Ignition System Components:
* Ignition Coil: This is a primary suspect. A faulty coil won't produce the high voltage needed for spark. Testing involves checking for primary and secondary resistance (using a multimeter) and visually inspecting for cracks or damage.
* Ignition Module (Control Module, Igniter): This is the "brain" that controls the coil. A faulty module will prevent the coil from firing. These often require specific testing equipment or replacement based on symptoms.
* Distributor Cap and Rotor: These components distribute the high voltage to the spark plug wires. Cracks, corrosion, or worn contacts inside the cap or on the rotor will prevent spark. Visual inspection is key here.
* Spark Plug Wires: Worn, damaged, or poorly connected wires can interrupt the high voltage path to the spark plugs. Look for cracks, fraying, or loose connections.
* Spark Plugs: Fouled, worn, or improperly gapped spark plugs will prevent or weaken the spark. They should be inspected and replaced as needed.
* Crankshaft Position Sensor (CKP): This sensor tells the ignition module when to fire the coil. A faulty CKP sensor will prevent sparking. Testing involves checking for voltage and resistance (multimeter).
* Cam Position Sensor (CMP) (if equipped): Similar to the CKP sensor, but for the camshaft. A faulty CMP sensor can prevent proper timing and spark. Requires testing with a multimeter.
Other Potential Issues:
* Ignition Switch: A faulty ignition switch might not be sending power to the rest of the ignition system.
* Wiring Harness: Damaged or corroded wiring anywhere in the ignition system can interrupt the electrical flow. Careful visual inspection is needed.
* Fuses and Relays: Check the fuses and relays related to the ignition system.
* Security System (if equipped): An aftermarket or factory security system could be preventing the engine from starting.
Troubleshooting Steps:
1. Check for Power: Make sure the battery has sufficient voltage and the ignition switch is supplying power to the ignition system.
2. Visual Inspection: Carefully inspect all components mentioned above for visible damage or corrosion.
3. Basic Testing: Use a multimeter to check for voltage and resistance in various parts of the ignition system. A repair manual will give specific test points and values.
4. Spark Test: The most direct way to check for spark is using a spark tester. This involves disconnecting a spark plug wire, attaching the tester, and cranking the engine. A strong spark should be visible.
Important Note: Working on the ignition system involves high voltage. Take necessary safety precautions. If you're not comfortable working with car electrical systems, it's best to take your Jeep to a qualified mechanic. A repair manual specific to your 1991 Jeep Cherokee will be invaluable during this process.
