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Rotor-arm diagnosis begins with the path the spark is trying to take
The coil pulse enters the distributor cap through its centre tower, crosses a sprung carbon brush to the rotor, travels along the arm and arcs to an outer post. A fault anywhere in that series can leave the rotor visibly marked.
Good diagnosis separates the failed component from the electrical stress that caused its failure.
Ignition distribution sequence
| Stage | Component | Normal event | Possible loss |
|---|---|---|---|
| 1 | Coil and switching device | Magnetic field collapses to create high voltage. | Weak supply, poor saturation or internal coil fault. |
| 2 | Coil lead and centre tower | Pulse reaches distributor cap. | Open lead, corrosion or flashover. |
| 3 | Carbon centre brush | Spring contact feeds rotating hub. | Missing, worn or stuck brush. |
| 4 | Rotor conductor | Current reaches outer arm. | Open resistor, loose strip or punctured insulation. |
| 5 | Rotor-to-post gap | Arc selects the indexed cylinder post. | Wrong height/radius, erosion or bad phasing. |
| 6 | HT lead and plug | Spark crosses the plug gap under compression. | High resistance, fouling or excessive gap. |
Mechanical indexing
The drive feature fixes rotor angle as well as retention
The distributor is driven at camshaft speed, one revolution for every two crankshaft turns on a four-stroke engine. A flat, slot, moulded key or screw fixes the rotor's angular relationship to the shaft.
If it is not fully seated, its height and angle can both be wrong. Never reshape a key to make a near-match fit.
Rotor-to-cap geometry
At the firing instant, the arm must overlap the intended cap electrode with a designed air space. A broad contact accommodates mechanical and vacuum advance without touching the post. Too much distance raises required voltage; actual contact creates debris and timing scatter.
Use rotor and cap references intended for one distributor variant. Mixed components can appear normal until the engine is under load.
Rotor variants
| Variant | How to recognise it | Extra function | Selection risk |
|---|---|---|---|
| Plain conductive arm | Direct metal path. | Basic distribution. | May omit required suppression. |
| Suppressed rotor | Embedded resistor or marked value. | Reduces radio-frequency energy. | Wrong resistance weakens spark or increases interference. |
| Rev-limiter arm | Spring-loaded weight/contact assembly. | Interrupts delivery above calibrated speed. | Speed and rotation direction are specific. |
| Bolted carrier type | One or more small screws. | Positive location on a carrier. | Wrong screw length can strike internals. |
| Push-on rotor | Keyed moulded socket. | Tool-free service. | Cracks if pressed onto wrong profile. |
| Shielded/special-energy type | Additional moulding or electrode geometry. | Matches a particular cap/coil strategy. | Generic substitution changes clearance. |
Resistance is a design value, not simply “low is good”
A resistor can damp electromagnetic interference and limit peak current. Check end-to-end resistance against data at stable temperature with clean probe contact. Both an open circuit and an incorrect low-resistance substitute can be faults.
A multimeter applies only a small test voltage. It cannot prove that moulded insulation remains sound at ignition voltage.
High-voltage tracking
Moisture or dirt reduces surface insulation. Repeated discharge leaves a carbon trail, often a pencil-thin black mark from centre conductor towards the shaft. Once carbonised, cleaning cannot reliably restore dielectric strength.
Replace the affected rotor and cap as condition requires, then correct water, oil or excessive-voltage causes.
Interpreting physical evidence
| Evidence | Interpretation | Follow-up |
|---|---|---|
| Small uniform tip erosion | Expected ageing may be present. | Compare interval, gap and cap posts. |
| Deeply pitted/melted end | Abnormally high firing requirement or wrong geometry. | Plugs, leads, coil-to-cap path and part match. |
| Burn mark beneath rotor | Voltage escaping to distributor shaft. | Replace and inspect coil output/earth cleanliness. |
| Scrape around circumference | Rotor has touched cap. | Shaft play, end float, cap seating and height. |
| Crack at keyed socket | Forced fit, ageing or shaft irregularity. | Confirm distributor number and drive condition. |
| Loose electrode/rivet | Heat or bond failure. | Remove debris and replace before running. |
| Oil or green deposits | Contamination is providing a leakage path. | Find seal, ventilation or water entry. |
Symptoms under different conditions
A complete internal short can remove spark from every cylinder. A longer-than-designed air gap may allow idle but misfire when cylinder pressure raises voltage demand. Surface leakage often worsens in damp weather.
An engine-speed limiter creates a repeatable cut near its set threshold; fuel starvation can feel similar but needs different evidence.
First checks for a no-start
Confirm cranking speed and engine mechanical synchronisation. Use an approved spark tester on the coil output and then at a plug lead. Spark into the cap but none leaving it directs attention to brush, rotor, cap and grounding paths.
Do not position bare leads near fuel vapour. Disable injection if the diagnostic routine calls for extended cranking.
Load-related misfire investigation
Inspect plugs for excessive gap and leads for resistance, heat damage and routing. A rotor that passes an ohmmeter test can still flash over under pressure demand. Secondary oscilloscope patterns help identify high firing voltage or a common burn-time problem.
Use a capacitive or high-voltage probe intended for the ignition design, never an ordinary oscilloscope lead.
Rotor phasing checks
Phasing can be checked with a marked or purpose-made viewing cap and timing light where an approved procedure exists. Observe how the rotor aligns through advance range. Keep clear of rotating parts and contain the high voltage.
Incorrect phasing can follow distributor removal, trigger-plate modification, timing-belt work or a mismatched arm; turning the distributor may change base timing without solving the relationship correctly.
Distributor shaft and advance mechanism
Radial bush wear varies the post gap and trigger timing. Excess end float changes rotor height. Test movement to workshop limits, not by expecting zero perceptible motion on every design.
Mechanical weights, springs and vacuum advance must move without making the rotor contact the cap. Service only with suitable lubricants in specified locations.
Cap assessment alongside the rotor
Examine the centre brush length and spring action, outer electrodes, vents, clips, cracks and tracking. A burnt centre recess can starve the rotor even when the arm is new.
Do not scrape deep trenches or coat the inside with water-displacing spray. Replace a cap beyond service limits.
Identification before removal
Record engine and distributor stamped numbers; several distributors may have been fitted to one vehicle model. Photograph lead routing and cap orientation. Compare supplier dimensions and resistance rather than relying on plastic colour.
For a limiter, convert distributor speed to engine speed correctly and confirm rotation direction.
Removal procedure
Switch off, remove the key and allow hot exhaust parts to cool. Disconnect the battery only if service data requires it and stored settings are understood. Release the cap without pulling HT leads by their cables.
Lift a push-fit rotor along the shaft axis. For screw-fixed types, use a correctly fitting driver and prevent the screw falling into the distributor.
Installing without damage
Compare overall height, contact radius, keyed angle and any suppressor marking. Align the socket and press at its reinforced centre; force on the arm can crack it. Fit the original-specification screw and locking feature.
Turn the engine by hand only where safely authorised to confirm clearance. Refit the cap fully in its locating notch.
Firing order and timing
If leads were removed, route them from the correct number-one post in the specified rotation sequence. Crossing two leads can cause backfire and catalyst damage. Keep them separated according to the original clips to limit inductive crossfire.
Set base timing only in the required diagnostic condition, with vacuum lines or electronic advance handled as the manual states.
Safety and UK roadworthiness
Ignition pulses can injure and may interfere with implanted medical devices. Fuel vapour and a deliberate spark test are a dangerous combination. Keep guards, insulated tools and rated test equipment in place.
A severe misfire can overheat a catalytic converter and increase emissions. Warning-lamp and emissions consequences may be relevant to UK MOT inspection; road safety and catalyst protection warrant prompt repair regardless.
Common mistakes
- Selecting an arm without identifying the distributor manufacturer.
- Using shaft fit as the only compatibility check.
- Grinding the contact to remove pitting.
- Ignoring the cap's worn centre brush.
- Refitting a tracked rotor after cleaning the mark.
- Applying grease or general spray to dielectric surfaces.
- Dropping a retaining screw into the advance mechanism.
- Changing distributor position without a timing procedure.
Practical rotor-arm FAQs
Q: Why is a distributor rotor also called a rotor arm?
A: Both names describe the rotating conductor inside the cap.
Q: Should the arm touch each cap post?
A: No; ignition voltage normally crosses a designed gap.
Q: Can physical fit prove compatibility?
A: No; height, radius, phase, resistance and limiter design also matter.
Q: What does a black line beneath the rotor indicate?
A: It commonly indicates carbon tracking and high-voltage leakage.
Q: Can resistance testing prove the rotor is good?
A: It finds some faults but cannot reproduce operating voltage stress.
Q: Why does misfire appear mainly under load?
A: Higher cylinder pressure demands more voltage and exposes weak insulation or excess gaps.
Q: What is rotor phasing?
A: It is rotor-tip alignment with the intended cap post when the spark occurs.
Q: Can a rotor arm limit engine speed?
A: Some contain a calibrated centrifugal cut-out.
Q: Should the tip be filed?
A: No; replace a worn part and diagnose abnormal electrical stress.
Q: Can distributor-shaft play damage a new rotor?
A: Yes, movement can vary clearance or cause cap contact.
Q: Does rotor replacement require timing adjustment?
A: Not if the distributor position and trigger are undisturbed.
Q: Is pulling a live lead a safe test?
A: No; use an enclosed rated spark tester.
Q: What completes the repair check?
A: Correct fit, cap/lead order, stable spark and fault-free operation under load.
