Polini Thor electrical wiring
by Had Robinson
updated December 1, 2019
If the wiring is undisturbed, it will rarely be an issue. However, the connectors used by Miniplane, Polini, and other Italian manufacturers are fragile and may fail. They have to be examined carefully if there are problems.
Below is a photo of a coil ground (black) wire which had a failed connection between the ring terminal and the wire itself. The insulation of the wire is still connected to the terminal but that is all. Engine vibration caused the copper wiring to fatigue and break right at the connector. It worked for quite a while until the space between the broken wires became sufficiently wide that the voltage could not jump the gap and the ignition failed. It was easy to repair with a quality terminal.
Polini uses the IDM ignition system which also has a 70 Watt alternator built-in. Below is a photo of a new style connector and wires coming out of the front of a typical Thor engine. The black wire is from the magneto and connects to the kill switch and as well as to blue wire from the coil. The connector plugs into any 12V motorcycle/lawn tractor voltage regulator e.g. the regulator used on Kohler garden tractor engines. The blue wire is ground. The white wire is the charging wire from the alternator. NEVER CONNECT THIS WIRE DIRECTLY TO A BATTERY! This wire must be connected to a regulator first. The red wire (no connection here) connects to the regulator via the connector and would be connected to a 12V battery via the ring terminal. If the charging system is not being used, the red wire should be connected to ground so the wire does not flop around. Honda small engines have a similar charging system which can be used to troubleshoot and setup the Thor system (including some others).
The ignition consists of the coil (usually located just above the carburetor) and the magneto (located inside the front of the engine and is not visible). The magneto supplies a +70VDC pulse to the blue primary wire on the coil at just the right moment (a little before top dead center of the piston). The pulse is greatly amplified by the coil and becomes +15,000V at the spark plug that ignites the fuel/air mixture inside the cylinder.
The magneto and charging system coils on the front of the engine are very durable and rarely need attention. It is the wiring from the magneto to the coil OR from the coil connections to ground OR the kill switch circuit that cause problems.
Periodically examine the wiring and be certain that each connection is solid and that there are no broken or loose wires/connectors. Be certain the male and female connectors are fully pushed together. If the engine suddenly quits, this is the place to start.
Here is the factory wiring schematic for the Thor engines. The white wire in the schematic photo is not labeled and very hard to see but it is there. The charging system requires the addition of an ordinary motorcycle voltage regulator to charge an onboard 12V battery. The new style regulator is commonly found on lawn tractor engines world wide. What is important are the connections of the ignition coil (the magneto wire in the above photo). If any of these are not connected properly, the ignition coil will not function and there will be no spark.
Please refer to the schematic above regarding the following information:
1. The black secondary wire with the spark plug boot must be firmly connected to the spark plug. The other end must have a secure connection to the coil. Ignition coil secondary resistance is 9K Ohms ±10% (spark plug terminal to engine ground) Can be 5K Ohms on newer models.
2. The black wire labeled "A" must be grounded to the engine. The coil will not work if this wire is loose. In fact, pilots may notice a shock through the metal parts of the throttle cable including the kill switch if they attempt to start the engine with this wire unconnected. This wire is the ground wire for the coil.
3. The blue wire labeled "B" is the primary wire of the coil and is connected to the magneto on the front of the engine and to the kill switch. The wire labeled "C" (it can be any color) is the one connected to the kill switch. To measure the magneto resistance, disconnect connectors "B" and "C" and measure the value from either female connector to ground. The value should 290 Ohms. The male connector "B" goes to the ignition coil primary. The resistance (all models except the 190) should be 1K+ Ohms (blue wire to engine ground). The coil has electronics in it which makes this value difficult to measure so any value greater than 1K Ohms is OK (except an infinite value). If you get no value switch the Ohmmeter leads. It is rare that there is a problem with the coil itself. Breaks in the wires going to the coil are the most common ignition problem.
4. The "D" (ground), "E" (battery) and the white wire (alternator) connections are for the optional voltage regulator and are not used unless there is a battery.