How To Test The Ignition Coil (1987-1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma)

The ignition coil can be checked easily and quickly with a few simple tools and in this tutorial, I'll explain how to test it with four easy tests.

All four ignition coil tests are explained step by step so you can easily determine if the ignition coil is defective and causing the engine not to start.

You can find this tutorial in Spanish here: Cómo Probar La Bobina De Encendido (1987-1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 2.5L Chevrolet S10 Pickup: 1987, 1988, 1989, 1990, 1991, 1992, 1993.
• 2.5L GMC S15 Pickup: 1987, 1988, 1989, 1990.
• 2.5L GMC Sonoma: 1991, 1992, 1993.

Symptoms Of A Bad Ignition Coil

The most common symptom you'll see when the ignition coil fails is an engine no-start problem.

To be a bit more specific, the engine isn't going to start because none of the spark plugs are receiving spark.

Where To Buy The ignition Coil And Save

The ignition coil isn't an expensive ignition system component and you can pretty much buy it anywhere.

The important thing to remember when buying an ignition coil is to buy a brand name component. The original equipment manufacturer is AC Delco.

The following ignition coils are from known Automotive brand names.

Ignition Coil Connector Circuit Descriptions

GREY CONNECTOR
Terminal	Wire	Description
A	White (WHT)	Tach signal output
B	Pink (PNK)	12 Volts (input from ignition switch)

BLACK CONNECTOR
Terminal	Wire	Description
C	White (WHT)	IC control signal
D	Pink (PNK)	12 Volts (output to ICM)

Ignition System Basics

The ignition system of your 2.5L Chevrolet S10 pickup (GMC S15 pickup, GMC Sonoma) is a simple mechanical distributor ignition system consisting of the following components:

• An ignition coil.
• A mechanical distributor.
• An ignition control module.
• A pickup coil (CKP sensor).
• Spark plug wires.
• Spark plugs.

In a nutshell this is how the ignition system works:

1. 12 Volts reach the ignition coil on the gray connector terminal labeled with the letter B (in the illustration above). The wire that connects to this terminal is a pink (PNK) wire.
2. As these 12 Volts enter the ignition coil, they also exit on the pink wire of the black connector (terminal labeled with the letter D of the black connector). These 12 Volts now feed power to the ignition module.
3. As the engine starts cranking, the pick-up coil (which is the crankshaft position sensor) starts generating its position signals, which are sent to the ignition control module.
4. The ignition control module (ICM) now starts to activate the ignition coil across the white wire of the black connector (terminal labeled with the letter C in the illustration above).
5. The ICM activates the ignition coil to start sparking by turning ON and OFF (interrupting) the Ground circuit.
6. You and I can check the ignition coil by checking that it's getting power and that the ICM is supplying the ignition coil activation signal.

TEST 1: Testing The Ignition Coil High Tension Wire For Spark

The very first thing we need to do to start troubleshooting the ignition coil is to check for spark.

Specifically, we'll connect a spark tester to the ignition coil high voltage wire and have a helper crank the engine.

If the ignition coil is working properly, the spark tester should spark.

This is a very simple test, but it needs to be done using a dedicated spark tester.

Any other method of testing for sparks can result in you chasing ghosts and wasting time and money.

Alright, these are the test steps:

1. 1

   Disconnect the high tension wire from the center tower of the distributor cap.
   
   NOTE: Leave the other end of the high tension wire attached to the ignition coil.

2. 2

   Attach the HEI spark tester to the spark plug wire.

3. 3

   Connect the spark tester to the battery negative (-) post using a battery jump start cable (see the illustration above).

4. 4

   Have your helper crank the engine while you observe the spark tester from a safe distance.

5. 5

   You'll get one of two results:
   
   1.) The spark tester will spark.
   
   2.) The spark tester will NOT spark.

Let's interpret your test results:

CASE 1: The spark tester sparked. This test result tells you that the ignition coil and its high tension wire are OK.

If the engine is not starting, you can conclude that the ignition coil and its high tension wire are not behind the problem.

CASE 2: The spark tester DID NOT spark. This test result confirms that a no-spark issue is causing the engine not to start.

The next test is to check for spark directly on the ignition coil tower. Go to: TEST 2: Testing The Ignition Coil Tower For Spark.