them close together. Leaving a gap between the two (hysteresis) means a bit of electrical noise or a sagging supply can't make the input bounce between states. For a fuel-switching function, that bounce would mean injectors turning on and off, so it's worth getting right.
Worked examples
| Switch behavior | Switch On Voltage | Switch Off Voltage | Result |
|---|---|---|---|
| Input sits at 5 V, switch pulls it to 0 V when LPG is selected | 0.5 V | 4.5 V | LPG counts as on below 0.5 V and only reads off again once it climbs past 4.5 V |
| Input sits at 2.5 V, switch raises it to 12 V when LPG is selected | 4.5 V | 3.0 V | LPG counts as on above 4.5 V and reads off once it falls back under 3.0 V |
Before you choose your numbers, measure the actual voltage at the ECU input in both switch positions with the engine running. Charging voltage and wiring losses can shift the levels a little from what you'd expect on paper.
Where the switch signal comes from
Most LPG systems have a fuel selector switch or a controller output that shows when gas is active. Use a signal that is only on when the LPG system is actually fueling the engine. If the LPG controller does its own changeover (for example starting on gasoline and switching to gas once warm), take the signal from the controller's changeover output, not just the dash switch. Otherwise the Elite can cut gasoline before the gas system is ready, and the engine will stall.
Testing
- With the engine off, flip the LPG switch and watch the LPG switch state in ESP. It should change cleanly with no flicker.
- Start on gasoline, then select LPG. Confirm the injector duty goes to zero and the fuel pump stops.
- Switch back and confirm the pump runs and injection resumes without a stumble.
- Log a few changeovers and look for any bounce in the switch state.
Troubleshooting
| Symptom | Usually |
|---|---|
| LPG state never changes | Wrong edge on an SPI, or pull-up off with a ground-switching switch |
| State flickers on and off | AVI thresholds too close together, or a poor ground at the switch |
| Stalls when changing to LPG | Switch signal arrives before the gas system is actually fueling |
| Engine runs rich on LPG | The input isn't reading on, so the injectors are still firing alongside the gas |
Related reading
The fuel pump function controls the pump this feature switches off, and digital and Hall effect sensor types explains the SPI input settings in more depth. If you're setting up a dual-fuel vehicle and want a second opinion, book a free consultation.
