Don’t make the mistake I made in buying a new set of ignition coils just because you tested the originals exactly as described in the HONDA CB550 WORKSHOP SERVICE REPAIR MANUAL and concluded that they were no good. Like all mistakes I’ve made, this one too, was due to misinterpretation and ignorance.
On the subject of ignition coils testing, the CLYMER manual (HONDA 350-550 cc FOURS 1972-1978) leaves you dumbfounded. It reads like a dictionary that under “redundant” says, “See redundant”. It advices that “an easy way to test for a defective coil is to substitute a known good one”. Good luck with finding one. And then going through all the work of installing it on the bike.
There is an easier way indeed.
The HONDA WORKSHOP MANUAL provides the essential information in greater detail. It directs you to check for continuity between the wires of the primary coil (yellow and black/white, and blue and black/white) as well as for continuity between each coil’s high voltage leads (the ones that the spark plugs are connected to) and provides the following advice: “If there is no continuity, the coil is open and must be replaced”.
Simple enough, isn’t it? Shouldn’t take more than a few minutes. So, without much further thought, I got my digital “multimeter”, set it to resistance mode, turned the buzzer on and checked for continuity as described above. Yellow and black/white: the buzzer sounded loud and clear. Continuity. Blue and black/white: same sound, same assurance that there was perfect continuity. High tension leads. No sound. I tried again, and again, and again. No sound. No sound = no continuity. Conclusion? The ignition coils are bad and must be replaced.
Yes, we are all creatures of habit. When the digital multimeter has worked perfectly well for many years in adjusting points or in checking any other electrical continuity, we develop an instinct. Sound = continuity, no sounds = no continuity. As simple an interpretation of continuity as that.
But, continuity doesn’t mean sound or no sound. And it doesn’t work that way either. There is, in the physical world, a connection (continuity) between all things (barring some divorced couples, of course). Continuity is not an on-off, either-there-or-not, thing. It is in all possible values in between. When you connect your digital multimeter’s leads to a piece of wood, there is certainly a physical connection, i.e.“continuity”: they are connected to each other through the piece of wood. But electrical current is not going to flow from one of the leads to the other because the wood puts out too much resistance and the electrical current cannot overcome it. On the other hand, connect the leads to a piece of metal and the electricity flows easily from one to the other because there is so much less resistance.
Yes, it is all about resistance.
There is always “continuity”. Some with little resistance, some with more resistance. Only if the resistance is infinite, can we say that, in effect, there is no “continuity”.
So, when I heard no sound when I connected the two high tension wires to the digital multimeter that did not mean there was no continuity. It meant the continuity was there, but with a relatively high resistance. As high as 15,000 Ohms. It turns out this a perfectly normal and expected resistance value for a set of coils of this type. And it turns out the digital multimeters do not produce a “continuity” sound, if the resistance exceeds a certain value! There is an explanation for the above, of course, but let’s get to the essence of this post.
HOW TO TEST HONDA CB550 IGNITION COILS:
- Unscrew the spark plugs boots from the coils high tension lead wires. Connect the digital multimeter to the two high tension wires coming out of one of the coils. (Strip a little of the high tension leads’ insulation if needed.) DO NOT TURN ON THE BUZZER. The resistance should read around 14,700 Ohms (14.7 kΩ). Repeat with the other coil.
- Now test each of the spark plug boots individually. The resistance should read around 5,000 Ohms (5 kΩ). They are made with a built-in 5 kΩ resistor. Discard any that read much higher resistance.
- Screw the spark plugs boots back on and check the high tension wires again. The resistance should now read around 24 kΩ to 25 kΩ (14.7 kΩ from the coils + 10 kΩ from the two spark plug boots combined). This will ensure that the connection between the spark plug boots and the high tension leads is good.
- To check the primary coil’s resistance, connect the digital mutimeter to the yellow and black/white wire on one of the coils. The resistance should be only around 4-5 Ohms. And if the buzzer is turned on, you will hear a beep. Repeat with the blue and black/white wires of the other coil.
The above values for resistance were obtained from a set of original HONDA CB550F coils known to be working since they came off a perfectly running bike. Another set of original coils I tested gave very similar values.QUESTIONS?