Testing the ignition coil, on your 4.0L equipped Jeep to see if it's bad and causing your Jeep to not start (or not), isn't hard and it's something you can do yourself without expensive diagnostic equipment.
In this tutorial I'll show you how to do it in a step-by-step way and with two simple tools: a spark tester and a 12 volt test light.
To help you navigate this article a little easier, here are its contents at a quick glance:
- Important Suggestions.
- Symptoms Of A BAD Ignition Coil.
- How Does the Ignition Coil Work?
- TEST 1: Testing The Hight Tension Wire.
- TEST 2: Testing for Spark Directly on the Ignition Coil Tower.
- TEST 3: Verifying The Ignition Coil Is Getting Power.
- TEST 4: Testing The Switching Signal.
- Where To Buy The Ignition Coil.
- More 4.0L Jeep Tutorials.
You can find this tutorial in Spanish here: Cómo Probar la Bobina de Encendido (1991-1997 4.0L Jeep) (at: autotecnico-online.com).
TIP 1: Two of the ignition coil tests in this tutorial require that you crank the engine... so, you've got to be careful and use common sense as you take all necessary safety precautions.
TIP 2: As a major safety precaution, keep your helper waiting outside the vehicle till you need him (or her) to crank the engine. Your helper should stay outside the vehicle and away from the ignition switch before you set up the test.
TIP 3: All of the tests in this tutorial are on-car tests, although the photos I'm using show the ignition coil off of the vehicle.
Symptoms Of A BAD Ignition Coil
The ignition coil is the one tasked with creating spark for all of the 6 cylinders of your 4.0L Jeep engine.
So, when the ignition coil fails, the most obvious symptom your Jeep is gonna' experience is a cranks but does not start condition.
Here are some more specific symptoms your Jeep will experience:
- You won't see spark firing from all of the spark plug wires.
- The fuel injectors will still work and spray fuel into the cylinders.
- The PCM will still activate the ignition coil.
- This means that the ignition coil does receive the switching signal but fails to fire spark.
- Crank sensor is good.
How Does the Ignition Coil Work?
The ignition coil is an amazingly simple device that is a key component of the ignition system on your 4.0L Jeep.
And since it's an amazingly simple component... it's also a cinch to diagnose when it has failed.
The following is a very simple explanation of how it works.
It all starts when you turn the key and start cranking the engine... and in a nutshell, this is what happens:
- As the engine starts to crank the crankshaft position sensor (on the transmission's bell housing) starts to generate it's crank signal.
- The PCM (Powertrain Control Module = fuel injection computer) gets the crank signal and then activates the ASD (Auto Shut Down) Relay. Once this relay becomes activated, the ignition coil gets power (12 Volts).
NOTE: The ignition coil only gets power with the engine cranking or engine running.
- Depending on your specific 4.0L Jeep, the wire that feeds power to the coil is a:
- DK GRN/BLK (dark green w/ black stripe) wire.
- DK GRN/ORG (dark green w/ orange stripe) wire.
- DK GRN/RED (dark green w/ red stripe) wire.
- At the same time the PCM activates the ASD Relay, it starts to switch the ignition coil ON and OFF by interrupting the ignition coil's primary voltage.
- Once the ignition coil gets this switching signal, it starts to spark away and as you're already aware, this spark gets delivered to the center of the distributor cap by a high tension wire.
- Depending on your specific 4.0L Jeep, the wire that feeds the switching signal to the coil is a:
- GRY/BLK (grey w/ black stripe) wire.
- GRY (grey) wire.
- BLK/GRY (black w/ grey stripe) wire.
- By this time, the fuel injection computer is also injecting fuel into the engine, which the spark will ignite... thus the engine starts!
OK, let's turn the page and start the first ignition coil Test...