How The Cam (CMP) Sensor Works

How To Test The Cam Sensor Diagnostic Trouble Codes: P0340, P0341

The camshaft position (CMP) sensor is a three wire Hall Effect type sensor that needs power and ground to produce a Signal.

The cam sensor is located within the distributor assembly and as such, it's rotated by the camshaft. This also means that a cam signal is created and sent to the PCM once every two revolutions of the Crankshaft.

Here, in a nutshell is what happens when you crank and start the engine:

  1. Power is supplied to the camshaft position sensor in the form of 12 Volts (ignition/battery source)
  2. ground is provided internally by the Powertrain Control Module (PMC, which is the technical name for the Fuel Injection Computer).
  3. As the distributor rotor turns,
    1. The vane (underneath and attached to the rotor) interrupts the Hall Effect sensor.
    2. When this vane, on the reluctor wheel, crosses the cam sensor, the cam sensor produces one 12 Volt Pulse.
    3. When the vane, on the reluctor wheel, completely passes the cam sensor, the 12 Volt pulse goes to 0 Volts.
  4. This On/Off 12 Volt pulse will continue as long as the engine is turning.

The best way to test the cam sensor is testing in action (such as the test I've written in this article). The cam sensor can not be tested by a simple multimeter resistance test.

CMP TEST 1: Cam Signal Test

How To Test The Cam Sensor Diagnostic Trouble Codes: P0340, P0341

The cam sensor (camshaft position sensor) is located inside the distributor on your Chevy (or GMC or Isuzu or Olds or Cadillac) mini-van or pick up or SUV. And as I mentioned at the beginning of the article... the cam sensor test is done with a multimeter in Volts DC mode.

So the very first thing you need to do, to start your cam sensor troubleshooting, is to verify that the cam sensor is either producing a cam signal or not. I'm gonna' ask you to manually rotate the engine with a 1/2 inch ratchet and the appropriate socket on the crank pulley... which means you can not use the starter motor to crank the engine.

Manually turning the engine by hand is important because this is the only way that this test can be completely accurate. I also recommend that you read this entire article first, especially familiarize yourself with the working theory of the camshaft position sensor found in the subheading How The Cam (CMP) Sensor Works. OK, here's the test:

  1. 1

    Disable the fuel injectors by unplugging the large connector (located right behind the Throttle Body) that supplies the fuel injectors, inside the plenum, with power and their activation signal. This is important, since it will keep the fuel injectors from injecting fuel as you're manually turning the engine.

  2. 2

    Disable the ignition system by disconnecting the ignition control module or the ignition coil from their electrical connector. This will keep the ignition system from creating and delivering spark to the spark plugs.

  3. 3

    Locate the cam sensor's connector and remove some of the plastic wiring loom protector so that you can gain access to the three wires inside.

  4. 4

    Using an appropriate tool, like a wire-piercing probe, pierce the wire identified by the letter B shown in the image viewer (this will be the middle wire of the connector).

    To see what a what wire piercing probe tool looks like, click here: Wire Piercing Probe.

    This wire (circuit) is the one that brings the cam signal to the Powertrain Control Module (PCM, or in layman's terms: Fuel Injection Computer).

  5. 5

    Connect the BLACK lead of the multimeter to a good ground point on the engine, preferably on the battery negative terminal. Steps are continued in the next page....

  6. 6

    Once your multimeter (and it's test leads) are set up, have a helper manually turn the engine using a 1/2 ratchet and the right socket on the crankshaft pulley... as you keep your eyeballs on the multimeter.

    Do not use the starter motor to crank up the engine, since your multimeter will not produce the same accurate result as manually turning the engine by hand. Also, the cam sensor must be connected to its connector.

  7. 7

    While your helper is turning the engine (manually), your multimeter should register and On/Off 12 Volt DC Signal.

    To go into more detail... if the cam sensor is working like it should, on your 4.3L (or 5.0L or 5.7) mini-van or pick up or SUV, your multimeter will register 0 Volts (which is the Off Signal) and when the cam sensor is activated, the multimeter will register 12 Volts DC (and this is the On Signal). This On/Off process should continue the whole time the engine is being manually cranked.

Let's interpret your test results:

CASE 1: The multimeter registered the On/Off 12 Volt Signal as the engine was manually turned. This is good, and let's you know that the cam sensor within the distributor is OK and not the cause of the cam sensor code lighting up the check engine on your instrument cluster.

I've had this happen to me before, where the cam sensor checks out good... and what really was happening was that this wire (circuit labeled with the letter B) had an open short and the cam signal was not reaching the PCM.

CASE 2: The multimeter DID NOT register the On/Off 12 Volt Signal as the engine was manually turned This means that the cam sensor is not producing a cam signal that the PCM can use.

With this test result alone, you can't condemn the cam sensor just yet. Two more important things have to be checked and they are: 1.) That the cam sensor is getting power and 2.) that it has a good ground. The next test will help you to verify the power circuit, go to: CMP TEST 2: Power Circuit.