OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Chainsaws and Concrete Cutters McCulloch Chainsaws McCullough chain saw won't start

Remember there is a ground (separate wire, or through the frame of the module) that connects to the engine frame, and also a kill wire that goes to the stop switch. If you *don't* ground the ground it won't fire, and if you *do* ground the kill wire it won't fire.

If there is a ground wire and it has become detached, you have to reconnect it properly. I haven't seen how it is constructed, so I can't guess whether it can be repaired.

I think the wire was connected ok but I suspect that as I was re-assembling the unit (because everything it so cramped) the earth wire has stretched a little and so was only hanging onto by a mere thread. Just hoping that we can re-attach/solder?

It is just a ground wire, so either soldering or clamping to the metal body of the module will probably work.

Thanks grumpy will give that a go.

The ground wire broke clean off from the module so soldering was going to be difficult. I managed to scratch some of the rubber(?)casing away from where the ground wire was (on the module) and even though there wasn't much wire to work with I managed to solder the broken ground wire on. It's not on very securely but there is continuity. I re-assembled part of the chainsaw and got a spark.

I'm just trying to understand this but why do we need the ground wire to get a spark? Does it complete the circuit?

Both the primary and secondary ignition circuits rely on working against ground. Here is a circuit diagram for a standard Kettering ignition system:


In the case of a magneto such as yours, the magnet in the flywheel passing the coil in the module generates the voltage shown as 12 V in the diagram. In an electronic magneto, instead of having the breaker points shown, a simple transistor circuit acts as a switch, opening the primary circuit when the magnet passes the module. So, a fairly low voltage appears across the left or primary side of the coil, and then the bottom of the primary is suddenly disconnected from ground by the transistor circuit. The sudden collapse of the current flowing in the primary causes a magnetic pulse in the laminated steel core or yoke of the module (the part that has to be close to the flywheel magnet) and that magnetic pulse causes a momentary very high voltage to appear in the secondary winding of the coil (the right side). That voltage will be around 15,000 volts. Because the bottom of the secondary is grounded, and so is the steel body of the spark plug, current flows through the high tension lead to the spark plug, jumps the spark plug gap, and returns to the coil through its ground connection. So, neither the primary nor the secondary circuit can work without a secure ground connection.

Thanks Grumpy, that makes sense.