1. The Basics - Easy Checks:
* Battery: Is the battery charged? Test the voltage with a multimeter. A low voltage can prevent the ignition system from functioning properly. Start with this; a dead battery is the most common reason for no spark.
* Battery Connections: Are the battery terminals clean and tight? Corrosion can severely impede current flow. Clean them with a wire brush and baking soda solution.
* Ignition Switch: Try turning the key all the way to the "start" position and holding it there. Does anything else happen (like the starter engaging)? If not, your ignition switch itself might be faulty.
* Fuses: Check the fuse box for any blown fuses related to the ignition system. Your owner's manual will show you which fuses control what.
* Grounds: Poor grounding can prevent current from flowing properly. Check the main ground connection from the engine block to the chassis for corrosion or looseness.
2. Ignition System Components:
If the basics check out, move to the components directly responsible for spark:
* Coil: The ignition coil transforms low voltage from the battery into the high voltage needed for the spark plugs. You can visually inspect it for damage (cracks, burns). A multimeter can test the coil's resistance (check your repair manual for the specific values). A bad coil is a very common cause of no spark.
* Ignition Control Module (ICM): (Some 1990 Jeeps use this, some don't - check your repair manual). The ICM controls the coil's firing. Testing this requires a multimeter and some knowledge of automotive electronics.
* Distributor (If equipped): If your Jeep has a distributor, several things can go wrong:
* Rotor: Check for cracks or burning.
* Distributor Cap: Look for cracks, corrosion, or burnt contacts.
* Pick-up Coil (inside the distributor): This sends a signal to the ICM (or directly to the coil in some systems). Testing this is more advanced and often requires a specialized tool or oscilloscope.
* Spark Plugs and Wires: Inspect the spark plugs for fouling, breakage, or excessive wear. Check the spark plug wires for cracks, damage, or loose connections. Use a spark plug tester to test for spark at the plugs. A weak spark suggests a problem elsewhere, while no spark points directly to the wire or coil (or further upstream).
* Crankshaft Position Sensor (CKP Sensor): This sensor tells the computer the engine's rotational position. A faulty CKP sensor prevents the computer from sending the signal to fire the coil.
3. Computer/ECM (Engine Control Module):
If your Jeep has fuel injection, the ECM controls many aspects of the ignition system. A faulty ECM is possible, but less likely than the simpler problems above. Diagnosing ECM problems is advanced and usually requires a scan tool or specialized knowledge.
Important Considerations:
* Repair Manual: A factory service manual for your specific year and model Jeep is invaluable. It contains wiring diagrams, testing procedures, and specifications for all components.
* Safety: Always disconnect the negative battery terminal before working on any electrical components.
* Professional Help: If you're not comfortable working on your car's electrical system, it's best to take it to a qualified mechanic. Incorrectly diagnosing and fixing electrical problems can lead to further damage.
Start with the easiest checks (battery, connections, fuses) and progress to the more complex components. The order above generally reflects likelihood and difficulty. Remember to consult your repair manual for your specific Jeep model.
