Tech Tip: Common Causes of Ignitor Module Failure
The procedures in this technical sheet involve electrical testing and diagnosis of the ignition system, including the ignition coil, ballast resistor, charging system, distributor wiring, grounds, and Ignitor module. If you are unsure of any procedure, lack the proper tools, or are uncomfortable performing electrical diagnosis, do not proceed and seek out a qualified Classic British Repair Facility.
Incorrect testing, improper wiring, incorrect coil resistance, inadequate grounding, or leaving the ignition switch ON with the engine not running may cause excessive current flow, overheating, or permanent damage to the Ignitor module, ignition coil, distributor, or vehicle wiring. Always switch the ignition OFF before disconnecting or reconnecting wiring unless specifically instructed otherwise. Exercise caution when performing tests with the engine running, as rotating components and hot engine components can cause serious injury.
The person performing the work assumes all responsibility for the use of the information contained in this document.
Condition
Ignitor module repeatedly fails, overheats, or stops functioning after installation.
Symptoms
One or more of the following symptoms may indicate an Ignitor module failure or an underlying ignition system fault:
- Engine will not start
- Engine stalls unexpectedly
- Intermittent ignition failure
- Ignitor module becomes excessively hot
- Repeated module failures after replacement
- Loss of spark after the engine reaches operating temperature
Diagnostic Procedure
The following inspections and tests are intended to identify the root cause of Ignitor module failure before replacing the module.
1. Ignition Switch Usage Verification
Procedure
- Determine whether the ignition switch may have been left in the ON position while the engine was not running.
- Review vehicle operation history with the owner or technician.
Test / Action | Result / Observation | Likely Cause | Remedy |
Verify ignition switch usage history | Ignition left ON with engine not running | Continuous current flow overheated the Ignitor module | Replace failed module and avoid leaving ignition ON when engine is not running |
Verify ignition switch usage history | No evidence of ignition being left ON | Proceed to Test 2 | Continue diagnosis |
2. Coil Primary Resistance Test
Procedure
- Turn ignition OFF.
- Disconnect all wires from the ignition coil.
- Measure resistance between the coil (+) and (-) terminals using an ohmmeter.
- Record the reading.
Test / Action | Result / Observation | Likely Cause | Remedy |
Measure coil primary resistance | Approximately 3.0 ohms | Coil resistance suitable for most Ignitor I applications | Proceed to Test 3 |
Measure coil primary resistance | Significantly below 3.0 ohms | Low-resistance coil or missing ballast resistor causing excessive current draw | Install proper ballast resistor or replace coil with correct specification |
Measure coil primary resistance | Open circuit or extremely high resistance | Defective ignition coil | Replace ignition coil |
3. Ballast Resistance Verification
Procedure
- Determine whether the vehicle was originally equipped with a ballast resistor or resistance wire.
- Verify whether the resistor is present and functioning.
- Confirm whether the installed coil requires ballast resistance.
Test / Action | Result / Observation | Likely Cause | Remedy |
Verify ballast resistor installation | Ballast resistor present and system configured correctly | Ballast circuit functioning normally | Proceed to Test 4 |
Verify ballast resistor installation | Ballast resistor missing, bypassed, or incorrectly wired | Excessive current flow through Ignitor module | Install or restore required ballast resistance |
Verify ignition coil type | Low-resistance or performance coil installed without ballast resistor | Module overheating and premature failure | Install proper ballast resistance or replace coil |
4. Charging System Voltage Test
Procedure
- Start the engine.
- Measure battery voltage with the engine running.
- Observe charging system output voltage.
Test / Action | Result / Observation | Likely Cause | Remedy |
Check charging system voltage | 13.8–14.5 volts | Charging system operating normally | Proceed to Test 5 |
Check charging system voltage | Above 14.5 volts | Overcharging condition damaging electronic ignition components | Diagnose and repair charging system, voltage regulator, or alternator |
5. Ignition Coil Inspection
Procedure
- Inspect the ignition coil housing.
- Look for signs of leakage, cracking, or overheating.
- Evaluate ignition performance after the engine reaches operating temperature.
Test / Action | Result / Observation | Likely Cause | Remedy |
Inspect ignition coil exterior | No visible damage | Coil appears serviceable | Proceed to Test 6 |
Inspect ignition coil exterior | Oil leakage from housing | Internal coil failure | Replace ignition coil |
Inspect ignition coil exterior | Cracked housing | Coil breakdown due to age, heat, or vibration | Replace ignition coil |
Inspect ignition coil after operation | Excessive coil temperature | Coil internal fault or incorrect primary resistance | Verify resistance and replace coil if required |
Monitor ignition performance when hot | Spark loss or engine stalls after reaching operating temperature | Internal heat-related coil failure | Replace ignition coil |
6. Ground System Inspection
Procedure
- Inspect distributor ground integrity.
- Inspect engine-to-chassis ground straps.
- Inspect battery ground connections.
- Verify all ground connections are clean and secure.
Test / Action | Result / Observation | Likely Cause | Remedy |
Inspect ground connections | Clean and secure grounds | Ground system functioning normally | Proceed to Test 7 |
Inspect distributor ground connections | Loose, corroded, or damaged | Increased resistance and voltage spikes stressing module | Clean, repair, or replace ground connections |
Inspect engine-to-chassis ground straps | Missing, loose, or corroded | Electrical instability and poor grounding | Repair or replace ground straps |
Measure voltage drop across grounds | Excessive voltage drop present | High-resistance ground path | Repair affected ground circuit |
7. Distributor Wiring Inspection
Procedure
- Inspect all wiring inside the distributor.
- Check for pinched wires, damaged insulation, or contact with moving components.
- Verify wiring is routed away from grounded metal surfaces.
Test / Action | Result / Observation | Likely Cause | Remedy |
Inspect distributor wiring | Wiring in good condition | Wiring not contributing to failure | Proceed to Test 8 |
Inspect distributor wiring | Pinched or chafed insulation | Intermittent short circuit damaging module | Repair or replace damaged wiring |
Inspect distributor wiring | Wiring contacting ground metal surfaces | Ground short damaging module circuitry | Correct routing and repair wiring |
8. Spark Plug Wire Inspection
Procedure
- Verify the type of spark plug wires installed.
- Confirm resistor-core or spiral-wound suppression wires are being used.
- Verify solid-core copper wires are not installed.
Test / Action | Result / Observation | Likely Cause | Remedy |
Inspect spark plug wire type | Resistor-core or spiral-wound wires installed | Ignition wire type acceptable | Proceed to Test 9 |
Inspect spark plug wire type | Solid-core copper wires installed | Excessive electromagnetic interference (EMI) affecting Ignitor operation | Replace with suppression-core or spiral-wound wires |
9. Replacement History Review
Procedure
- Determine whether previous Ignitor modules were replaced without diagnosing the ignition system.
- Review service history if available.
Test / Action | Result / Observation | Likely Cause | Remedy |
Review replacement history | Multiple modules failed after replacement | Underlying ignition system fault not corrected | Complete all diagnostic procedures before replacing another module |
Review replacement history | No abnormalities found during inspection | Prior module may have failed due to age, overheating, or previous operating conditions | Replace module and verify system operation |
Operational Reminder
Do not leave the ignition switch in the ON position when the engine is not running. Depending on distributor position, continuous current flow through the ignition coil and Ignitor module can rapidly generate heat and permanently damage the module.
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