This is the HOW TO that I wish I could have found the first time I went to test an ignition coil. Note that there are lots of different types of ignition coils. Some methods of testing may differ for different types of ignition coils. This will cover the testing specifically the types of ignition coils found on XJs. As the name implies, ignition coils use coils of wires to take a relatively low input voltage (12V in our case) and create a high enough voltage to cause a spark to jump a gap (spark plug) at just the right time. In a sense our ignition coils are a special case of a transformer. When two coils of wire are in close proximity to each other and a current is run through one of the coils, the dark magic of electromagnetic induction will create a current in the other coil. This same basic concept is what makes wireless charging work on your phone. The actual coils of wires are embedded in the ignition coil unit itself, but if you cut it up you would see them. When we measure the ignition coils, what we are really doing is trying to determine the condition of those wire coils embedded inside the ignition coil unit. If there is something wrong on the inside, we should see resistance measurements that are out of spec. Things that could go wrong could include a short circuit in one of the coils, or a broken wire in one of the coils. Enough chit-chat, let's just test the ignition coils. Here are today's test subjects. This is a set of ignition coils that I acquired in a box of goodies taken from an XJ650 Maxim. As you can see, one of the plug caps is already missing. I tested these when I got them, and one of the caps tested bad - which is why I suspect these ended up in a parts box in the first place - and I threw it away already. Here is what we will use to test the coils. These are digital multi-meters (DMM). Neither one of these is overly expensive. We will be measuring Ohms (resistance), symbolized by the Greek letter capital Omega (Ω). The one on the left has only one Ohm setting, meaning with will automatically adjust depending upon whether it is reading a small number of Ohms, or thousands of Ohms. The one on the right requires the user to set the range depending upon the expected amount of Ohms. You will need to understand your specific DMM to properly make the measurements. We are going to be making measurements of just a few Ohms all the way up to tens of thousands of Ohms. The next thing we need to know is what the specs actually are. A good online reference for XJs is http://xj4ever.com/catalog/d-6-ignition.html), but of course service manuals should have this info too. Like all transformer-ish things, our ignition coils have a primary coil (the input) and a secondary coil (the output, in our case spark plug wires). The primary side of the coil are the wires with the connectors that come from the TCI unit. This picture shows how I am going to put the probes into the connector in order to test the primary coil. For the XJ650, the specs say that when we stick these probes in we should measure 2.5 Ohms plus or minus 10%, so the acceptable range is 2.25-2.75 Ohms. It was really hard to hold the probes in the the connector with one hand and get a photo with the other hand. I was able to hold the probes with one hand, but you can't see the connector. The good news is that the measurement is 2.6 Ohms, so we are within spec! Now on to the secondary coil. This picture shows how to measure - one probe in one plug wire, one probe in the other. It's hard to see, but the meter is indicating a "K", which means "kilo" (x 1,000) Ohms. So the reading is actually 12,460 Ohms. More good news, the XJ650 spec for the secondary coil is 11,000 Ohms plus or minus 20% (8,800-13,200 Ohms), so again, this one is within spec. Now on to the plug caps. This picture shows how to measure the plug caps - one probe in one end, one in the other. This cap is measuring 4,920 Ohms, which is within the spec of 5,000 Ohms plus or minus 20%. When you install one of the plug caps, the resistance should be the sum of the secondary coil plus the cap. Here we have 12,460 Ohms + 4,920 Ohms ~= 17,450 Ohms. In this picture, you can see that total resistance in the secondary side with both 5K Ohm plug caps installed should be the sum of the resistance of the output coil plus the resistance of the two 5K Ohm caps. In our case, the math is roughly: 12,460 Ohms + 4,920 Ohms + 5,000 Ohms ~= 22,700 Ohms. Note that originally these bikes used non-resistor (zero Ohms) spark plugs, so fully installed, the total resistance in the secondary coil would be 22,700 Ohms. If we used non-resistor (zero Ohms) caps and resistor plugs (5K Ohms), we would end up with the same net total resistance on the secondary side.