Testing a 543-020 Non-ballasted Ignition Coil

The procedures in this technical sheet involve electrical testing and diagnosis of the ignition system, including the 543-020 ignition coil, HT lead, ignition wiring, grounds, and related components. 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.

The 543-020 ignition coil is an unballasted 3 Ω coil designed to operate on a full 12V ignition system. Installing or testing this coil on a ballasted ignition system, or incorrectly wiring the ignition circuit, may result in reduced spark output, poor engine performance, overheating, or damage to the ignition coil and other ignition components. Always verify the vehicle's ignition system configuration and coil specifications before installation or testing.

Incorrect testing, improper connections, or failure to properly ground the ignition system during spark testing may result in electrical shock, arcing, damage to the coil, distributor, ignition module, or vehicle wiring. Keep hands, tools, clothing, and other objects away from moving and high-temperature engine components while the engine is cranking or running. Always switch the ignition OFF before disconnecting or reconnecting ignition wiring unless specifically instructed otherwise.

The person performing the work assumes all responsibility for the use of the information contained in this document.

Important Application Note: The 543-020 ignition coil is an unballasted 3 Ω coil.

• It is designed to operate on a full 12V ignition system. Important: Before testing the coil, confirm that the coil is receiving a full 12V supply and not a reduced voltage caused by a ballast resistor or resistor wire.
• If installed on a ballasted ignition system (using a ballast resistor or resistor wire), spark output will be reduced.
• A weak spark caused by incorrect vehicle wiring can mimic the symptoms of a faulty coil.
• Always verify ignition system configuration before condemning the coil.

Symptom Identification

Common symptoms that may indicate an ignition coil, HT lead, or wiring issue include:

• Weak or intermittent spark
• Hard starting or no start condition
• Engine misfire
• Poor engine performance under load
• Yellow/orange spark instead of a strong blue spark
• Spark present at times but inconsistent during cranking

Difference definition between Ballasted and Non-Ballasted coils: 

Non-ballasted coil: Is a coil that does not require a ballasted resistor or resistance wire They are designed to operate directly from the vehicle's full battery voltage (typically about 12 volts). It usually has a primary resistance of around 3 ohms and does not require an external ballast resistor or resistance wire.

Ballasted coil: Is a coil that requires a ballasted resistor or resistance wire. As they are designed to run at a reduced voltage during normal operation, typically around 6–9 volts. It has a lower primary resistance (often around 1.5 ohms) and works with a ballast resistor or resistance wire in series. (For more information on Ballast and Non-Ballast coils go to www.youtube.com/user/MossMotorsCom look for electrical video Ballast Ignition Systems)

Test Procedures

1. Primary Resistance Test (Critical for This Coil Type)

Procedure:

1. Disconnect the coil.
2. Place a multimeter across the + and – terminals of the coil.
3. Measure resistance.

Teste Results:

• Ohms reading: 3.0–3.5 Ω: Good coil
• Ohms reading less than 1 Ω: Wrong coil fitted or coil has internal short circuit
• Very high resistance or open circuit: Internal winding failure/open circuit.

2. Secondary Resistance Test (Important for Screw-In HT Lead Coils)

Procedure:

1. Unscrew the HT lead from the coil tower.
2. Place one meter probe inside the coil tower (contacting the internal spike).
3. Place the second probe on either the + or – terminal.
4. Measure resistance.

Expected Reading:

Note:

• Do not measure through the HT lead unless necessary.
• HT leads often add 3–10 kΩ of resistance, which can produce misleading results.

Test Results:

• 8,000–10,000 Ω (8–10 kΩ): Secondary winding is within specification
• Significantly outside good specifications: Possible internal fault
• Open circuit: Secondary winding failure

3. HT Lead Test (Always Rule Out the Lead Before Condemning the Coil)

Procedure:

1. Remove the HT lead.
2. Measure resistance from one end of the lead to the other.
3. Record the reading.

Test Results:

• A few thousand ohms (kΩ): Lead is generally serviceable
• Open circuit: Faulty HT lead requiring replacement

4. Spark Test (Real-World Performance Check)

Procedure:

1. Reconnect the coil and HT lead.
2. Connect a spark tester or a properly grounded spark plug.
3. Crank the engine while observing spark quality.

Test Results

• A strong, bright blue or blue-white spark: Coil and ignition output appear healthy.
• Yellow/Orange spark: Weak coil output, voltage supply issue, poor connection.
• Intermittent spark: Loose connection, failing component, wiring faulty or coil issue. 

Recommended Corrective Actions

If Coil Voltage Is Low

• Verify the coil receives a full 12V supply.
• Inspect ignition wiring and connectors.
• Check for unwanted voltage drop in the circuit.

If Primary Resistance Is Out of Specification

• Replace the coil.
• Verify the correct coil model is installed.

If Secondary Resistance Is Out of Specification

• Replace the coil after confirming the HT lead is not affecting the measurement.

If the HT Lead Fails Testing

• Replace the HT lead before replacing the coil.
• Retest the ignition system after lead replacement.

If Spark Is Weak or Intermittent

• Verify coil supply voltage.
• Check all ignition connections and grounds.
• Test and replace faulty HT leads as required.
• Confirm the correct coil type is installed.