What the trigger system actually does
Every fuel and spark event the ECU schedules is timed against crank angle. To know that angle, the ECU watches two signals:
- Trigger (you'll also hear crank signal or reference): a toothed wheel and sensor that tell the ECU how fast the engine is turning and, on patterns like missing-tooth wheels, where the crank sits within one revolution.
- Home (also called sync, cam signal or G signal): usually on the cam, turning once every two crank revolutions. It tells the ECU which half of the 720-degree four-stroke cycle it's in, so it knows which cylinder is on compression. That's what makes fully sequential injection and direct-fire ignition possible.
At a bare minimum the trigger needs one tooth per spark event to run an engine. More teeth give finer resolution, and a home signal with several teeth is required when the ECU has to measure cam position for continuously variable cam timing.
The quick way: Select Trigger System
If your engine is a common one, start with the Select Trigger System button. It opens a list of predefined setups, and choosing one fills in every trigger setting and a known TDC angle in one go. That saves a lot of guesswork, and it's where we start on almost every OEM engine.
Once you've used a predefined setup, the TDC Angle field becomes TDC Angle Offset. You're no longer entering the full angle; you're entering a correction from the angle the preset already applied. You still verify it with a timing light, covered below.
Trigger configuration settings
| Setting | What it does | Our advice |
|---|---|---|
| Trigger Type | Tells the ECU which crank and cam pattern to expect. Selecting one shows an information panel describing the pattern. | Pick your engine if it's listed. For aftermarket wheels use Generic Missing Tooth or Generic Multi-tooth. |
| Trigger Signal Location | Whether the trigger wheel turns with the crank or with the cam (a distributor, for example) | Grayed out means the chosen trigger type already defines it |
| Number of Teeth | Tooth count on a generic wheel, counting missing teeth as if they were present | A 36-1 wheel is entered as 36, not 35 |
| Number of Missing Teeth | How many teeth are absent from the gap (Generic Missing Tooth only) | Count the gap physically, don't assume |
| TDC Angle | Angle from the cylinder 1 reference tooth to cylinder 1 TDC, anywhere from 0 to 720 degrees | Always confirm with a timing light in Ignition Lock mode |
| TDC Offset Angle Table Enable | Turns on a table that corrects locked timing drift across the RPM range | Only needed if a timing light shows timing moving with RPM |
| RPM Filter | Smooths the RPM value used in every calculation | Haltech recommends 3 to 10 ms on Nexus. Aim for a clean, crisp RPM trace in logs. |
| Quick Start | With direct fire and sequential injection, runs wasted spark and semi-sequential fuel below Max Cranking RPM once the ECU has at least half sync (enabled by default) | Leave it on unless you run CDI ignition or methanol |
Two extra options, Custom Missing Teeth Detection Size and Tooth Count Til Start, appear for Elite ECUs only. They help engines whose cranking speed swings a lot by adjusting how the ECU recognizes the gap and by counting some teeth before it tries to sync. On Elite the RPM filter is also a 0 to 4 scale rather than a time constant, with 1 recommended (2 for noisy triggers).
If your engine isn't in the trigger type list, Haltech takes requests to add patterns in future firmware. In the meantime, most aftermarket wheels fit one of the generic types.
Setting TDC angle properly
- Put the ECU in Ignition Lock mode so it commands a fixed timing figure.
- Aim a timing light at the crank pulley marks and compare what you see to what the ECU commands.
- Adjust TDC Angle (or TDC Angle Offset) until the two agree.
- Raise RPM and check again. If timing walks as RPM climbs, that's usually sensor or ignition delay. Use the TDC Offset Angle Table, and on Nexus look at the ignition system's Time Offset setting for ignition delay compensation.
A TDC angle that's even a few degrees off means every number in your ignition map is wrong by that amount. We never tune under load until commanded and actual timing match.
Setting up the trigger and home sensors
The Trigger and Home inputs have the same set of sensor options. Get these right and most sync problems disappear.
Sensor type
- Reluctor (variable reluctance, VR, magnetic): no power supply, usually two wires plus sometimes a shield. It makes its own AC voltage, swinging positive as a tooth approaches and negative as it leaves, so the output looks like a sine wave. Choosing Reluctor brings up the arming voltage table, and on Nexus you can let the ECU pick the arming threshold automatically from recent peak voltages with Automatic Reluctor Threshold.
- Hall Effect (also optical or digital): powered, three or more wires, and it snaps between an on and an off voltage to give a square wave. The ECU uses a 1.0 V window centered on 2.5 V, so on a falling edge the signal has to drop through 3 V and then 2 V to count.
- Custom: only for when Haltech tells you to use it. It exposes advanced sensor properties.
Not sure which you have? Our Hall effect or reluctor guide walks through telling them apart.
Edge
Rising or falling edge picks which transition of the signal counts as the tooth. The right choice is found by testing and depends on the pattern; for listed engines Haltech can usually supply it. A wrong edge often shows up as timing that drifts with RPM or intermittent sync loss.
Pull up
Hall sensors pull their signal to ground and need a resistor feeding it up to 5 V. Reluctors don't need one. Your choices:
| Option | Resistor | Use it when |
|---|---|---|
| Disabled | None | The sensor has its own pull up, you've fitted an external one, or you're sharing the signal with another ECU (the OEM ECU already provides it) |
| Enabled | 1.2k ohm to 5 V | The normal choice for a standalone Hall sensor |
| Strong | 440 ohm to 5 V | Sensors that need a firmer pull up to give a clean signal |
Ground reference
For reluctor and custom sensors only. With it enabled, the ECU measures the sensor against signal ground instead of the Trigger- wire. Use it when the sensor's second pin goes to signal ground in the harness rather than back to the ECU's minus input, or when the sensor type calls for it.
Home signal synchronization options
The home input has two settings the trigger doesn't.
Minimum RPM works with Quick Start. Below this speed the ECU ignores the home signal, running semi-sequential fuel and wasted spark. Once RPM passes it, the home signal is used and the ECU steps up to sequential and direct fire. It resets when the engine stops.
Synchronization Mode decides when the home signal is trusted:
- Always: the standard choice. Every trigger and home event must be seen every cycle. A missed or extra event loses sync, bumps the Trigger System Error Count, and the ECU re-syncs at the next home event. A count that keeps climbing means you have a signal problem to chase.
- Until Full Sync: after eight clean consecutive engine cycles, the ECU drops the home signal and runs on the crank signal alone. Only certain generic, missing-tooth and extra-tooth patterns support it.
- Conditional: stops using the home signal above a set RPM or throttle position, then picks it back up seamlessly when you drop below those values.
These exist for engines where the cam signal can't be trusted at high RPM and load, such as big drag engines running a high-current magneto (MSD Pro-Mag style). With the home signal ignored, a single missed or extra crank tooth shifts your timing, and more missed teeth stack the error. That can damage an engine. Only use them with a crank sensor you've proven is perfect.
Input limits
The Trigger and Home inputs accept -5 to 5 V, sample at 1 million samples per second, withstand -75 to 75 V indefinitely, and handle signals up to 48 kHz. For reference, Haltech quotes an RB optical home signal at 15,000 RPM at about 45 kHz and a 60-2 wheel at 20,000 RPM at about 20 kHz, so real-world patterns sit comfortably inside that limit.
Troubleshooting trigger problems
| Symptom | Usually |
|---|---|
| RPM shows while cranking, but no sync and no fire | Wrong trigger type, wrong tooth count or a home signal that isn't arriving |
| Trigger System Error Count keeps climbing | Noise, wrong edge, arming voltage too low, or a pull up setting that doesn't suit the sensor |
| Timing light disagrees with the ECU | TDC angle not set, or set without Ignition Lock active |
| Timing moves as RPM rises | Sensor or ignition delay; use the TDC offset table or time offset |
| Hall sensor gives no signal at all | Pull up disabled with no external resistor, or no power to the sensor |
| Hard starting on a methanol or CDI engine | Try Quick Start disabled |
For noisy signals, the trigger glitch filter and trigger activation voltage settings are the next places to look.
Trigger system FAQ
How do I enter a 36-1 or 60-2 wheel?
Choose Generic Missing Tooth, enter the full tooth count including the gap (36 or 60) and the number missing (1 or 2). Then set TDC angle with a timing light.
Should I run a reluctor or Hall effect sensor?
Both work well on Nexus. Hall sensors give a clean square wave at any speed; reluctors are simple and tough but need a correct arming voltage table. Use what the engine or trigger kit was designed for.
Can I share a crank signal with the factory ECU?
Yes, with the pull up set to Disabled, since the factory ECU is already supplying one. If you're unsure about your setup, book a free consultation and we'll look at it with you.
