Honda GL1000 Coil and Spark Plug Cap Testing + Replacement Options

Coils are fabulously simple devices that work reliably and can last for decades with virtually no maintenance. I like to think of them as electricity “amplifiers.” When the breaker points open, the magnetic field produced by the primary windings collapses. Through the magic of induction, this collapse creates an energy force within the secondary windings of the coil. Since the number of windings is much greater on the secondary side, nominal battery voltage (roughly 12V) is multiplied to an incredible 8,000+ volts! This energy immediately finds a path to ground via the spark plugs…arcing helpfully in the process. Voila – we have ignition!

The stock GL1000 coils are considered fairly low performance items today. However, in the ’70s, they were considered an “upgrade” for folks tuning other brands and models of bikes. Combined with a good set of points, and condensers, the stock coils perform very well and are ultra reliable.

One weak aspect of the GL1000 coil design – the spark plug wires are integrated into the unit and are not separately replaceable.

Coil Testing

If your bike suddenly loses ignition on either the front or rear pair of cylinders, your coils are suspects. First though, eliminate the more common problems caused by the breaker points, condenser, and associated wiring.

Here’s how to test your coils on the bike. Remove the spark plug wires from all 4 spark plugs. With the ignition off, use your multi-test meter set to resistance mode.

First, measure the coil secondary resistance. Use the 10K range on your multi-meter.

You don’t necessarily have to remove the plug caps to check the secondary resistance. There is a 5000-ohm resistor incorporated into each spark plug terminal…mainly for electronic noise suppression. In later models, this is built into the plastic, but in most 75-77 models it is a separate resistor that can be removed by screwing out the “connector” inside the plug terminal. The resistor has a spring behind it (easy to lose!).

Measuring end-to-end with the spark plug caps attached, you should get a value of about 24,250 ohms between the sparkplug terminals, accounting for the two 5,000 ohm resistors plus the 14,250 in the coil itself. If the reading is high, you will have to determine if it is due to a bad cap(s) or a bad coil.

With the caps removed from the wires, the reading should be about 14,250 ohms. Both coils should be nearly the same. If one reads significantly more (say 17,500+ ohms) then it is probably on the path to failure.

Note: one coil fires the front two cylinders (#1 and #2) and the other coil fires the rear two cylinders (#3 and #4). So, when you do your secondary resistance tests, you will be testing through plug wires #1 and #2, then testing through plug wires #3 and #4 for the other coil. Some folks get confused by the fact that the pair of spark plug wires on each side of the bike are corralled by a retainer on the chrome carb stays. However, when you look at the plug wires on each side of the bike, remember that they connect to 2 different coils.

If the caps generate more than 5,000 ohms each when isolated and tested for resistance, you can try disassembling and cleaning the internal resistor. 

Correct orientation of the screw retainer, resistor, and spring

Sometimes, simply tightening the screw “connector” retaining the resistor will alleviate the problem. However, if you still have the original spark plug caps, treat yourself to some new ones. OEM Honda caps are very spendy. However, you can use NGK model #XD05F (or the improved version #XD05FP with more waterproof boots) which are functionally equivalent. The only difference is the raised white NGK logo on the side. To make the plug retainer seals fit better, grind off the NGK logos. You can buy a set of these caps here…

To test the primary side, set your multi-meter to the lowest scale for measuring resistance. You should get a reading of about 2 ohms. This is measured at the two bullet connectors on the short wires near the top of each coil. The important thing here is that you do not have an open connection (infinite resistance). Failures on this side of the coil are generally catastrophic. So, this test is usually perfect or terribly bad…no partial failures on this side.

If you find that the coils test OK for both primary and secondary resistance, but you still suspect coil problems, put the coils in an oven and heat them up to 200 degrees or so and test them again. Sometimes the resistance will dramatically increase when heated…revealing heat-related faults.

Curiously, there is no coil polarity to worry about on the GL1000 ignition setup unlike the ignitions for cars of this vintage. The stock wiring harness is configured so that you can only hook the coils up in one “direction.” However, you could reverse the leads and everything would still work fine.

Curiosity: Did you know that on this “wasted spark” design one side of the bike fires it’s spark plugs in reverse (side electrode to center electrode)? Strange but true! If you look carefully at worn spark plugs from a GL1000, 2 of the plugs will have worn center electrodes and the other 2 will have worn side electrodes.

Coil Replacement

If you need new coils and/or spark plug wires, here are your options:

• New OEM Honda coil assemblies. These come with integrated wires. Unfortunately, these are hard to find now.
• Aftermarket and high-performance upgrade coil assemblies (like Dyna). These are available from Randakk’s.
• Coils does not come with spark plug caps…use NGK part #XD05F (or the improved version #XD05FP with more waterproof boots). The only difference is the raised white NGK logo on the side. To make the plug retainer seals fit better, grind off the NGK logos. You can buy a set of these caps here

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