How do I test an outboard ignition coil with a multimeter?
My outboard has started misfiring under load, and one cylinder seems to have a weak spark. I have a digital multimeter: where should I put the probes, and what readings would show whether the ignition coil needs replacing?
Answer
Test an outboard ignition coil by disconnecting it and comparing the resistance of its primary and secondary windings with the workshop manual's specifications. A multimeter can reveal some open or shorted windings, but a coil that passes these checks can still fail when hot or under load.
Identify the coil and connections
Use the engine's model code and serial number to find the correct ignition test specifications and terminal diagram. Yamaha, Mercury, Suzuki, Honda and Evinrude/Johnson used different ignition systems across their ranges and model years. There is no universal resistance figure or probe arrangement.
Inspect the spark plug, plug cap, high-tension lead, coil casing and earth connection before measuring. Loose terminals, green corrosion, cracked insulation and black tracking marks can explain a misfire without a failed winding. A poor earth can also stop a sound coil working.
Before unplugging wiring, switch off the ignition, remove the kill-switch lanyard and disconnect the battery's negative terminal where fitted. Keep sparks away from petrol vapour and never handle ignition leads while cranking.
Establish whether you have a conventional separate coil, a twin-output coil or an electronic coil with an integrated igniter. Resistance testing may be unsuitable for an integrated unit unless its manual gives a procedure.
Measure the primary winding
- Photograph or label the wiring, then disconnect the coil's low-voltage connections to isolate it from the surrounding circuit.
- Plug the meter leads into COM and V/Ω, select resistance and choose the lowest suitable range, commonly 200 Ω on a manual-ranging meter.
- Touch the probes together and note their resistance. Use the REL function to cancel it, or subtract it from the coil reading.
- Place the probes across the primary terminals specified in the manual. Some coils have two terminals; others use one terminal and a designated earth connection or metal core.
- Hold the probes firmly on clean metal and record the stable reading.
Many conventional coils have a primary resistance of around 0.2 to 2 Ω, but some outboard coils fall outside that range. This is only an illustration. The model-specific specification determines whether yours passes.
Lead resistance matters. If the probes measure 0.3 Ω together and the coil measures 0.7 Ω, the corrected reading is 0.4 Ω. An ordinary multimeter may not measure extremely low resistance accurately enough to judge the coil.