TEST 1: Testing Spark At The Spark Plug Wire
The very first thing that you need to do is test for spark. I'm gonna' show you a very specific way to do it so that you can diagnose a bad ignition coil with 100% accuracy. This method requires that you use a dedicated spark tester. Now, not just any, but one called an HEI spark tester (don't have an HEI spark tester? Need to buy one? You can buy it here: OTC 6589 Electronic Ignition Spark Tester).
In the photo above you can see one. It's attached to the end of the battery jump cable. This is a very inexpensive tool that you can buy just about anywhere and this article assumes you have one or have gotten one. If you'd like some more info about the HEI spark tester, click here: HEI Spark Tester.
OK, the starting point for this test is to test for spark at the spark plug wire boot as shown in the photo above. I recommend testing all of the spark plug wires even if you just have one ignition misfire code registering on the vehicle's computer.
Let's get started:
- 1
Remove the spark plug wire from the spark plug.
- 2
Insert the HEI spark tester into the spark plug wire boot.
- 3
Connect the HEI spark tester to the battery negative (-) post with a battery jumper cable.
Using a battery jump start cable to Ground the spark tester to the battery negative (-) post will guarantee a correct test result. - 4
Have your assistant crank the vehicle.
- 5
You're gonna' get one of two results:
1.) Spark
2.) No spark. - 6
Repeat this test on the remaining spark plug wires.
Let's take a look at what your test results mean:
CASE 1: You got spark from all of the spark plug wires. This is the correct test result and it lets you know that all of the ignition coils and the spark plug wires are working fine.
You have just eliminated these ignition system components as the source of the misfire condition.
CASE 2: You got NO spark from one spark plug wire. Your next step is to check for spark directly on the spark plug cable's ignition coil tower.
For this test go to: TEST 2: Testing Spark At The Ignition Coil Tower.
CASE 3: You got NO spark from 2 spark plug wires that connect to the same ignition coil (each ignition coil has two towers). Your next step is to check for spark directly on both ignition coil towers that feed spark to those 2 spark plug wires.
For this test go to: TEST 3: Testing Spark At Two Adjacent Towers Of Same Coil.
CASE 4: You got NO spark from 2 spark plug wires that DO NOT connect to the same ignition coil (each ignition coil has two towers). Your next step is to check for spark directly on both ignition coil towers that feed spark to those 2 spark plug wires.
For this test go to: TEST 2: Testing Spark At The Ignition Coil Tower.
CASE 5: You got NO spark from any of the spark plug wires. The most likely cause is that the ignition control module or the crankshaft position sensor has gone bad.
It's nearly impossible for all of the ignition coils or spark plug wires to fail at the same time. To test the ignition control module and the crankshaft position sensor, see this tutorial: Testing The Ignition Module And Crank Sensor (GM 3.1L, 3.4L).
TEST 2: Testing Spark At The Ignition Coil Tower
The most common problem with this type of ignition coil is that one of its towers will fire off spark but the other will not. This result is a dead giveaway that this particular ignition coil is defective.
So, in this step you're gonna' test for spark at the ignition coil tower of the spark plug wire that did not fire off spark in TEST 1.
We're gonna' attach the spark tester to the ignition coil tower using a small piece of vacuum hose (see the photo above).
IMPORTANT: This test and its interpretation only applies when 1 of 2 spark plug wires that connect to the same ignition coil did not spark.
These are the test steps:
- 1
Remove the spark plug wire that did not fire off spark from its ignition coil tower.
- 2
Connect the spark tester to the tower with a short piece of vacuum hose.
NOTE: As you can see in the photo, this is a very short piece of vacuum hose that permits the end of the spark tester to touch the metal part of the coil tower. - 3
Connect the HEI spark tester to the battery negative (-) post with a battery jump start cable.
- 4
Once you're clear of the engine compartment, have your helper crank the vehicle while observing the HEI spark tester for spark.
- 5
You're gonna' get one of two results:
1.) Spark
2.) No spark.
Let's take a look at what your test results mean:
CASE 1: The spark tester sparked. This test result tells you that the ignition coil is good.
You can also conclude that the spark plug wire that did not fire off spark (in TEST 1) is bad since it can no longer transmit the spark that the coil is producing.
Replacing the spark plug wires as a set will eliminate the misfire problem.
NOTE: This conclusion (CASE 1) applies only if the other spark plug wire (that's attached to this same ignition coil) sparked in TEST 1.
CASE 2: The spark tester DID NOT spark. This test result tells you that the ignition coil is bad.
Here's why: The ignition coil is designed to fire off spark from both of its 2 towers simultaneously. Since you have confirmed that one tower is not firing off spark while the other tower is (from your test results in TEST 1), you can conclude that the ignition coil is defective and needs to be replaced.
NOTE: This conclusion (CASE 2) applies only if the other spark plug wire (that's attached to this same ignition coil) sparked in TEST 1.
