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Concepts: Camshaft vs. Crackshaft Position Sensor?

I am just diving back into cars after many years.
Trying to fill in gaps of conceptual knowledge.

I understand that these 2 sensors are part of a fuel injection system.
It is no longer purely mechanical with lifters, rocker arms, valves, etc.

What is the difference between them?
I have read they both are used to determine when to fire the spark plug.
(Something the timing belt used to do?)

Can you share the basic concepts behind these 2 sensor’s roles?

They both just tell the engine’s computer where the engine is in the stroke and are used to control spark and fuel delivery. They’re often just little magnets pointed at a toothed wheel on the cam & crankshafts. The teeth pass the magnet & so put out an electrical signal. There are also hall effect sensors

In general, I would find a good book - or I have also found this to be an excellent website for general auto knowledge including all of the contemporary engine control systems etc:

Here’s their page on the cmps and ckps:

The crankshaft and camshaft are sync’d to each other with the timing belt. Some cars have variable timing, but for most econobox type cars once the timing belt is installed, there is no adjustment possible between the pistons and valves, they remain in sync. Furthermore, the crankshaft is sync’d to the pistons moving up and down, while the camshaft is sync’d to the valves opening and closing. What is adjusted however is at what time the spark plug fires in this cycle, and when in the cycle the fuel injector releases it’s pulse of gasoline. The engine computer decides this timing, based upon various inputs such as the rpm, engine vacuum, air temperature, coolant temperature, which accessories are operating, etc. The computer then decides when to inject the fuel and when to fire the spark plug with respect to the piston and valve. The way the computer knows in which part of the cycle the piston and valve are is by reading (respectively) the crank and camshaft sensors. These sensors (like is said above) use a magnet to convert the piston position and the valve position to an electrical signal which is then input to the engine computer.

Camshaft position senors and crankshaft position sensors basically do similar jobs, synchronize the engine motion for the ECM. However, crankshaft position sensors cannot determine TDC on their own. The crankshaft makes two rotations per cycle, so each vain in the crankshaft sensor wheel will pass twice. The camshaft(s) turns exactly half a fast, so they rotate once per cycle, making it possible to detect TDC.

I’ve seen engine management systems use either one as a primary. But the systems that rely on the crankshaft sensor tend to use a ‘wasted spark’ ignition system, where the coil fires two cylinders at a time, one on the power stroke, and the other on the exhaust stroke of the opposite cylinder. The EFI is non-sequential in this case.

Those that use the camshaft sensor can provide true sequential fuel injection, since they can detect the intake stroke for each individual cylinder, and can use an ignition system that fires one coil per cylinder, since it can detect the power stroke for each individual cylinder.

Many car systems use both, but typically only rely on one as a primary sensor, and the other as a back-up or error checking sensor.


So, both are related, and just sense the position of different (yet related/synched) compnents.

Are these a common repair item on Japanese cars?
On BMW’s, this is a very common repair before 100k

I’ve not had to replace many cps or cks on Asian cars. Of course, since they are simple Hall effect sensors, they tend to either work or not work. Intermittent problems are rare with these type of sensors, and tend to be bad connectors when they occur.

Most engines can run without the camshaft sensor being functional. The crank sensor is the main one that the computer uses to determine where the engine is in terms of what piston is where on its stroke. But since the pistons move up and down twice for every firing of a cylinder, the cam sensor is necessary for the precise determination of where the engine is in the combustion cycle. Without a cam sensor working, you can experience long crank times before the engine fires, and fuel economy will suffer because the computer will be firing ‘blind’ at the back of the intake valves continually. Cutting-edge direct-injection engines may not run at all without a working cam position sensor. In theory, if the computer could know whether the pistons are on the power or intake stroke, the cam sensor wouldn’t be necessary, as long as it remained ‘synced’ with what was going on. But the computer relies on input from the cam sensor to precisely fire the injectors at the right time and for the right duration (pulse width) Otherwise, driveability, performance, and mileage will suffer.

Crank vs. cam… Is one consistently harder to replace than the other?

Camshaft sensors tend to be easy. They tend to be located in the valve cover, since most engines these days are overhead cam design.

Crankshaft sensors can be a pain, since the crankshaft is located on the bottom of the block. I’ve seen some located at the front of the engine under the front balancer/pulley, but many are located at the rear reading the back of the crankshaft. These are typically harder to reach.

I don’t have experience with this kind of repair, but from what I gather on my early 90’s Toyota (which has electronic ignition, and a distributor and rotor and cap to switch the high voltage to the plug wires) these sensors are located in the distributor housing rather than directly on the camshaft or crankshaft. They sense the position of the distributor shaft. The distibutor shaft is synched to the crankshaft through the gearing mechanism that turns the distributor shaft and rotor. On my car, thesesensors have never needed repair and continue to work perfectly. Unlike cars with true crankshaft sensors, positioned right on the crankshaft, I do need to set/verify the distributor timing as part of bi-annual routine maintenance however.