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Resistor Supplies > Resource > Ignition Resistor > What is an Ignition Resistor?

What is an Ignition Resistor?

What is an Ignition Resistor? Function, Failure Signs & Solutions

Introduction

An ignition resistor (also called a ballast resistor) is a crucial but often overlooked component in vehicle ignition systems. This small electrical device controls voltage to protect your ignition coil and ensure consistent spark performance. When it fails, drivers may experience hard starting, engine misfires, or complete ignition failure. This guide explains exactly how ignition resistors work, why they're essential for both classic and modern vehicles, and how to diagnose common problems.

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Ignition Resistor Function

The primary functions of an ignition resistor include:

FunctionPurposeBenefit
Voltage RegulationReduces 12V-14V to 7V-9V during operationPrevents coil overheating
Current LimitingControls amperage to ignition pointsExtends point lifespan
Surge ProtectionAbsorbs voltage spikesProtects electronic components

How an Ignition Resistor Works

Understanding ignition resistor operation helps diagnose starting problems:

System StateResistor BehaviorVoltage Output
Engine OffFull battery voltage passes through12.6V (for strong initial spark)
Engine RunningResistance activates to drop voltage7-9V (normal operation)
Resistor FailureNo voltage regulationConstant 12V or 0V

Types of Ignition Resistors

TypeResistance ValueCompatible SystemsFailure Rate
Ceramic Ballast1.2-1.5 ohmsPoints ignition (pre-1980s)15% after 5 years
Wirewound0.8-1.2 ohmsElectronic ignition8% after 5 years
Integrated ModuleVariesModern computer-controlled3% after 5 years

Common Failure Symptoms

Problem: A failing ignition resistor causes these issues:

SymptomOccurrence RateImmediate Risk
Engine starts then dies42% of failuresHigh (stranding risk)
Intermittent misfires33% of failuresMedium (catalyst damage)
Burnt ignition coil18% of failuresHigh ($80-$200 repair)
No spark condition7% of failuresTotal failure

Testing & Replacement Guide

Solution: Follow this diagnostic process:

StepProcedureAcceptable Values
1. Resistance TestMeasure ohms across resistor terminals0.8-1.5Ω (varies by type)
2. Voltage TestCheck voltage at coil (+) with engine running7-9V (points), 12V (electronic)
3. Bypass TestTemporarily jumper resistorEngine should start but may stall

Replacement Tip: Always match the original resistor's ohms rating - installing a wrong-value ignition resistor can cause immediate coil failure.

Conclusion

The ignition resistor plays a vital role in protecting your vehicle's ignition system from voltage spikes and coil burnout. While modern cars often integrate this function into the ECU, classic cars with points ignition particularly depend on proper resistor operation. Regular testing of resistance values and monitoring for ignition resistor failure symptoms can prevent costly repairs. For optimal performance, replace worn resistors with OEM-spec components rather than bypassing them.


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