The last few days have seen a number of posts where the Ignition coil needed testing, at least for resistance.
On the 360º engines (Bonneville and T100 models) the ignition coil secondary or HT winding is double-ended and isolated from ground. This means that without dismantling anything important we can measure the resistance and continuity of the 3 main items in the HT circuit:
1.- The HT (High Tension) side of the coil
or secondary winding. This being the thinnest wire within the coil, most likely to break or get damaged, punctured or burned insulation as it's the most highly stressed part due to the high voltages involved, (>20Kilo volts).
2.-The plug caps
which contain 5000 Ohm resistors that can fail sometimes. These resistors are fitted for radio interference suppresion.
3.-The HT cables
that on our bikes are made with copper conductor cores and not likely to break, but we can check them just the same as the connections at both ends can come adrift from the coil terminals or the plug cap terminals.
Once again we need a digital multimeter obtainable for a few dollars in lots of places. Set the range switch to a suitable resistance (Ohms), 50K, 100k, 200k, etc and insert the probes into the disconnected plug caps. Ensure the metal part of the probe can reach the internal contact. Luckily the waterproofing rubber fitted to the plug cap often serves to hold the probe in position leaving both hands free. If this is not so you'll have to hold the probes in there while performing the test.
Here's a diagram showing how the 360º twins HT circuit is arranged, and showing the connections of the Ohmmeter:
The reading on the meter should come to a total of around 18 to 20 Kilo Ohms for carburetted models and some 30-32 Kilo Ohms for EFI models.
If an open circuit is detected (meter reading infinite resistance), sometimes symbolised on the display with a "1", first of all check the plug caps are attached properly to their cables.
The connection consists of a kind of self-tapping screw fixed where the cable enters the cap and sometimes the cable can become "unscrewed" from it. Hold the cable firmly and twist the cap to "screw" it back into the HT cable.
If OK go on to the coil posts where the HT cables are attached. This connection consists of a sort of spike inside shrouded coil terminals. The cable is fitted with a pressure gland and plastic nut that forces the cable onto the spike.
If the previous items are OK then it's safe to assume the coil's secondary or HT winding is broken internally.
If you get a suspiciously low resistance reading the chances are that the internal insulation of the coil winding has broken down leading to an internal short circuit.
The resistance of the coil secondary only, should be in the region of 8-9K for the carburetted coils and around 20-22k for the EFI ones.