I've been to my evening class tonight and we got to strip down an engine. Removed cylinder head, manifolds, camshaft and related gubbins. The engine was an 8 valve, normally aspirated Ford 1600.
When looking at how the 'toothed belt' connected the camshaft and crankshaft, it got me thinking about some of the threads I have read on here about sensors.
On this engine, the camshaft drove the distributor which set the timing and order of firing. This I understand is replaced by the electronic ignition on a modern car and I guess must take it's signal to fire from the camshaft sensor? Thus, if the camshaft sensor fails, the car wouldn't 'know' what position the pistons were in and would result in the spark being fired at completely the wrong time or not at all.
If the engine 'knows' what position the camshaft is in, and this is connected directly to the crankshaft by the toothed belt, what is the point of the crankshaft sensor? And what are the consequences of it's failure.
Not bad reasoning but, not how it truely works.....
The ECU uses crank position to determine when to fire the ignition because the most important thing about spark is to fire at the correct piston position (and the timing varies with load and revs). Hence its actualy the crank shaft sensor which gives the position (hence the term CPS - Crank Position Sensor).
So, why the need for a cam sensor?
Well, its used simply to determine which cylinder is next to fire and on earlier wasted spark systems this is purely to allow sequential injection (firing the fuel injector as the inlet valve opens) and on later coil per plug cars it also allows the correct coil to fired as well. But, the key thing to remember is that the Cam sensor plays no part in the timing.
SO, what happens when it fails.....
Two key things......the injectors can now no longer fire sequentialy as you dont know which cylinder is on the inlet stroke and which is on the power stroke so the ECU adopts a limp mode and fires both of the injectors at the same time whilst limiting the revs to 4000rpm.
Similarly on a coil per plug setup, you dont know which cylinder is on the compression stroke, this is no big deal to a wasted spark setup but, on a coil per plug arrangement you now fire two coils at the same time and again limit the revs.
Of course there are some minor variations to operation in limp mode dependent on who wrote the software.