Ignition Coil Guide – 6V vs 12V vs Ballast Resistor Coils

Choosing the correct tractor ignition coil is essential for reliable starting and smooth engine performance. Using the wrong coil can cause hard starting, overheating, weak spark, or premature coil failure.

Here’s how to choose the right one.

🔋 6 Volt Ignition Coil

Used on:

• Original 6V tractor electrical systems

• Early Ferguson & vintage tractors

• Unconverted classic models

Key Points:

• Designed for 6V battery systems

• Must NOT be used on 12V systems

• Lower output compared to 12V coils

If your tractor is still running its original 6V system, use a 6V ignition coil only.

⚡ 12 Volt Ignition Coil (Non-Ballast Type)

Used on:

• Factory 12V tractors

• 6V tractors converted to 12V

• Most modernised classic tractors

Key Points:

• Designed for full 12V supply

• No ballast resistor required

• Higher output for improved starting

• Common upgrade for classic tractors

Example: Bare Co B5212 12V Lucas Style Coil

This is the most common choice for 12V conversions.

🔧 12 Volt Coil With Ballast Resistor

Used on:

• Certain factory 12V systems

• Applications requiring reduced running voltage

Key Points:

• Requires an external ballast resistor

• Receives full voltage during starting

• Reduced voltage while running

• Prevents coil overheating

If your tractor has a ballast resistor fitted, you must use a compatible ballast-type coil.

How to Identify What You Need

✔ Check battery voltage (6V or 12V)
✔ Confirm if a ballast resistor is fitted
✔ Review wiring diagram
✔ Check existing coil markings

If unsure, match the replacement coil to your current system setup.

Why Coil Selection Matters

Using the correct ignition coil:

• Improves cold starting

• Provides stronger spark

• Reduces misfiring

• Prevents overheating

• Extends ignition component life

Common Applications

• Ferguson TE20 (6V original / 12V conversion)

• Massey Ferguson MF35

• Ford 2000 / 3000

• Case IH & David Brown

• Other classic agricultural tractors

SEO Keywords This Supports

• 12V ignition coil tractor

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Ferguson TE20 12V Conversion Guide – Upgrade from 6 Volt to 12 Volt

Many classic Ferguson TE20 tractors were originally fitted with a 6-volt electrical system. While authentic, 6V systems can suffer from slow cranking, hard cold starts, and limited electrical performance.

Upgrading to a 12V conversion improves starting reliability, charging performance, and overall electrical efficiency.

Why Convert a TE20 to 12 Volt?

Upgrading to 12V provides:

✔ Faster engine cranking
✔ Improved cold starting
✔ Brighter lights
✔ Better charging performance
✔ Easier battery sourcing
✔ Improved reliability

A 12V conversion is one of the most common and beneficial upgrades for Ferguson TE20 tractors.

What You Need for a TE20 12V Conversion

1️⃣ 12 Volt Battery

Replace the original 6V battery with a 12V battery of suitable size and capacity.

2️⃣ 12 Volt Ignition Coil

Install a 12V ignition coil, such as:

Bare Co B5212 – 12V Lucas Style Screw-In HT Coil

• Higher output voltage

• Improved starting performance

• Original screw-in HT terminal

• Ideal for TE20 conversions

If using a non-ballast coil, no resistor is required.

3️⃣ Alternator Conversion

Replace the original generator with:

• A 12V alternator

• Updated mounting bracket

• Correct wiring setup

Alternators provide stronger charging output and improved reliability.

4️⃣ Replace Bulbs

All 6V bulbs must be replaced with:

• 12V headlight bulbs

• 12V warning lights

• 12V dash bulbs

5️⃣ Starter Motor

In most cases, the original 6V starter motor can operate on 12V:

• Provides stronger cranking

• Avoid prolonged cranking to prevent overheating

Some owners choose to fit a purpose-built 12V starter.

Ballast or Non-Ballast Coil?

For most TE20 12V conversions:

✔ Use a 12V non-ballast ignition coil
✔ No resistor required
✔ Simpler wiring
✔ Reliable performance

Only use a ballast coil if your wiring system includes a resistor.

Common Questions

Will a 12V conversion damage my engine?

No — if installed correctly, it improves starting and reliability.

Is it reversible?

Yes — conversion does not permanently modify the engine block.

Does it affect originality?

Yes, for show tractors originality may matter. For working tractors, 12V is generally preferred.

Recommended Agline Products for TE20 12V Conversion

• 12V Ignition Coil (Lucas style screw-in)

• Headlight bulbs 12V

• Electrical connectors

• Battery cables

• Charging system components

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Ferguson TE20 Troubleshooting Guide

Hard Starting – No Spark – Weak Spark

If your Ferguson TE20 is difficult to start, has no spark, or runs roughly, the issue is often within the ignition or electrical system. Use this guide to diagnose common problems and identify the correct replacement parts.

🚜 Problem 1: Hard Starting (Slow Cranking)

Possible Causes:

✔ Weak battery
✔ Poor earth connection
✔ Undersized battery cables
✔ Failing starter motor
✔ Incorrect ignition coil (6V vs 12V mismatch)

What To Check:

• Confirm battery voltage (6V or 12V system)

• Clean all earth connections

• Check heavy-gauge battery cables

• Ensure correct ignition coil is fitted

🔧 Recommended Upgrade:
A 12V conversion with a high-output ignition coil (e.g., Bare Co 12V Lucas-style coil) greatly improves cold starting performance.

⚡ Problem 2: No Spark

Possible Causes:

✔ Faulty ignition coil
✔ Worn contact points
✔ Condenser failure
✔ Broken HT lead
✔ Incorrect wiring
✔ Faulty ignition switch

How To Test:

1. Remove spark plug lead

2. Hold near engine block

3. Crank engine

4. Check for strong blue spark

No spark usually indicates:

• Failed ignition coil

• Points not opening

• Condenser failure

🔧 Common Fix: Replace ignition coil with correct 6V or 12V unit.

🔥 Problem 3: Weak Spark / Misfiring

Possible Causes:

✔ Coil overheating
✔ Incorrect coil for system voltage
✔ Dirty or worn spark plugs
✔ Poor HT lead connections
✔ Failing condenser

Weak yellow spark often indicates:

• Incorrect coil voltage

• Coil breaking down internally

• Ballast mismatch

🔧 For 12V conversions:
Use a 12V non-ballast coil unless a ballast resistor is fitted.

🔧 Coil Selection Quick Reference

Using the wrong coil can cause:

❌ Overheating
❌ Weak spark
❌ Short coil lifespan
❌ Hard starting

🔎 Additional TE20 Ignition Checks

• Check distributor cap for cracks

• Inspect rotor arm

• Ensure points gap is correct

• Confirm timing is properly set

Recommended Agline Products for TE20 Ignition Repairs

• 12V Lucas-style ignition coil

• Contact points & condenser

• Spark plugs

• HT leads

• Headlight bulbs (12V conversion)