Two signals, two different jobs
Before an ECU can fire a spark plug or an injector at the right moment, it has to know two things: how fast the engine is turning, and exactly where it is in its cycle. Haltech gets that from two inputs, and uses its own names for them.
| Trigger | Home | |
|---|---|---|
| Usually from | The crankshaft | The camshaft |
| The ECU uses it for | Engine speed and ignition timing | Working out which part of the four-stroke cycle the engine is in |
| How critical | Everything depends on it | Still has to be right, but it isn't used for timing |
Haltech says "Trigger" and "Home" rather than "crank sensor" and "cam sensor" for a good reason: the trigger isn't always on the crank. Some engines take it from a distributor or a cam gear. The names describe what the signal does, not where it lives.
Why the home signal is needed at all
In a four-stroke engine the crank turns twice for every turn of the cam. That's why a cam gear has twice the teeth of the crank gear. The first crank revolution covers intake and compression, and the second covers power and exhaust. A crank sensor on its own can't tell which of those two revolutions it's in, because the crank looks exactly the same both times.
A cam-driven signal solves that, because the cam only goes round once per full cycle. Without it, the ECU has to fire each coil and injector once every revolution (wasted spark and batch fueling). With it, the ECU can run fully sequential injection and ignition. And since the ECU isn't calculating timing from the home signal, having it on the cam is fine.
Why a cam-mounted trigger is a weakness
Manufacturers often combine trigger and home into a single sensor on the cam. It saves money, space and wiring. Nissan's CAS, including the one on the RB26, is the classic example. It's fine for a stock engine. It's not great for a performance build.
The trouble is the belt or chain between the crank and the cam. It stretches, and under high load and rpm it vibrates. So the cam, and a trigger mounted on it, doesn't follow the crank perfectly. The ECU ends up timing sparks from a signal that jitters, and the timing you tuned isn't quite the timing the engine gets. We tune ignition in crank degrees, so it makes sense to measure the crank directly. That's why a crank-mounted trigger wheel is one of the first upgrades on a serious build. Our Nissan CAS facts and fiction article covers that system in more detail.
Common tooth patterns
Every manufacturer has its own idea of how many teeth each wheel should have. A few examples:
| Engine | Trigger | Home |
|---|---|---|
| Nissan CAS, 6-cylinder | 360 slots | 6 pulses |
| Nissan CAS, 4-cylinder | 360 slots | 4 pulses |
| Toyota, non-VVT-i | 12 teeth on the crank | 1 tooth |
| Toyota VVT-i | 36-2 on the crank | 3 teeth on the cam |
| Most current engines | 60-2 on the crank | A multi-tooth pattern unique to each maker |
The industry has mostly settled on a 60-2 crank wheel with a multi-tooth cam wheel, and it's easy to see why. Lots of crank teeth mean more accurate timing. A cam wheel with several uniquely spaced teeth lets the ECU work out engine position quickly, so the engine starts fast. It also gives the ECU enough cam position information for sequential operation and variable cam timing control.
Getting it right on your engine
- Find your engine's trigger page. We have write-ups like the 2JZ / 1JZ and LS2 / LS3 trigger pages.
- Check the sensor type and pinout. Our guide to working out Hall effect sensor pinouts helps with unknown sensors.
- Use a trigger scope or log to confirm the ECU sees a clean signal before cranking the engine for real.
- Verify the TDC offset with a timing light once it runs. Don't assume the base map is right.
Trigger FAQ
Do I need a home signal?
Some engines can run without one, but without it you can't run sequential injection or coil-on-plug sequential ignition, and you can't control variable cams properly.
Is a crank trigger worth fitting on a stock engine?
On a mostly stock engine at stock power, the factory setup is usually fine. Once you're adding boost and rpm, a crank trigger removes a real source of timing error. Stuck on trigger setup? Book a free 30-minute consultation.
