In this tutorial I'm going to explain how to test the ignition coils on the 2002, 2003, 2004, and 2005 4.2L Chevrolet TrailBlazer and GMC Envoy.
Testing the ignition coils, to find out if you have a defective one, isn't hard and I'll explain how to test them to find out in a step-by-step manner.
Contents of this tutorial:
- Symptoms Of A Bad Ignition Coil.
- Ignition Coil Pin Outs.
- TEST 1: Testing The Ignition Coil For Spark.
- TEST 2: Making Sure The Ignition Coil Is Getting Power.
- TEST 3: Swapping The Ignition Coils.
- TEST 4: Other Common Misfire Causes.
- Where To Buy The Ignition Coil And Save.
- More GM 4.2L Test Tutorials.
You can find this tutorial in Spanish here: Cómo Probar Las Bobinas De Encendido (2002-2005 4.2L Chevrolet TrailBlazer) (at: autotecnico-online.com).
NOTE: You can find the ignition system wiring diagram here: Ignition System Wiring Diagram (2002-2005 4.2L Chevrolet TrailBlazer).
Symptoms Of A Bad Ignition Coil
As you're already aware the ignition coil's job is to supply spark to its cylinder. This spark then ignites the air-fuel mixture within the cylinder.
Without this spark the end result is a dead cylinder that is not producing power.
So when an ignition coil fails the very first thing that you're going to notice is a misfire problem when the engine is running.
You're also going to see the check engine light lit up by a specific misfire trouble code. You'll see one or more of the following:
- P0300: Random Cylinder Misfire
- P0301: Cylinder #1 Misfire
- P0302: Cylinder #2 Misfire
- P0303: Cylinder #3 Misfire
- P0304: Cylinder #4 Misfire
- P0305: Cylinder #5 Misfire
- P0306: Cylinder #6 Misfire
Besides a misfire trouble code stored in your Chevrolet Trailblazer or GMC Envoy's fuel injection computer you're going to see one or more of the following symptoms:
- Bad fuel mileage.
- The check engine light flashes when the misfire is occurring.
- Rough idle.
- Lack of power as you accelerate under load.
- Engine misses (misfires) as you accelerate it's under load.
- Smell of raw fuel coming out of the tailpipe.
Ignition Coil Pin Outs
|Ignition Coil -Cyl #1|
|Ignition Coil -Cyl #2|
|Ignition Coil -Cyl #3|
|B||LT BLU||Activation Signal|
|Ignition Coil -Cyl #4|
|B||DK GRN/WHT||Activation Signal|
|Ignition Coil -Cyl #5|
|B||DK GRN||Activation Signal|
|Ignition Coil -Cyl #6|
|B||LT BLU/WHT||Activation Signal|
TEST 1: Testing The Ignition Coil For Spark
Testing an ignition coil simply involves removing it from its place on the engine valve cover and attaching a spark tester to it.
Then the engine is cranked to see if the spark tester sparks. If the spark tester sparks, then you can conclude that the ignition coil is OK (not defective). You can also conclude that it's not behind the dead cylinder's misfire problem.
If the spark tester DOES NOT spark then you can conclude that the ignition coil is defective and behind the misfire problem (although it's a good idea to make sure it's getting power and an activation signal).
IMPORTANT: To get the most accurate test result from your ignition coil test you need to use a dedicated spark tester. If you don't have one and need to buy one then I recommend the HEI spark tester. You can see what this tool looks like and where to buy one here: The HEI Spark Tester (The Best Spark Tester On The Market) (at: easyautodiagnostics.com).
These are the test steps:
Remove the ignition coil from its place on the valve cover.
Connect a spark tester to the ignition coil.
Reconnect the ignition coil to its electrical connector (if you disconnected it to remove it).
Ground the spark tester with a battery jump start cable directly on the battery negative terminal.
Have a helper crank the engine while you observe the spark tester.
The spark tester will do one of two things: Spark or Not Spark.
Let's take a look at what your test results mean:
CASE 1: The spark tester sparked. This is the correct and expected test result and let you know that the ignition coil that you're testing is not defective.
This is a test result also confirms that this specific ignition coil is getting power and an activation signal from the fuel injection computer.
If the cylinder is misfiring, take a look at the following section for more testing suggestions: TEST 4: Other Common Misfire Causes.
CASE 2: The spark tester did not spark. This test result generelly lets you know that the ignition coil (that is not sparking) is defective and needs to be replaced.
Before condemning the ignition coil as defective you need to make sure that it's getting power and an activation signal. The next step is to make sure it's getting power, for this test go to: TEST 2: Making Sure The Ignition Coil Is Getting Power.