Contact Breaker

Points control energy stored in the ignition coil

With the contacts closed, battery current builds a magnetic field in the coil primary winding. At the firing point, the distributor cam opens them. The rapid field collapse induces high voltage in the secondary winding, which reaches the appropriate plug through the rotor and cap.

The system is simple but dynamic. Coil current, contact opening speed, dwell, condenser value and distributor accuracy all influence spark energy and timing.

Components in a conventional ignition system

Dwell angle is more informative than static gap

Set dwell before setting timing

Dwell is the distributor rotation during which the points remain closed. Increasing contact gap reduces dwell; decreasing gap increases it. A feeler gauge establishes a workable starting point with the rubbing heel on a cam peak, but a dwell meter measures the system while operating.

Changing dwell changes the instant the points open, so it changes ignition timing. Complete gap or dwell adjustment first, then set timing.

Why the condenser matters

When the contacts begin to open, the coil primary tries to maintain current and would arc strongly across the small gap. The condenser temporarily accepts charge, reducing arcing and forming an oscillating circuit with the coil. Its capacitance must suit the system.

A shorted condenser can prevent primary interruption and produce no spark. An open or poorly earthed condenser allows severe arcing. Metal transfer from one contact face to the other can provide clues, but correct diagnosis includes wiring and coil checks.

Identify the distributor, not just the car

Ballast-resisted and unballasted coils

Some systems use a lower-resistance coil with a series ballast during normal running, then bypass it for a stronger spark while cranking. Others supply a coil designed for full system voltage. Mixing them can overheat points and coil or reduce spark energy.

Measure primary resistance with suitable equipment and compare the complete circuit with wiring data. Coil labels are useful but not conclusive after decades of replacement and rewiring.

Contact condition provides diagnostic evidence

Symptoms and diagnostic priorities

A failing low-tension circuit can cause cranking without firing, intermittent cut-out, misfire at high rpm or weak spark under cylinder pressure. Heat may worsen a coil or condenser fault. Fuel starvation and carburettor problems can feel similar, so establish whether spark disappears when the symptom occurs.

Use an appropriate spark tester rather than holding a lead. Verify battery cranking voltage, coil positive feed, primary switching and a sound distributor earth. High-voltage ignition can cause injury and damage electronic test equipment.

Distributor shaft and base-plate checks

Radial shaft movement changes the point gap as the cam turns, producing timing scatter that fresh points cannot cure. Check all lobes and compare dwell stability. Axial movement can also shift timing on helical-drive distributors.

Vacuum advance rotates a movable base plate. Its braided earth lead must remain flexible and conductive throughout travel. A fractured lead may fail only when manifold vacuum moves the plate.

Safe removal and installation

The insulating washers are functional parts

The coil-negative conductor and moving contact must be insulated from the distributor body until the points close. A shoulder bush and washers often isolate the terminal post. Assemble one washer on the wrong side and the circuit may remain earthed continuously.

With the points open and ignition off, continuity testing can confirm isolation. Rotate or move the vacuum plate while testing to expose a chafed flexible lead.

Initial gap setting

Rotate the engine in its normal direction until the rubbing heel rests at the top of a cam lobe. Use a clean feeler gauge of the specified thickness and adjust until it slides with the stated light drag. Tighten the screw while preventing the plate from moving, then recheck.

Do not file a new contact set. On used points, a feeler gauge spanning a pit and peak can misrepresent effective separation, which is another reason to check dwell.

Running dwell and timing

Connect instruments with correct polarity and away from belts and fans. Compare dwell at idle and raised speed; large variation indicates mechanical instability. Once dwell is correct, set base timing with vacuum connections handled exactly as service data specifies.

Confirm centrifugal advance progresses with rpm and returns freely. Test vacuum advance with a controlled vacuum source and verify the diaphragm holds. Excess advance can cause knock and engine damage; retarded timing creates heat and poor response.

Starting current and coil temperature

Leaving the ignition switched on with closed points holds continuous coil current. The coil and contacts can overheat, and some coils may leak or rupture. Do not leave the system energised during adjustment.

After commissioning, check that the coil is appropriate and not becoming abnormally hot. Measure voltage on both sides of any ballast during running and cranking rather than bypassing it by assumption.

Electronic conversion considerations

A contactless trigger can reduce routine dwell wear, but it must match distributor, coil resistance, supply voltage and polarity. It does not repair shaft play, seized advance or a damaged cap. Retain clear wiring records and any original components needed for diagnosis.

Some electronic modules need full supply while the coil remains ballasted. Follow the conversion instructions exactly; connecting both to whichever terminal is convenient can make the module unreliable.

Maintenance and roadside diagnosis

At specified service intervals, inspect gap/dwell, faces, heel, distributor cam, cap, rotor and advance. A small clean spare set and correct tools can be useful on an older vehicle, but roadside work must be away from traffic with the ignition isolated.

Do not repeatedly sand burnt points without correcting the condenser or current fault. Abrasive debris left between contacts prevents reliable low-resistance closure.

MOT, emissions and roadworthiness

Ignition misfire can raise exhaust emissions, overheat a catalytic converter where fitted and make an engine stall in traffic. Secure wiring and correct timing are therefore more than refinement issues. A vehicle must meet the applicable emissions and roadworthiness requirements for its age and class.

If fuel vapour is present, stop before creating sparks. Repair leaks and ventilate safely before ignition testing.

Practical contact-breaker FAQs

Q: Are contact breakers the same as points?
A: Yes. Both terms describe the mechanical primary-current switch.

Q: Should gap or timing be set first?
A: Set gap and dwell first, because they change the firing point.

Q: Why do new points need a condenser check?
A: A faulty or mismatched condenser can rapidly burn the new contacts.

Q: Can points be selected by registration alone?
A: Distributor number is safer because several units may fit one model.

Q: What closes the points?
A: Their spring closes them after each distributor-cam lobe passes.

Q: Why does the gap become smaller?
A: Rubbing-heel wear, loose adjustment or cam/shaft faults can close it.

Q: Can oil be put on the contacts?
A: No. Only a tiny specified lubricant belongs on the cam/heel interface.

Q: What causes timing scatter?
A: Shaft play, worn cam, loose base plate or unstable point opening.

Q: Does a spark in open air prove ignition is healthy?
A: No. Cylinder pressure demands more voltage, so weak systems can still misfire.

Q: Can a ballast resistor be removed?
A: Not unless the complete coil system is correctly redesigned.

Q: Why does the coil overheat with ignition left on?
A: Closed points allow continuous primary current without normal switching.

Q: Will electronic ignition cure distributor wear?
A: No. Mechanical play and advance faults remain.

Q: Is high-voltage ignition testing dangerous?
A: Yes. Use an insulated purpose-designed tester and safe procedures.