Dr.Hess' How To Diagnose Your Charging System

Buy about a $10-20 digital multi-meter. I do not recommend the auto-ranging style for these tests. You also don't need a $500 model. The super cheap ($2 or "free with the coupon") ones may or may not work, and it is more important to have one that does work, so spend about $20. Fully charge the battery (overnight on a 1 Amp charger).


Scale on DC Volts, around 20V max voltage scale. Nominal readings are given in brackets. Check voltage across battery terminals (12.8). Turn bike on. Check voltage (less than previous, ~12.0+, depending on headlight, accessories). Start bike and let idle. Check voltage (could be 12.0 to 15). Rev to about 2500. Check voltage (should be more than observed with bike on but motor not running, and more than with bike off. Ideally between 13 and 15.) Turn high beam on. Should be about the same, give or take a little. If the voltage is over 15 or 15.5-ish with a headlight on, I'd consider replacing the regulator and/or checking all grounds (battery to frame, regulator to frame in particular). If the battery voltage with the headlight on, bike not running is less than around 11.5-ish, I’d replace the battery. If it’s 10, it’s past it’s prime.

Scale on AC Volts, generally a lower setting (might be 20V, or 100V). Check across the battery when the bike is running. If you get any significant reading, like 10V or 15V AC, the diodes in the regulator are shot. This is a pretty rare failure point, but it has happened to me. Another sign of this is a strange behaving tachometer.


If you pass the above tests, your system is most likely fine, including the regulator and stator. If you don't pass, then:

Bike off. Meter set on Ohms, medium-ish scale, like 20K or 200K Ohms max scale, depending on your meter. Pull stator plug. Ground the meter black lead to a good chassis ground, like a bolt or even the battery negative. With the red lead, touch a different part of the bike, like the engine case at an unpainted part or another bolt. Meter should read low ohms, like 0. If it doesn’t, you didn’t ground the black lead. With the red lead, touch each contact on the motor side (stator) of the plug (the part stuck in the case). Depending on if your case has a male or female plug, if you can't see the metal part/pin of the plug, you can put a paper clip in the hole and touch the paperclip with your meter red. Meter reading should be infinity on all pins. If it isn't, your stator is shorted to the case, replace.

The following is for single phase systems. I don't have a multi-phase and haven't had to diagnose anyone else’s, so I haven't dug into those systems.

Set meter to lowest ohm scale, like 200 Ohms, typically. (Note: Not 200K ohms). Check resistance between the two stator plug pins. Should be fairly low. My Book says 0.2-0.4 ohms. The spec is in your shop manual. If it is infinity, stator is blown open - Replace. If it is 0, stator is shorted to itself – Replace. I hesitate to suggest the shorted to itself test, as the novice electrician with a cheap meter is likely to screw this up. Cheap meters may not be able to determine that low of a resistance, and the zero may not be correct on them. What you can do, if you want to put any faith in this (shorted to itself) test is to first touch your two meter leads together and note the reading. It could be 0, or 0.4 or even 1 or 2 ohms. Then when you test across the two stator pins, you should have whatever you just measured plus 0.2 to 0.4 ohms. So, a reading of 2 ohms with the leads shorted and 2.4 ohms with the leads on the stator is within the book’s spec. It is very rare for the stator to short to itself, so I wouldn’t worry about that one too much. Much more common for them to blow open or short to the case.


Set meter to AC Volts, 100V scale. Attach each meter lead to a stator pin. You may need to rig up some type of temporary plug. It is important that nothing can short to ground or to each other accidentally, or you will blow the stator if it wasn't blown before. An old plug off of your last regulator is a good way to do it, but, get creative and be careful. I can do it holding the leads on the pins once the bike is running, but I don't like to. Start bike. Voltage should vary with engine speed. Specs are in your shop manual, but 35V at a couple thousand RPM is probably about right. My book says 19-26 V / 1K RPM.

If you passed that stator test and failed the first test, your regulator is shot. If you failed any part of the stator test, replace both regulator and stator. Cheaping out on parts is a good way to have to go back and do it again. I prefer to use Genuine H.D. parts here (regulator and stator) because I don't want to have to keep doing it.

 

Jolly well done Dr H, as usual. This deserves to be a sticky!

 

Thanks, Tup. I asked Eve to sticky it. I had to figger this out back when I rode a shovelhead. That thing would eat electrical systems for lunch.

 

Is Eve still around? She's been very quiet of late - concerned she hasn't fully recovered from a recent bad patch, poor thing.

 

Excellent post for technical stuff. Copied and saved for reference...thanks.

 

Dr. Hess,
Thanks for posting this info. I have it printed and filed and my factory service manual : )

 

Darn just compared this to your old charging test and its the same. Thought I was going to update my notes, maybe you found something new.
Thanks Dr Hess

 

I hope Eve is ok she usually responds much quicker than this.
Doc you might have to ask Terry.

 

Yeah, I'll PM Terry.

I added a few lines to my notes, but the basics are the same.

 

sticky...sticky...sticky!!!