How to Build
CDI and Magneto Coil Tester
by Starfire

The tester can be constructed in a plastic or wooden box. It tests  any CDI under actual working conditions, and will dynamically test the high voltage magneto coils, giving a visual indication of their condition...good or bad. It requires few parts and is very easily assembled. The tester has been used on the H2, KH400, H1 late and H1 early CDI units. In the case of the early H1's, this tester will replace the "B" box and test the "A" box thoroughly.

Click here for diagram
Click here for diagram 2

How it works:

The mains, for safety is fed into a small mains transformer to provide isolation. For the low powered 110 volt nations, 110 volt to 110 volt transformer is used. A smallish 1/2 to 1 amp transformer is fine. The high powered nations with their full allocation of volts will require a 240 to 110 volt job.

The High voltage AC output is fed into the rectifier box on the bike, as with the H2, or direct into the CDI as in the case of the KH400/H1E etc. A second output is made to the AC side of the rectifier to further test Magneto coils in situ, the small incandescent light bulb as used in a fridge or similar is used as an indicator.
A third AC voltage 180 degrees out of phase with the High voltage output is used to trigger the CDI at the mains frequency of 50/60 cycles. On a triple, this is the equivalent of 1 cylinder firing at 3600 rpms or so. it his is handy to have the CDI  left running while being externally heated or cooled to diagnose a suspected heat related fault.

How to use:

To test a CDI, the CDI  Magneto coil inputs and trigger inputs should be disconnected from the harness. In the case of the H2 for example, this will be the Green and white from the mag coils, and the three whites from the magneto. All other wires should be left in place. Switch on the ignition. The RED (high voltage ) wire from the tester should be connected to the WHITE from  the CDI rectifier box. The BLACK wire from the tester should be securely connected to the engine. The chosen trigger wire (one of the three whites from the CDI) can be connected to the WHITE from the tester, each one in turn, to trigger each cylinder. The WHITE  wire from the tester is safe to touch as it is a small 2.8 volt peak to peak signal..... just sufficient to trigger the CDI. Before testing, remove the sparkplugs from the engine to prevent spontaneous starting, lay these carefully on the cylinder heads so the spark gaps are visible. DO NOT run these tests without plugs!!!!
With the tester switched on, the chosen plug should fire without missing, giving an even buzz. This also tests any suspect ignition coil.

If the trigger coils are suspect, reconnect the three trigger wires only back to the harness, and kick the bike. With the tester on, the plugs should spark in sequence. This tests the trigger /signal/pickup coils.

A second way to test the Low speed coil in situ, or a unknown spare, again using the H2 as an example, the tester should be connected as follows:
The tester GREEN is connected securely to the engine, or the stator casing if testing a spare off the engine, or the BROWN in the harness if testing off the bike. The tester RED is connected to the WHITE in the harness, this is the Low speed coil. Disconnect the harness WHITE from the rectifier box first if on bike testing. With the tester switched on, the light bulb will tell the condition of the Low speed coil. If very bright, the Low speed coil has shorted turns, or a short circuit in the harness. If the bulb remains dark, the Low speed coil has an open winding or a bad joint in the harness. If the bulb glows dimly, the Low speed coil is ok.

Always SWITCH OFF the tester before connecting/disconnecting wires. DO NOT fiddle with any wires with the tester switched on. When used with care, this tester can save much time and is safe to use.