The difference between Ballast and non Ballast coils.

ALL TESTS ASSUME COIL IS CORRECTLY FITTED 

Non-Ballast Ignition Coil:

  • No Ballast Resistor Used.
  • Voltage at Coil: Designed to run at full 12 volts continuously.
  • No Start Bypass Needed: It’s built to handle the full voltage without damage.
  • Common In: Most classic cars up to the 1970s.

Ballast Ignition Coil:

  • Used with a Ballast Resistor (or a resistance wire).
  • Voltage at Coil: Normally runs at 6 to 9 volts during engine operation.
  • Starting Boost: During startup, the ballast resistor is bypassed so the coil gets a full 12 volts to help with cold starts.
  • Purpose of Ballast Resistor: Limits current after starting to protect the coil and prolong its life.
  • Common In: Fords from 1968, many Triumph models, and widely used from the early 1980s.
⚠️ If you remove the ballast resistor and run 12V full-time to a ballast coil, it will overheat and eventually fail.

How to test coil type :
Set you volt meter to OHM and read between the + and - terminals with no other wires connected

  • Resistance Check (Primary winding):
    • Ballast Coil: ~1.5 ohms or higher
    • Non-Ballast Coil: ~3 ohms (higher resistance)
    • Low resistance coil for factory electronic systems ~0.6 -1.1 ohms
  • Labeling: Some coils are marked “Use with Ballast Resistor” or “12V” (non-ballast).

 

How to test if your ignition has a hidded resistor:

1. Turn ignition ON.

2. Remove all wires from negative side of coil 

3. Manually ground the coil negative terminal (–) to earth . Use a short jumper wire to a clean chassis point.

This simulates the contact breaker points being closed.
It puts the coil under load, just like when the engine is running 

4. Now measure voltage at the coil + terminal (red probe on coil +, black probe to body earth).

~12V or near Battery voltage = Non-ballast system
~6–9V = Ballast system = ballast system

5. Remove the earth jumper from coil – after the test so you don’t overheat the coil.

 

Why this matters:

Without grounding the coil negative, the circuit is “open,” and the coil may show near battery voltage even on a ballast system.

By grounding it, you replicate real operating conditions and get the true voltage drop across the ballast resistor.