The most important sensors on your engine are the Crankshaft and Camshaft Position Sensors. These sensors have to be wired and configured correctly in order for the engine to start, let alone run well.
We refer to the crankshaft position sensor as the “Trigger” signal. This is the one that the ECU uses to measure engine RPM and calculate out the ignition advance.
We refer to the camshaft position sensor as the “Home” signal. The ECU uses this sensor to determine where the engine is up to in its 2 or 4 stroke full cycle.
Keep in mind that not all Trigger signals are mounted on the crankshaft, they can also be mounted in a distributor drive or on a cam gear – so technically they’re not a “Crank Sensor” and why we don’t refer to them as such.
Manufacturers often save space, money and wiring complexity by combining both the Trigger and Home signal into one unit. A perfect example of that is the CAS system found on Nissan engines including the 6 cylinder RB26.
These engines have a cam mounted system which integrates both the trigger and home in one unit – good for the manufactures – not so great for the performance enthusiast.
The mechanical connection between the camshaft and crankshaft (in this example the timing belt) stretches and has harmonic resonance issues under high load resulting in timing accuracy that is less than desirable.
Consequently the signals received by the ECU can be rather flakey causing the ECU to make adjustments based on an inconsistent signal.
It’s for this reason that a crank mounted trigger is desirable. It is measuring exactly what the crankshaft is doing and because in the performance world we measure and tune ignition timing in crank degrees – it seems odd to measure it in cam degrees.
The Home signal might be somewhat little less critical than the Trigger signal but it still needs to be right.
Because the ECU is not calculating ignition timing off this signal, having it on the camshaft isn’t a problem. In fact, this is the only way to achieve sequential fueling or ignition on your 4 stroke engine.
Remember that a 4 stroke crankshaft spins twice, while the camshaft spins once (take a look at the crank and cam gears on your engine, notice the cam gears are twice as big as the crank gear with double the number of belt teeth.) Remember the 4 strokes; intake, compression, (one crank revolution) power and exhaust (the second crank revolution). Without a unique cam position sensor we wouldn’t know if the crank was on the intake/compression stroke or the power/exhaust stroke.
Next, let’s take a look at the number of Home and Trigger teeth on your engine. Different manufacturers do it in all different ways, Nissan have a 360 pulse trigger and a 6 pulse home for 6 cylinders, while they have a 360 pulse trigger and a 4 pulse home for 4 cylinders.
Toyota have a 12 tooth trigger on the crank and a single tooth home on non-VVTI engines, while the VVTI engines have a 36 tooth wheel with two missing on the crank while they have 3 teeth spaced 90 degrees apart on the cam.
In more current engines trigger systems seem to be heading towards 60 teeth with 2 missing on the crankshaft as the trigger, and a unique multi-tooth pattern as the home on the camshaft. This combination offers the best ignition accuracy because of the high number of trigger teeth, the fastest engine startup and enough home information to achieve sequential operation and variable cam control position feedback.