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The module turns coil primary current on and off
Current flowing through the coil primary builds magnetic energy. At the commanded ignition point, the module interrupts it rapidly and the collapsing field induces high voltage in the secondary winding.
Switching speed, peak current and dwell determine stored energy and heat. The module must match both coil and controller strategy.
Ignition architectures
| System | Module location | Trigger source | Service distinction |
|---|---|---|---|
| Distributor electronic ignition | Distributor body or remote heat sink. | Hall, inductive or optical pickup. | Module may control dwell internally. |
| Remote multi-channel igniter | Engine bay bracket. | ECU logic commands. | One module switches several coils. |
| Coil pack with module | Integrated in coil assembly. | ECU or crank signal. | May require complete pack. |
| Smart coil-on-plug | Power transistor inside each coil. | Low-current ECU command. | No separate igniter. |
| ECU-switched coil | Driver inside engine controller. | Controller logic. | External “module” may not exist. |
Dwell builds magnetic energy
Longer charging is not always a stronger spark
Dwell is the time or crank angle for which primary current flows. It must be long enough to reach useful current but short enough to avoid saturation and heat.
Battery voltage, coil inductance and engine speed affect required dwell. Some modules calculate it; others follow an ECU command.
Current limiting
Power transistors may regulate primary current once a threshold is reached. This protects both coil and switch during low-speed or extended key-on conditions.
A module designed for a high-resistance coil can overload with a low-resistance coil. Never add or remove a ballast resistor without the wiring specification.
Inductive and Hall pickup signals
| Sensor type | Signal form | Module role | Testing concern |
|---|---|---|---|
| Variable reluctance | AC waveform whose amplitude rises with speed. | Shapes threshold and timing edge. | Polarity and air gap. |
| Hall-effect | Switched digital voltage/current. | Supplies or interprets logic. | Power, pull-up and channel type. |
| Optical | Digital slots interrupt light. | Conditions clean trigger. | Contamination and supply. |
| ECU logic command | Square pulse defining dwell/firing. | Power amplification only. | Probe without loading driver. |
Heat generation and mounting
The transistor dissipates energy during switching and current limiting. A metal base transfers heat to a distributor or dedicated aluminium bracket.
Corrosion, thick old paste, a bent plate or missing screws raises thermal resistance. The module may work cold and fail after heat soak.
Thermal compound
Where specified, compound fills microscopic air gaps; it is not adhesive and more is not better. Use the exact electrically insulating or conductive product called for.
Some modern modules use a dry pad or sealed mounting and prohibit paste. Follow component instructions rather than habit.
Earth paths
A module can earth through its connector, mounting plate or both. Paint or anodising beneath an intended earth creates voltage drop under primary current.
Measure ground voltage while switching, not only continuity with the ignition off. Restore corrosion protection around, but not through, the defined contact.
Failure symptoms and alternatives
| Symptom | Module possibility | Alternative | Diagnostic priority |
|---|---|---|---|
| No spark on all channels | No switching or module supply. | Crank signal, ECU permission, main relay. | Check trigger and power first. |
| One dead channel | Failed multi-channel driver. | Coil, plug, lead or wire. | Compare waveforms. |
| Hot cut-out | Thermal transistor failure. | Crank sensor, relay or coil. | Capture during fault. |
| Coil overheats | Driver held on/excess dwell. | Wrong coil or key-on time. | Stop power promptly. |
| Weak high-load spark | Limited primary current. | Plug gap, coil, voltage or mixture. | Primary/secondary testing. |
| No tachometer | Lost switching output possible. | Instrument or network fault. | Trace signal source. |
Power-supply testing
Measure module and coil voltage during cranking and when hot. A high-resistance relay, ignition switch or connector can pass unloaded voltage but collapse under current.
Use voltage-drop tests across positive and earth paths. Never substitute a larger fuse to overcome a drop.
Cranking synchronisation
Scan-tool rpm and synchronisation flags help, but an oscilloscope shows missing or distorted pickup edges. Compare crank and cam signals with ignition command.
An immobiliser may allow rpm data yet inhibit coil drive. Check authorisation and injection together.
Primary-current waveform
A current clamp shows ramp slope, peak current, limiting plateau and collapse. A shallow ramp can indicate low supply or high primary resistance; excessive peak suggests wrong coil or failed limiting.
Clamp orientation and bandwidth matter. Compare channels under the same battery voltage and temperature.
Primary-voltage waveform
Use a differential or attenuating probe rated for inductive spikes. The pattern can show switch dwell and coil oscillation.
A standard meter or oscilloscope input connected directly may be damaged. Observe equipment grounding rules to avoid shorting the circuit.
Secondary ignition condition
Excessive plug gap, open lead or lean cylinder raises firing voltage and stresses coil/module insulation. A fouled plug lowers voltage but can lengthen burn demand.
Repair secondary causes before fitting a driver. Catalyst damage can follow prolonged misfire.
Coil testing
| Check | What it can show | Limitation |
|---|---|---|
| Primary resistance | Open or gross short where specified. | Very low values need lead compensation. |
| Secondary resistance | Open winding on applicable coils. | Smart coils may not permit test. |
| Current ramp | Inductance and saturation behaviour. | Needs known module dwell. |
| Output stress test | Ability to jump a calibrated gap. | Must use enclosed approved tester. |
| Channel substitution | Whether fault follows coil. | Only if parts and coding are interchangeable. |
Part identification
Use VIN, engine code, distributor or ignition-system number and full module markings. Production revisions can change dwell control without changing connector shape.
Count channels and verify each pin function from the wiring diagram. Do not rely on wire colour across model years.
Connector and harness condition
Inspect terminal tension, corrosion, heat darkening and insulation hardened by engine temperature. Primary circuits need firm low-resistance contacts.
Use repair terminals and crimp tools specified for current and sealing. Twisted wires or household connectors introduce resistance and electromagnetic noise.
Safe removal
Switch off, remove key and wait for controller shutdown. Disconnect the battery if the procedure requires it, preserving data only by an approved method.
Keep fuel systems closed and high-tension components discharged. Label similar coil connectors before release.
Mount preparation
Remove old compound without gouging either face and check bracket flatness. Clean intended earth contact while protecting surrounding metal against corrosion.
A warped mounting base cannot be pulled flat by the module screws without stressing the electronics.
Installation controls
| Stage | Action | Failure prevented |
|---|---|---|
| Confirm part | Match system number and channel strategy. | Wrong dwell/current. |
| Prepare interface | Flat, clean and correct thermal material. | Hot failure. |
| Position module | Seat without sliding contamination into paste. | Air gaps and bent pins. |
| Tighten | Even sequence and specified torque. | Cracked housing or poor heat contact. |
| Connect | Lock terminals and restore shielding. | Intermittent trigger. |
| Route | Clear exhaust, ignition leads and moving parts. | Heat/EMI/abrasion. |
Commissioning
Verify power and earth before cranking, then observe synchronisation and primary current. The engine should start without prolonged raw-fuel cranking.
Warm to the condition that previously caused failure and monitor dwell, module temperature trend and misfire data. Check for stored codes after a controlled road test.
Common mistakes
Errors include replacing the module after finding no spark, fitting a mismatched coil, omitting heat-sink compound, painting an earth face and probing primary spikes with unsuitable equipment.
Another is leaving the ignition on with a system that holds coil current, overheating both components before first start.
UK safety and emissions context
Misfire can overheat a catalyst and raise emissions. Do not drive with a flashing engine warning or raw-fuel smell.
Failed electronic modules and coils should enter suitable electrical recycling. Protect sharp terminals and residual contamination.
Practical ignition-module FAQs
Q: Does every petrol engine have a separate module?
A: No. It may be inside the coil or engine controller.
Q: Can connector shape identify the part?
A: No. Channel, signal and current strategy must match.
Q: Does no spark prove the module failed?
A: Check supply, trigger, authorisation, coil and wiring first.
Q: Why can failure happen only when hot?
A: Semiconductor or sensor behaviour can change with temperature.
Q: Is thermal compound always required?
A: Use only the interface material specified for that design.
Q: Can extra compound improve cooling?
A: A thick layer can insulate; apply the stated amount.
Q: Why test primary current?
A: Its ramp reveals coil loading, dwell and limiting behaviour.
Q: May a spark be tested against the engine by hand?
A: Use an enclosed calibrated tester and safe fuel controls.
Q: Can a bad plug damage the module?
A: Secondary faults can increase coil and driver stress.
Q: Is continuity enough for the module earth?
A: No. Measure voltage drop while current is switching.
Q: Can ballast resistance be bypassed?
A: Only if the exact ignition design specifies no ballast.
Q: Why preserve the mounting bracket?
A: It may be the heat sink and electrical earth.
Q: What verifies the repair?
A: Correct waveforms, stable hot operation and no recurrent misfire.