Ignition Coils Under The Intake Manifold Plenum

How To Test 2-Wire Coil-On-Plug (COP) Ignition Coils

Some vehicles have some or all of the Coil-On-Plug ignition coils under the intake manifold plenum. Here the plenum has to be removed to test them.

If you need to remove the intake manifold's plenum, to access some or all of the ignition coils, you should:

  1. Disable the fuel pump or the fuel injectors. If this isn't done, the engine will back-fire thru' the open intake manifold possibly hurting you and or damaging components on the vehicle.
  2. Avoid accidentally dropping something into the open runners of the intake manifold. Be careful this doesn't happen or the engine will be in a world of hurt if something does fall into a runner and into the cylinder.
  3. If you have this kind of vehicle and your mechanical skills aren't at the necessary level to remove the plenum, then consider taking it to a shop or to an experienced mechanic/technician.

Testing the ignition coils under the plenum may seem complicated, difficult, and/or risky on these types of vehicles. The truth of the matter is that if you're careful and use common sense, this test can be done without any complications or problems.

How To Test The Coil-On-Plug Switching Signal

Testing the Coil-On-Plug for the Switching Signal is also a very, very easy thing to do.

This testing technique only applies to the two wire Coil-On-Plug ignition coils. The three and four wire Coil-On-Plug ignition coils can not be tested in this manner.

Testing for the Switching Signal is just another way of saying: Lets test to see if the ignition control module (whether it's in the ECM or not) is signaling the Coil-On-Plug ignition coil to start sparking away.

On the 2-wire Coil-On-Plug ignition coils, we can use a test light, or an LED light. Now, this Switching Signal can not be tested using a test light on all cars and trucks. The rule of thumb is that the newer the vehicle, the more the likelihood that it needs to be tested using an LED. How can you find out for sure? Well , the only way is trying it out, therefore I recommend that you always use an LED light. You can buy one at your local Radio Shack for about 2 dollars (US), at your auto parts store, or any electronics shop (like Fry's, Radio Shack, etc).

You can see an example of this LED light tool here: The LED Light Test Tool and How To Make One.

This test is performed after the spark and the Power Circuit Test. An LED Light is used to test for the presence of the Switching Signal. Since you'll be working around a cranking engine, use common sense and take all safety precautions. This is how it's done:

  1. Step One.
    1. Disconnect the Coil-On-Plug connector to be tested...
      1. And install the red lead of the LED in the Power Circuit of the Coil on Plug's connector.
      2. Insert the black lead of the LED into the Switching Signal circuit of the Coil-On-Plug's connector.
  2. Step Two.
    1. Have an assistant crank the engine while...
      1. You observe the LED light for it to flash ON and OFF as the engine is cranking.
      2. If you're testing a MISFIRE condition, the engine will start.
      3. If you're testing a no start condition, of course the engine will not start.
  3. Step Three.
    1. If you're testing a no start condition:
      1. Test several more ignition coils.

The most common result of testing for the Switching Signal is that it is indeed present. So logically, if the Switching Signal is present and 12 Volts are present on the Power Circuit, and the Coil is not sparking, the Coil-On-Plug ignition coil is bad. As easy as that! Again, this will be the most common result of your test 9 times out of 10. But for that 1 out of 10 that doesn't fit this profile you can read the following:

Analyzing The Results

If there's no flashing ON and OFF of the LED Light at one Coil-On-Plug ignition coil and if 12 Volts are present on the Power Circuit, then:

  1. The most likely cause is that the ignition control module is FRIED (on externally mounted ignition control modules somewhere in the engine compartment).
  2. If the ignition control module is located in the ECM (Fuel Injection Computer), the ECM is bad. This is rare, but it does happen. Before condemning the ECM, there are a few other tests that have to be performed that are beyond the scope of this article.
  3. There probably is an open-circuit problem in the circuit between the Coil and the ECM. This is especially true if the other coils are sparking and/or are Flashing the LED Light ON and OFF. Remember to always test more than one or two coils for comparison purposes.

If the LED Light comes on and stays on continuously only on one Coil-On-Plug ignition coil... by this I mean that the LED Light does not Flash ON or OFF but stays lit the whole time the engine is being cranked. Then:

  1. The most likely cause is that the ignition control module is FRIED (on externally mounted ignition control modules somewhere in the engine compartment).
  2. If the ignition control module is located in the ECM, the ECM is bad. This is rare, but it does happen. Before condemning the the ECM, there are a few other tests that have to be performed that are beyond the scope of this article.
  3. Or there's a short to Ground on the Switching Signal circuit. This would apply to both of the above ignition module configurations.

If there's no flashing ON and OFF of the LED Light at ANY of the Coil-On-Plug ignition coils and if 12 Volts are present on the Power Circuit, then:

  1. The most likely cause is that the crankshaft position sensor is bad.