Specialist Guide to Ignition Coils

Chainsaws and Brushcutters: Theory, Diagnostics, and Spare Parts

Welcome to the world of small two-stroke engine mechanics. The ignition coil is the "beating heart" of these machines: it has the titanic task of stepping up the input voltage to a level (thousands of volts) high enough to allow the current to "jump" the gap between the spark plug electrodes, creating the spark.

1. Internal Structure and High-Voltage Physics
To understand why a coil fails, we must analyze how it is built. It is all based on Faraday-Lenz's Law (Electromagnetic Induction).

Composition: The exterior is made of insulating material. Internally, the system is a step-up autotransformer consisting of two windings:

Primary Circuit: A few hundred turns of thick copper wire where energy accumulates.

Secondary Circuit: Thousands of turns of ultra-fine wire that elevate the voltage.

Insulation: They can be oil-bathed (outdated technology) or, more commonly, sealed in epoxy resin, which protects the wires from extreme vibrations.

Connections: They feature two essential terminals: Positive (power supply) and Ground (to close the circuit).

Construction Types: Focus on "Closed Secondary"

In professional equipment, the most common type is the closed secondary (or Ohmic-inductive). The energy leaves via the high-voltage lead, discharges through the spark plug, travels through the engine block, and returns to ground. For these coils, the use of inductive ignition cables is mandatory.

2. Technological Evolution
Mechanical Coils (Points): Now considered antiques. A mechanical breaker opened the circuit to generate the spark.

Electronic Coils (Transistorized): The current standard. An integrated electronic module determines the exact ignition timing (phase), ensuring reliability and zero mechanical wear.

3. Malfunction Symptoms and Diagnostics
Testing a coil requires precision to avoid replacing a functioning part (costs range from €30 to €100).

Warning Signs:

The "Heat Problem": The machine starts fine but stalls after 10 minutes and won't restart until it cools down (micro-fractures in the windings expanding with heat).

Weak Spark: It must be blue/purple. If it is yellow or orange, the coil is failing.

Misfiring at High RPM: The module can no longer manage the frequency of discharges.

Step-by-Step Testing Guide:

Tools: Multimeter (set to Ω), Feelers gauge, Spark plug wrench.

Visual Inspection: Check that the cable isn't frayed and the spark plug boot makes solid contact. Disconnect the Kill Switch (stop wire): if the engine starts, the problem is the switch, not the coil.

Primary Circuit Test: Measure between the stop wire connector and the metal core (ground). Expected value: 0.5 – 2.0 Ω.

Secondary Circuit Test: Measure between the inside of the spark plug boot and the metal core. Expected value: 2.0 – 8.0 kΩ.

Air Gap Adjustment: The distance between the coil poles and the flywheel magnets should be 0.3 – 0.4 mm.

4. Where to Buy: Bricoutensili.com
Once the fault is confirmed, the next step is choosing the right replacement on bricoutensili.com.

Why choose Bricoutensili?

Compatibility: A vast catalog for brands such as Stihl, Husqvarna, Oleo-Mac, and Efco.

Quality: Materials resistant to high temperatures to prevent premature failure.

Support: Technical assistance to identify the correct OEM code.

How to identify the correct coil:

Note the Make and Model and look for the OEM Code (often embossed on the plastic, e.g., 1130 400 1302). Always visually compare the mountings and cable position with the photos on the website.

5. Installation Pro-Tips
When you receive your spare part from bricoutensili.com:

Clean the Contacts: Thoroughly clean the area on the cylinder where the coil grounds.

Wiring: Ensure the stop wire is positioned correctly so it isn't pinched or severed by vibrations.

Extra Maintenance: It is good practice to replace the spark plug along with the coil for a 100% efficient ignition system.