3

On a vehicle with variable valve timing (VVT), the need for a camshaft position sensor (CMP) is obvious. However, on an older engine without VVT, what purpose does the CMP serve? The position of the cam vs crank is fixed, so it seems redundant to have both a CMP and crank position sensor (CKP).

I am diagnosing a intermittent misfire on a 2005 Chevy Malibu 3.5 liter (LX9, no VVT). No check engine light, already did the obvious stuff (plugs/wires/coil). I would like to understand what the CMP and CKP are used for to help my diagnosis.

masospaghetti
  • 922
  • 5
  • 15

2 Answers2

4

Because a cam shaft rotates at half the speed of the crank.

The ECU needs to know which phase the cam is in in relation to the 4-stroke cycle, I.E is it on "Suck" stroke or "Bang" stroke, in which the CAM is 180 degrees different but the Crank is the same.

Why?

Because the ECU needs* to know when to fire the injectors.

Not so much for spark, as a lot of engines run "Wasted spark" ignition whereby the plugs fire on the exhaust stroke as well as the combustion stroke, or a distributor which rotates at the same speed as the cam. Although some run fully sequential ignition too which also requires the cam sensor.

*Some older engines don't have cam sensors, they simply fire the injectors multiple times during the cycle, which will eventually get sucked in - These have the injectors further away from the valve.

Digital Lightcraft
  • 665
  • 4
  • 13
3

The crankshaft rotates twice per rotation of the camshaft, so simply having a crankshaft position sensor DOES NOT tell the ECU the exact position of the valves and at which point in the combustion cycle the engine is in. The piston can be a top dead center and either be at the end of the compression stroke or end of the exhaust stroke.

Without a camshaft position sensor, the car may run, but will take multiple tries to start as the ECU must guess which of the two possible positions the camshaft is in. The "guessing" is achieved, as the engine rotates during start attempts, it will coincide after some number of attempts with the position that corresponds to the position the ECU believes the system to be at.

Once the engine successfully starts, the ECU can use the crankshaft signal to deduce the camshaft position (since it guessed the initial position correctly)

sebastian323
  • 156
  • 3