Ballast Resistors Explained + Ballast Testing Procedure

Like most battery/coil setups of this vintage, early GoldWings feature a “wasted spark” ignition design. This clever design was chosen by the engineers to save weight and space. Thus, only 2 coils are required to fire 4 spark plugs. All without a distributor as in most cars of this era.

This minor miracle is possible because each coil fires 2 spark plugs simultaneously. One plug fires on the cylinder that is just completing its compression stroke and the other fires the cylinder that is just completing its exhaust stroke. It should be rather obvious why one of these sparks is considered “wasted.”

Curiously, 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.

Since every spark event has to fire across 2 spark plug electrodes, it’s imperative that all components be in top condition. When 2 spark plugs share the same coil, a slight degradation in one spark plug can affect the other plug’s performance.

The stock ignition system on a GL1000 is comprised of a carefully matched set of components. The principal ones are the coils, plugs, plug wires / caps, ballast resistor, points, and dual condensers. The battery, wiring harness, stator, rectifier, voltage regulator and associated switchgear are in the mix as well. All of these components must function harmoniously for good results. Points-type ignition systems are relatively simple to maintain and diagnose. They are also highly reliable and easy to repair when they do break. However, beneath the apparent simplicity of the design lies some very complex electrical theory.

I won’t attempt to explain the physics here, but I will share a functional explanation of how the system works and how the parts are interrelated. The main focus of this tech tip is to explain the function of the ballast resistor as well as necessary test procedures. There is a simple test to diagnose the total failure of the ballast resistor (bike won’t run). I will also cover testing for partial failure (bike will run, but has poor low speed ignition performance). Finally, I will cover a less expensive substitute ballast resistor option that offers slight ignition performance improvements and better idling characteristics.

First, the ballast resistor is the long, white ceramic thing attached to the left side of the coil assembly. Basic function: the ballast resistor is a resistor contained in the primary ignition circuit wiring that lowers voltage after the engine is started to reduce wear on ignition components. Less well understood is the fact that ballast resistors provide appropriate compensation within a circuit for external demands placed on that circuit. One example of this would be to compensate for temperature changes.

Honda Ballast ResistorOEM Honda Ballast Resistor (part #34930-371-003)

The coils, ballast resistor, points and dual condenser of a GL1000 are optimized for proper ignition function by coils that are designed to deliver full secondary circuit energy pulses to the spark plugs with primary input circuit voltage of approximately 7-9 Volts … not the 12 Volts that you might expect.

This is so you will have “full” power to the coils during start-up…if the coils were “expecting” 12 volts they would be disappointed! The nominal 12 Volt battery voltage is reduced considerably during start-up by the heavy effort to energize the starter and spin the engine…hence the reduction to around 7V as the “normal” state of affairs for these coils (at idle). Sometimes, you might hear geezers like me refer to GL1000 coils as “6-volt” coils. Don’t get hung up on the exact values – the relative values are what’s important here.

Once the engine fires, the alternator (stator actually) comes online and battery voltage is regulated to approximately 14.5 Volts at normal cruise RPMs (voltage is less at lower RPMs). Without intervention, the coils which are optimized for 7-9 V would now be seeing the primary input voltage of 14+ volts. Clearly, this would cause problems of overheating and reduced life for ignition components. This is where the ballast resistor enters the picture.

Through clever wiring, the power to the coils is always routed through the ballast resistor unless the starter is spinning – in which case the ballast is by-passed. The resistance value of the ballast resistor is calibrated to deliver the 7V or so expected by the coils at idle. During starting, the ballast is by-passed and the full available voltage is routed to the coils. The beauty of this design is that you get quick starts AND long life for the components. I won’t go into the theory, but the ballast cleverly compensates somewhat for the ignition requirements required by changes in temperature and engine rpm.

What can go wrong?

Catastrophic failure of the ballast resistor

This is easy to diagnose. The symptom is a bike that starts easily but dies as soon as you release the starter button. If any of you owned Chrysler cars during the 60s and 70s, you are no doubt familiar with this common problem! Fortunately, the Honda ballast is very reliable and catastrophic failure is rare.

To test your ballast, remove the 2 attached wires from the terminals and check for resistance across the terminals (ignition off). Set your ohmmeter to the lowest scale (Rx1). A good (cold) reading for the GL1000 Honda ballast is exactly 3 ohms. Catastrophic failure will yield infinite resistance (no continuity). Any value other than 3 ohms is a sufficient reason for replacement.

If you are stranded by the roadside without test equipment and suspect a bad ballast, you can disconnect the 2 wires and temporarily reconnect them to each other (by-passing the ballast). If the engine then starts and continues to run when the starter button is released, you’ve successfully diagnosed the problem. It would then be OK to ride home (or to an auto parts store for a cheap substitute…covered later) with the bypass. This is not a permanent solution as your coils are at risk of overheating and your points will burn up quickly operating on 14+V as explained above.

“Out of Range” problems with the ballast resistor…one cause of poor idle in GL1000s

Sometimes a ballast resistor will test correctly at 3 ohms under a static, cold resistance test per above, but perform incorrectly under dynamic load on the bike. In such cases, the actual delivered resistance can deviate from the design spec.

The deviation is almost always toward more rather than less desired resistance. Too little resistance in use is rare…I’ve never actually seen this. It is of course theoretically possible and the expected outcome would be premature pitting of the points. However, the usual cause of prematurely pitted points is a bad condenser assembly (or poorly grounded condenser).

If the resistance is too high under load, this would result in the coils receiving less input voltage than they are expecting. The resultant spark at the spark plugs would be less vigorous than normal. This reduced voltage would be especially troublesome at idle speeds (since the alternator isn’t doing much at idle). If you have a bike which runs flawlessly above a certain speed – say 1500 or 2000 RPMs but refuses to idle smoothly AND you can absolutely rule out ALL other ignition and carb issues, then you MAY have a bad ballast resistor. (Note: there are many other higher probability causes of poor idling. A few of these are: weak battery, faulty charging system, improper ignition timing, burned or badly adjusted points, poor compression, misadjusted idle mixture screws, blocked idle jets and nozzles, vacuum leaks, poor synchronization, etc., etc.)

Here’s a way to test with your ballast under load conditions with a multimeter.

  1. First, test for static resistance per above…should be exactly 3 ohms, otherwise, toss the ballast.
  2. Start with a fully charged battery.
  3. Disconnect the fuse to the headlight. Also, disconnect any heavy load accessories which come on with the ignition. Do this to keep from draining your battery during the testing which follows. Also, heavy drains would distort the voltage readings I mention below.
  4. If your ballast resistor has the terminals pointing to the left side of the bike, proceed to step 5, if the terminals point to the right side of the bike, remove the air cleaner assembly for access (for whatever reason, bikes were assembled differently by the factory on this point).
  5. Pull the 2 connectors to the ballast back slightly…. they should still be connected to the ballast terminals.
  6. Turn kill switch to “on.”
  7. Turn ignition switch to “on.”
  8. Using pin probes on your multi-test meter, set to appropriate scale for 12V and check voltage at both ballast resistor terminals. Your negative (black) lead will be attached to a convenient ground. Your positive (red) lead will be touched to the input terminal on the ballast (wires remain connected to ballast). The top is usually the input side, but sometimes the wires are reversed. In any case, one side should show a value around 11.5V (your nominal battery voltage…reduced a bit by the load of the taillight, front running lights, instrument lights, etc.) and the other side (output) should show approximately 5-6 Volts.
  9. If the “high” reading is much lower than 11.5V, this indicates a problem with the battery, ignition switch, kill switch, starter switch, main fuse, battery cables and / or associated wiring.
  10. If the high reading is lower than expected and also the same as the low reading, this means that both of your ignition contact points are “closed” and both coils are energized at the same time. That is “bad” and should never happen with properly adjusted points, but can happen when the adjustment is out of whack. For now, remove the points cover and rotate the engine until you can verify that only one set of points is closed, then repeat the test detailed in #8. Later, you will want to make sure your points and timing are properly adjusted as explained here.
  11. If the high reading is normal (around 11.5V) and the low reading also reads the same value, this means that both of your ignition contact points are “open” and neither coil is energized. Ignore the “why” of this curiosity, for now, remove the points cover and rotate the engine until you can verify that only one set of points is closed, then repeat the test detailed in #8. Later, you will want to make sure your points and timing are properly adjusted as explained here. By the way, the reason there is no voltage reduction when the points are open relates to the fact that the ballast resistor offers no resistance at all unless there is an actual load on the circuit…potential loads don’t matter to the ballast. The load is imparted by the primary windings in the coils which of course aren’t energized unless the points are closed.
  12. If your high reading is OK and the “low” reading is much lower than 5 V, you will probably have problems with poor ignition at idle speeds. The ballast should be replaced.
  13. The test can be repeated with the engine fully warmed up and idling at 1000 RPMs. Sometimes heat is the culprit that causes the ballast to go out of spec. and this test will reveal if this is your problem. You can put the headlight fuse back in for this test, but disconnect any heavy load accessories which come on with the ignition. Tested this way, the high reading should now be about 12.0 – 13.0V. The low reading should now be about 7-8V. Any value for the low reading outside this range is cause for replacement.

Substitute Ballast Resistor

This aftermarket substitute delivers slight improvement in low speed ignition performance and idling manners.

Caution: This substitute ballast carries the potential to accelerate the wear rate of your ignition components…especially the contact points. More heat will be generated in the ignition coils so there is an increased risk of coil failure as well.

Note: This substitute ballast is not recommended for bikes with electronic ignitions (like Dyna) even when the bike is equipped with stock coils. Follow the electronic ignition manufacturer’s instructions to the letter. Dyna’s recommendation regarding the retention / non-retention of the stock ballast depends partially on whether you have stock or non-stock coils. But there is an additional point of confusion – some Dyna implementations call for a “piggy-back” ballast to be used in conjunction with the OEM ballast resistor when retaining stock coils. Other implementations with stock coils provide a substitute DYNA ballast and the OEM ballast is not used.

NAPA makes a ballast resistor which is rated at 1.82 ohms (versus 3.0 ohms for the stock ballast). The NAPA part number is #ICR13.


NAPA Ballast ResistorNAPA (Echlin) Ballast Resistor #ICR13

The price for this part is about 1/3 the price of the OEM Honda part. This part has a similar shape to the OEM item except for a protruding mounting eye. It can be mounted using longer screws and spacer washers in the original bracket…the protruding mounting eye should face to the rear of the bike. Be very careful handling the ballast as the ceramic is very fragile. Be sure to use spacer washers to avoid squeezing the ceramic…it will break! Alternatively, you can simply zip tie the new ballast near the OEM unit using the protruding mounting lug.

Caution: Unlike the OEM ballast, this item has an “open” back which exposes the main element wire. It is critical that you position the ballast in such a way that open back does not abut anything that would be damaged by the considerable heat the ballast sheds during operation (this heat will melt the insulation from wires and zip ties so be careful).

Changing to this lower resistance ballast will slightly improve the idling ability of your GL1000. I’ve used this part on several of my GL1000s over the years with good results. With this NAPA ballast, I get a smooth idle all the way down to about 950 RPMs. I like this because I get more engine braking and crisper return to idle when I close the throttle.

This works because voltage to the points is boosted about 1-2 volts at idle. This results in better coil saturation and stronger spark at idle…where you really need it.

As I mentioned above, the downside is accelerated points wear. Not nearly as bad as if you were running without a ballast, but accelerated nonetheless. Also, there is some increased risk that your coils will overheat and fail prematurely. For me, the risk is acceptable for the performance gain delivered. Implement this mod at your own risk!

Contradictory Advice Regarding Ballast Resistors

I’m aware that there have been some contradictory advice regarding ballasts issued by various manufacturers of coils and electronic ignitions. My view is to side with the manufacturers of electronic ignitions on this point.

Here is a table which summarizes my best advice for ballast requirements vs. ignition and coil details:

Ignition Type Coil Type Ballast Resistor Requirements
OEM Breaker points OEM coils (2.0 ohm primary resistance) Retain stock 3.0 ohm ballast. Consider 1.83 ohm auto-replacement ballast per above discussion for improved performance at the expense of accelerated points wear and slight increased potential for premature early coil failure
OEM Breaker points Aftermarket high performance coils such as:
• Dyna
Retain stock 3.0 ohm ballast. Consider 1.83 ohm auto-replacement ballast per above discussion for improved performance at the expense of accelerated points wear and slight increased potential for premature early coil failure.
In this combination, the ballast is retained mainly to protect the points…not the coils.
Electronic ignition like Dyna-S OEM coils Follow the electronic ignition manufacturer’s directions regarding retention or replacement of OEM ballast.
Generally, Dyna will want you to end up with a ballast resistance value of 1.5 ohms. This has been accomplished in 2 different ways. Some Dyna installs have provided a “piggyback” resistor to be used in conjunction with the OEM ballast to provide the correct resistance value.
Other Dyna installs have provided a replacement ballast calibrated to 1.5 ohms. In this case, the OEM ballast is omitted.
Just be sure you end up with approximately 1.5 ohm total resistance.
Electronic ignition like Dyna-S Aftermarket high performance coils such as
• Dyna
Here’s where the contradictory advice shows up…
In this combination, Dyna recommends NO BALLAST. However, Accel recommends that the OEM ballast be retained.
I have generally sided with Dyna on this point. Modern high performance coils are designed to handle nominal 12V battery voltage (and cruise voltage of around 14.5V) without issue, so I think Accel’s advice is overly conservative. There’s no problem if you follow Accel’s advice and retain the OEM ballast, but you will give back some of the improved ignition performance you’ve paid for.
On my bike RC003, I have Dyna-S ignition + Accel coils and wires.
A compromise for those inclined to follow Accel’s advice might be to consider running an automobile replacement ballast with less resistance than the OEM 3.0 ohm unit. Auto replacement ballasts are available in a variety of values ranging from 0.85 ohms to 2.0 ohms.
Since there is a slight chance that Accel might know something about their coils that I don’t, I’ve decided to add a 1.60 ohm ballast to my rig just to be safe. I’m currently running a NAPA #ICR37 rated at 1.60 ohms
Here’s some commonly available automobile ballasts available from NAPA:
•#ICR23 – 1.20 ohms
•#ICR11 – 1.35 ohms
•#ICR34 – 1.40 ohms
•#ICR37 – 1.60 ohms
•#ICR35 – 1.80 ohms
•#ICR13 – 1.82 ohms
Finally, if you’ve upgraded to an electronic ignition (like Dyna-S) AND have aftermarket coils like Accel or Dyna – the spark plug gap should be increased to .035″ to take full advantage of the ignition improvements you’ve invested in.

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14 thoughts on “Ballast Resistors Explained + Ballast Testing Procedure

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  1. THANX!! Was killing modules in elderly Subaru with Hitachi ignition. This was a BIG help in figgering out why.

  2. Lo Mein sauce is made with a sesame oil base that the noodles are tossed in with
    garlic, ginger, oyster sauce and soy sauce to round out the slightly sweet and slightly spicy sacuce.

  3. Recently, I checked the stock ballast resistor on a 1978 GL1000, and found that the reading was exceeding 3 ohms. I decided to replace the part with the NAPA ICR13 replacement. When checking the new part to insure it was functioning, I found the resistance to be 2.2 (cold) right out of the box. I checked another replacement and found the same reading. At first I chalked this up to meter calibration, but a second meter produced the same result. I used a digital meter in both instances. My question is . . . the ICR13 spec is 1.82, I am finding 2.2, is this critical or is it a non-issue? By the way, I am running Accel 3 ohm coils (140403). Any feedback will be appreciated.

    1. First, if you have Accel coils and electronic ignition like Dyna/S, you can safely bypass the ballast resistor for better performance.

      Second, 2.2 ohms on the ICR13 part is fine. No worries on that reading.

      1. Thanks for quick reply. No Dyna ignition . . . yet! Glad to know that the 2.2 reading will work out. As usual you folks do a great job supporting those of us with vintage Wings.

        Thanks . . . Don

  4. Randak here’s how I mounted the napa replacement ballast by modifying the original bracket. I removed a rectangular window from the center with a Dremel cutoff tool allowing it to clear the mounting tab and hold it securely in place.

  5. Is it possible to run 2 NAPA #ICR34 resistors (1.40 Ohms) in series to make a 2.80 Ohm resistor circuit ? Thank You P.S. Your tech site help me keep my 1978 GL1000 running at its best.

    1. Yes, but be sure you understand the difference between SERIES and PARALLEL. Also, it is imperative that you TEST the resistance of the the combined resistors to measured the actual total resistance they achieve. Also make sure there is adequate air space around both. these things get very hot!

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