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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20 thoughts on “Honda GL1000 Coil and Spark Plug Cap Testing + Replacement Options

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  1. For me, the selection of coils is a bit confusing, unless the decision is to go with Dyna S ignition and the Dyna green coils (3 ohm). There, it’s pretty clear the ballast resister is not to be used. But, if you want to stay with points, you need to retain the ballast with green coils, and sadly there’s no advice for us how to mount the ballast – the original mount no good as these coils are bigger. Also, the 5 ohm coils also recommended with points, and while it is implied that ballast not be used, it isn’t clear. I called Dyna and the tech was not totally sure about not using the ballast in that situation, but “supposed” it would work. We could use assertive clarification that 5 ohm Dyna coils work with points without ballast, and advice on how to mount ballast if it is used with Dyna coils.

  2. Hey Guys is there a cheaper after market coil i can use for my gl 1000 and secondly do you need the resistor if you have electrinic ignition?

  3. Upgrading voltage coils was the best thing I ever did for my 1977 GL 1000. It was backfiring and emitting puffs of black smoke and getting lousy mileage. Instead of another carb overhaul, I installed Dyna Coils to go with my already installed Dyna electronic ignition. The original coils tested okay in terms of resistance, but I went ahead and replaced them anyway. HUGE difference. No backfiring, easier starts, smoother idle, more power, better throttle response. And thanks to Randakk for all the online advice (including about the ballast, which I have discarded with no ill effects). I also added a power relay to make sure the new coils got the correct voltage, as most old bikes have a wiring harness that just can’t deliver proper voltage. My bike feels like a new machine after this upgrade.

  4. Been reading though all these tech tips as I slowly rebuild my ’78, and the resistance requirements is throwing me off. I am planning on upgrading to the Dyna ignition and also get a set of the dyna coils, so i can replace my plugs wires. Curious if i would need extra resistors besides the NGK boots and DR8ES plugs. Also notice there’s different ohms ratings for the coils, curious what would be the best fit, 3 Ohms?

  5. Planning to upgrade my ignition system since i am rewiring a majority of the bike, (stupid mice nibbled plug wires and various other wires).
    Been reading through the tech tips, and what has me slightly confused is the resistance requirements.
    What i have planned was the following:
    Dyna 3ohm Coils and the Dyna S Ignition
    and use a set of the dyna plug wires, (some say “surpressed” not entirely sure if that refers to resistance)
    Plan to use DR8EA plugs (have a good stock of them), Would i need the NGK resistor caps as well ?


  6. Hi everybody.
    I’ve measured the secondary side of my gl1000 coils and between 1&2 is it 18.2k without caps. Between 3&4 is it 17.0k without caps. Can these coils be the reason to the backfiring and slow throttle respons from idle? Also, when riding with little throttle speed it’s like riding a horse.
    Thx from Norway

      1. Sorry about my late thx.
        I’m using dynatek ds1-3 ignition and I’ve bought a set with 3 ohms dyna coils.(not arrived yet)
        My question is now. Can this complete dyna ignition run without the ballast resistor?
        Thx again

  7. Hi There. I’m having a bad idle curse. Early 78GL1000 18K. All oem except 77 horseshoe exhaust and KN Airfilter. previous carb rebuild mechanic placed two small washers lifting each carb needle about 1.5mm similar, to what you recommend to for the ’77 models. that is still in the bike.
    I rebuildt and sync carbs (your rebuild kit and video are great!), valves adjusted, points and condensers OEM new , checked the timing advancer and is OK, and interchanged the OEM ballast resistor that reads 3.4OHM with the NAPA’s ICR13 which reads 2.6OHMs. Did not notice changes in idle. with the Napa’s, but ruled out that.
    I now wanted to rule out the coils. I am getting this readings:
    Primary test is 2.4 OHM cold and 2.8 at 200F
    Secondary without wire caps Cold: 15.4(cil1&2) and 15.5 (cyl 3&4). My NGK caps add 5 ohm. When Hot at 200F they read 18.9 and 19.1
    What do you think? Do you see problems with the coils? Would the lifted carb needles interfere?
    Thanks in advance!

  8. My 1976 GL1000 does not have a spring and resistor in the spark plug terminal. Is that normal?

    1. Should have spring and resistor …or a straight brass “distance piece” if the resistor was previously eliminated per this Tech Tip.

    1. Depends on the model of bike. If GL1000 – the answer is generally yes, but it depends somewhat on what type of spark plug wires you end up with and do they add “extra” resistance.

  9. Hi Randall

    I own a 79 GL1000, I have just bought a complete kit; Ignition + coils + Cables, i willl purchase the NGK caps to go along with that, my question is should I buy new NGK D8EA or DR8EIX to go along with that?

    Thanks a lot for your advice


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