How To Test The Ignition Coil Packs (1994-2005 3.1L V6 Buick Century)

The 1994-2005 3.1L V6 Buick Century uses a Direct Ignition System (DIS) with three individual ignition coil packs mounted on top of the ignition control module (ICM). Each ignition coil fires two spark plugs at the same time, so every coil pack serves a pair of engine cylinders.

In this tutorial, I'll show you how to test each ignition coil pack step by step, making sure it's firing spark and whether or not the ICM is sending it an activation signal —all without expensive diagnostic equipment.

You'll also learn how to check the spark plug wires so you can determine whether the misfire or no-start condition is being caused by a bad ignition coil or a faulty spark plug wire.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.1L V6 Buick Century: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005.

IGNITION COIL AND IGNITION CONTROL MODULE (ICM) WIRING DIAGRAM:

• ICM And 3X CKP Sensor Circuit Wiring Diagram (1994-1995 3.1L V6 Buick Century).
• ICM And 7X CKP Sensor Circuit Wiring Diagram (1996 3.1L V6 Buick Century).
• ICM And 7X CKP Sensor Circuit Wiring Diagram (1997 3.1L V6 Buick Century).
• ICM And 7X CKP Sensor Circuit Wiring Diagram (1998-2005 3.1L V6 Buick Century).

OTHER IGNITION SYSTEM DIAGNOSTIC TESTS:

• How To Test The Ignition Coil Packs (1994-2005 3.1L V6 Buick Century).
• How To Test The Ignition Control Module (1994-2005 3.1L V6 Buick Century).

Symptoms Of A Bad Ignition Coil Pack

A failed ignition coil pack can cause one or more of the following symptoms:

• Check engine light: The PCM will usually store one or more misfire trouble codes, such as P0300, P0301, P0302, P0303, P0304, P0305, or P0306. Other ignition system-related codes may also be present.
• Rough idle: The engine idles rough because one or more cylinders are misfiring.
• Bad gas mileage: Fuel mileage drops because the engine isn't firing on all six cylinders.
• Black smoke: A severe misfire can cause black smoke to come out of the tailpipe due to unburned fuel.
• Rotten egg smell: Unburned fuel from the misfiring cylinder overheats the catalytic converter, producing a sulfur or "rotten egg" odor. Continued driving under these conditions can eventually damage the catalytic converter.
• Lack of power: The engine hesitates or lacks power because one or more cylinders aren't contributing their share of power.

Basic Operating Theory Of Direct Ignition System

Before we start testing the ignition coils, it helps to understand a few basics about how this Direct Ignition System (DIS) works.

Here are the five most important things to keep in mind:

1. Each ignition coil needs battery power. With the ignition key in the ON position, all three ignition coils receive 12 Volts.
2. Each ignition coil is fired by the ignition control module (ICM). The ICM does this by switching the coil's Ground circuit ON and OFF. I'm gonna refer to this ON/OFF Ground is as the Switching signal.
3. No Switching signal means no spark. Even if an ignition coil has 12 Volts, it won't fire unless the ICM provides its Switching signal.
4. It's very uncommon for all three ignition coils to fail at the same time. Most ignition system problems involve a single bad ignition coil, a faulty spark plug wire, or an ignition control module problem.
   • Usually when you see all 3 ignition coils not sparking, it's a bad 7X crankshaft position (CKP) sensor.
5. Each ignition coil can be tested right on the engine. With the proper test procedure, you can accurately determine whether an ignition coil is good or bad without replacing any unnecessary parts.

As we go through the tests in this tutorial, you'll verify whether each ignition coil is producing spark and receiving its Switching signal using some basic tools (a multimeter and a spark tester). This will let you pinpoint the cause of the misfire or no-start condition with confidence.

What Tools Do I Need To Test The Ignition Coil Packs?

You don't need any expensive diagnostic equipment to test the ignition coil packs on your 1994-2005 3.1L V6 Buick Century. Here's what you'll need:

• Test light: You'll use it to verify the ignition coils are receiving battery power.
• LED test light: You'll use it to check for the ignition control module's Switching signal. If you don't have one, you can buy one here: Oznium Flush Mount 12V LED Light (Amazon affiliate link)
• Multimeter: A multimeter is needed for a few of the electrical tests in this tutorial. If you don't have one yet, take a look at my recommendations here: Tekpower TP8268 AC/DC Auto/Manual Range Digital Multimeter (Amazon affiliate link)
• A spark tester: A spark tester is the only accurate way of testing these ignition coils for spark. Don't substitute a regular spark plug, since it can lead to inaccurate test results. If you need one, this is the one I've been using my whole automotive repair career: OTC 6589 Electronic Ignition Spark Tester (Amazon affiliate link).
• Battery jumper cable: You'll use it during one of the ignition coil tests later in the tutorial.
• Helper: Someone will need to crank the engine while you perform several of the tests.
• Repair manual: Although this tutorial shows you how to diagnose the problem, a repair manual is helpful if you need removal, installation, or vehicle-specific repair procedures.

NOTE: A scan tool is a must have tool nowadays, but it isn't required to diagnose the ignition coils on your Buick Century.

Where To Buy The Ignition Coils And ICM And Save

The following links will help you comparison shop for the original AC Delco (Delphi) ignition module and ignition coils:

TEST 1: Testing Spark At The Spark Plug Wire

To get started, the first thing we're going to do is check for spark.

I recommend doing this test with the engine completely cold. If the engine has been running for any length of time, let it cool down before continuing.

When checking for spark, it's very important that you use a dedicated spark tester. You can use any spark tester you already have. If you don't own one, the one I've always used is the OTC 6589 Electronic Ignition Spark Tester (Amazon affiliate link) because it gives accurate and consistent results. Whatever spark tester you use, just don't substitute a spark plug for one.

In the test instructions below, I recommend checking all six spark plug wires for spark. Technically, you don't have to, but if you want to diagnose the ignition system without unnecessary guesswork, it's best to test all six spark plug wires.

One last recommendation. Use a spark plug wire puller pliers to disconnect each spark plug wire from its spark plug. This helps prevent the metal terminal inside the spark plug wire boot from pulling loose and getting stuck on the spark plug. Most auto parts stores have these available, or you can buy one here Performance Tool W80519 Adjustable Spark Plug Boot And Wire Remover (Amazon affiliate link).

CAUTION: Most of the tests you'll be performing are done while the engine is cranking. Keep your hands, tools, and clothing away from any moving engine components, and take all necessary safety precautions.

Let's get started:

1. 1

   Remove the spark plug wire from the spark plug.

2. 2

   Insert the spark tester into the spark plug wire boot.

3. 3

   Ground the 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. 4

   Have your assistant crank the vehicle.

5. 5

   You're gonna' get one of two results:
   
   1.) Spark
   
   2.) No spark.

6. 6

   Repeat this test on the remaining spark plug wires.

Let's take a look at what your test results mean:

CASE 1: All six spark plug wires fired spark. This is the correct test result. It confirms that all three ignition coils and all six spark plug wires are working properly.

You can rule out the spark plug wires, ignition coils, and the ignition control module as the cause of the misfire or no-start condition.

CASE 2: One spark plug wire did not fire spark. Your next step is to determine whether the problem is the spark plug wire or the ignition coil tower it (the spark plug wire) connects to.

Go to: TEST 2: Testing Spark At The Ignition Coil Tower.

CASE 3: Two spark plug wires did not fire spark and both connect to the same ignition coil. Since each ignition coil fires two cylinders, your next step is to check for spark directly at both towers of that ignition coil.

Go to: TEST 3: Testing Spark At Two Adjacent Towers Of The Same Ignition Coil.

CASE 4: Two spark plug wires did not fire spark, but they connect to two different ignition coils. We need to treat each no-spark wire as a separate problem and test each ignition coil tower individually.

Go to: TEST 2: Testing Spark At The Ignition Coil Tower.

CASE 5: None of the spark plug wires sparked. When all six cylinders lose spark, the problem usually isn't the ignition coils or the spark plug wires.

The most likely cause is a failed ignition control module or crankshaft position sensor. To continue diagnosing the problem, see: How To Test The Ignition Control Module (1994-2005 3.1L V6 Buick Century).

TEST 2: Testing Spark At The Ignition Coil Tower

If only one spark plug wire failed to spark in TEST 1, the next step is to find out whether the problem is the spark plug wire itself or the ignition coil tower feeding it.

To do this, you'll remove the spark plug wire from the ignition coil tower and connect your spark tester directly to that tower. We'll use a short piece of vacuum hose to securely attach the spark tester (see the photo above).

If the ignition coil tower sparks, you can rule out the ignition coil and conclude the spark plug wire is bad. If the tower doesn't fire spark, the ignition coil is defective.

IMPORTANT: This test only applies when one of the two spark plug wires connected to the same ignition coil did not spark in TEST 1.

These are the test steps:

1. 1

   Remove the spark plug wire that did not fire off spark from its ignition coil tower.

2. 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. 3

   Connect the HEI spark tester to the battery negative (-) post with a battery jump start cable.

4. 4

   Once you're clear of the engine compartment, have your helper crank the vehicle while observing the HEI spark tester for spark.

5. 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 confirms that the ignition coil tower is producing spark, which means the ignition coil is working correctly.

Since the spark plug wire connected to this tower did not spark in TEST 1, you can conclude that the spark plug wire is defective and should be replaced. I recommend replacing the entire set of spark plug wires.

CASE 2: The spark tester DID NOT spark. This tells you that the ignition coil isn't producing spark from that tower.

Because the other spark plug wire connected to this same ignition coil fired spark in TEST 1, you can conclude that the ignition coil is defective and needs to be replaced.

NOTE: These test results apply only if the other spark plug wire connected to the same ignition coil sparked during TEST 1.

TEST 3: Testing Spark At Two Adjacent Towers Of Same Coil

You've reached this test because the two spark plug wires connected to the same ignition coil did not fire spark in TEST 1.

Our next step is to figure out whether the problem is the ignition coil itself or the two spark plug wires. To do that, you'll check each ignition coil tower for spark individually using your spark tester.

If both ignition coil towers fire spark, the ignition coil is working properly and both spark plug wires are defective. If neither tower sparks, you can conclude that the ignition coil has failed.

Let's get started:

1. 1

   Remove one of the two spark plug wires from the ignition coil you're testing.
   
   NOTE: This should be one of the two spark plug wires that did not fire spark in TEST 1.

2. 2

   Connect your spark tester to the ignition coil tower using a short piece of vacuum hose (see the photo above).

3. 3

   Connect the spark tester to the battery negative (-) terminal with a battery jumper cable.

4. 4

   Have your helper crank the engine while you watch the spark tester.

5. 5

   You'll get one of two results:
   
   1.) The spark tester sparks.
   
   2.) The spark tester does not spark.

6. 6

   Remove the spark tester and reconnect the spark plug wire to the ignition coil tower.

7. 7

   Repeat steps 1 through 6 on the other ignition coil tower of the same ignition coil.

Here's what your test results mean:

CASE 1: Both ignition coil towers sparked. This confirms that the ignition coil is working correctly.

Since neither of the two spark plug wires fired spark in TEST 1, you can conclude that both spark plug wires are defective. Replacing the spark plug wires will solve the problem.

CASE 2: Neither ignition coil tower sparked. This usually means the ignition coil is bad, but we could also have a bad ignition control module on our hands.

To further troubleshoot this, our next step is to verify that the ignition coil is receiving battery power. Go to: TEST 4: Verifying That The Ignition Coil Is Receiving 12 Volts.

CASE 3: One ignition coil tower sparked but the other did not. This confirms that the ignition coil has failed and needs to be replaced.

Each ignition coil is designed to fire spark from both towers at the same time. Since only one tower is firing spark, you can confidently conclude that this ignition coil is defective and needs to be replaced.

TEST 4: Verifying That The Ignition Coil Is Receiving 12 Volts

You've reached this test because both towers of the same ignition coil failed to spark in the previous test.

Your next step is to verify that the ignition coil is receiving battery power. The results of this test will tell you whether the problem is the ignition coil itself, the ignition control module (ICM), or if you need to continue with the next diagnostic step.

To perform this test, you'll need to remove the ignition coil that isn't producing spark. This will expose the two male spade terminals that provide the ignition coil with battery power and its Switching signal.

IMPORTANT: The photos above show the ignition control module with all three ignition coils removed and the entire assembly off the vehicle. I'm showing it this way only to make it easier to identify the power and Switching signal terminals. Do not remove the ignition control module from the engine or disconnect its electrical connectors. For this test, only remove the ignition coil you're testing.

Alright, let's get the ball rolling:

1. 1

   Remove the ignition coil from the ignition control module. This is the ignition coil that didn't fire spark from either tower in TEST 3.

2. 2

   Set your multimeter to Volts DC.

3. 3

   Connect the black multimeter lead to the battery negative (-) terminal.

4. 4

   Touch the red multimeter lead to the male spade terminal marked with the + symbol (see the photo above).

5. 5

   Have your helper turn the ignition key to the ON position. Do not crank the engine.

6. 6

   Your multimeter should read 10 to 12 Volts.

Here's what your test results mean:

CASE 1: Your multimeter measured 10 to 12 Volts. This confirms that the ignition coil is receiving battery power.

The next step is to verify that the ignition control module is supplying the coil with its Switching signal. Go to: TEST 5: Verifying The Ignition Coil's Switching Signal.

CASE 2: Your multimeter did not measure 10 to 12 Volts. This confirms that the ignition coil isn't receiving battery power.

Since you've already verified that the other two ignition coils are producing spark, you know they're receiving battery power. This means the ignition control module has failed internally and isn't supplying power to this ignition coil. Replace the ignition control module.

TEST 5: Verifying The Ignition Coil's Switching Signal

This is the last test in the ignition coil diagnostic process. By the end of it, you'll know whether the no-spark condition is being caused by the ignition coil or the ignition control module (ICM).

In TEST 4, you verified that the ignition coil is receiving battery power. Now it's time to verify that the ignition control module is providing the coil with its Switching signal. Without this signal, the ignition coil will never fire spark, even though it's receiving 12 Volts.

A regular test light will work for this test, but I recommend using an LED test light. In my experience, the LED responds much better to the ignition coil's Switching signal and makes it easier to determine whether the signal is present while the engine is cranking. If you don't have one, you can purchase an LED test light here: Oznium Flush Mount 12V LED Light (Amazon affiliate link).

Leave the ignition coil removed from the ignition control module, just as you did in TEST 4, and perform the following steps:

1. 1

   Connect the red lead of the LED test light to the male spade terminal marked with the + symbol.

2. 2

   Connect the black lead of the LED test light to the male spade terminal labeled Signal.
   
   This is the terminal that carries the ignition coil's Switching signal.

3. 3

   Have your helper crank the engine while you watch the LED test light.
   
   NOTE: Don't worry whether the LED is ON or OFF before cranking begins or after cranking stops. The only thing that matters is whether it flashes while the engine is cranking.

4. 4

   The LED should flash ON and OFF while the engine is cranking.

Here's what your test results mean:

CASE 1: The LED flashed ON and OFF. This confirms that the ignition control module is supplying the ignition coil with its Switching signal.

If you've also confirmed the following:

• TEST 1: The two spark plug wires connected to this ignition coil did not spark.
• TEST 2: The ignition coil towers did not spark.
• TEST 3: The ignition coil is receiving battery power.
• TEST 4: The ignition control module is supplying the Switching signal.

...then you can conclude that the ignition coil is defective and needs to be replaced.

CASE 2: The LED did not flash. This confirms that the ignition control module is not supplying the ignition coil with its Switching signal.

If you've already confirmed that the ignition coil is receiving battery power in the previous test, you can conclude that the ignition control module has failed and needs to be replaced.

More 3.1L V6 Buick Century Diagnostic Tutorials

If this tutorial was helpful, there's plenty more, and you can find them in the following index:

• 3.1L V6 Buick Century Index of Articles.

Here's a sample of the tutorials you'll find in the index:

• How To Test The Fuel Pump (1994-2005 3.1L V6 Buick Century).
• How To Test The TPS With A Multimeter (1997-2005 3.1L V6 Buick Century).
• Common Causes Of Spark Plug Failure (1994-2005 3.1L V6 Buick Century).
• How To Test Engine Compression (1994-2005 3.1L V6 Buick Century).