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Troubleshooting your charging system


Troubleshooting your charging system

Buy a £10-£15 voltmeter from any car parts place and take it with you everywhere. If it won't help you because you never break down, it will help the next guy. Change the meter battery every year.


At winter's end and even now and again during the season, get that meter out.

With battery in place on bike, put the red lead to battery positive terminal and the black lead to earth terminal, after setting the meter to the DC voltage figure just higher than those which you are looking for (usually ''20'')

You should get a reading of 12.6-12.9 DC volts. Any less and you will encounter starting problems - most of the capacity of a Harley's big battery is for the starter motor. (Kick start only? You're still good to go until it drops below around 11 volts!)

I did read once that if 12.9 volts is a 100% charged battery, 12.5 is about 50% charged. Sobering....don't think that 12.2 is good. It's knackered.

If battery needs charging, use an OptiMate or similar - these deliver a SLOW trickle charge and can recover even a deeply discharged battery.

(Fast charging using car-type chargers will shorten the life of your battery. You should disconnect your battery if not riding over winter, and preferably leave it on an OptiMate or similar. Batteries do discharge slowly over time, especially when hooked up.)

Check on....with the bike running, confirm that the correct charge is getting to the battery - place meter leads as before and at fast idle the voltage should read around 13.5 volts DC.

Rev the bike - the voltage should go no higher than around 14.8. Fifteen volts and above is a no-no - this means the regulator is not regulating and the extra voltage will surely boil the battery dry while you're out on the road. Even refilling can be pointless due to plate damage caused by boiling....

Your regulator needs an excellent connection to ground to regulate, i.e. shunt the excess alternator current to earth.

Some regulators are earthed by body by being securely connected to the frame, some through a wire to frame (which acts as ground in either case). Check this connection if you are reading too much (15 or over) DC voltage at the battery.

Use star-type washers that 'dig in' with either method - AFTER removing any paint or powdercoat. Bare metal gives the right connection.

If the voltage does not rise, that's not good either. Check it a second way by turning on the lights - they should brighten slightly as you rev the bike because the charging voltage is rising to the ideal 14.5-14.8.

If not, the connection from regulator to battery may have failed (see below).

And there's more.....If you are getting normal charging voltage but when you rev harder said voltage plummets, even to zero, in all likelihood the regulator is breaking down at the higher rpms. Replace.

Also, if the regulator is trying to charge into a defective battery (and cells or plates can fail without warning due to age or too much current coming in) it will immediately go to max output, then if it continues at this level, the output circuit will fail, which renders the regulator useless.

Tricky blighter, Johnny Regulator......
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It's very often an electrical/charging fault.

If exhaust bangs and pops and makes firecracker sounds before the bike stops dead, the first likely culprit is/are loose battery terminal/s. Always carry a 10mm ring spanner for your battery bolts - often the ONLY metric bolt on your Harley.

If they are tight, check the one critical wire that provides power - it goes from the regulator all the way up to or right next to the battery positive terminal. If that connection fails, the bike will run on battery power alone until that's all used up and the battery has almost certainly trashed itself trying to keep you in the wind despite being cut adrift from the charging system.

If those connections and all other relevant ones (Kill switch wires intact at switch inside handlebar? Main circuit breaker dead, or pinging due to short circuit on another wire?) are all good, proceed....

Turn the key off. Place voltmeter to correct DC voltage scale and put leads across battery terminals.

It's gonna show far less than the magical 12.6 or so volts ... OR .. it will show ''good'' voltage until you turn the key ON and the voltage will plummet (even down to ZERO volts), showing the battery can't hold a load. You won't even get a peep out of the horn. Turning key on should only drop the reading a smidge, if that...

It's dead forever in that case.

Once you've been recovered home and recharged your injured battery, or if it failed to hold a load and your club mates have ridden back to your stranded bike with a new battery from the nearest shop, you gotta work out if it was just the battery, or something else.

If you just replace the battery and the regulator is at fault.....the same thing will happen in about 200 miles. Gets expensive, just replacing the battery if that's only the symptom, not the fault....but then, sometimes the battery AND regulator will commit suicide together. It's a lottery as to the death toll ...


Do the battery voltage one (see above) with the new battery in place and the engine running.

Delve further......

Ignition off. Undo the regulator plug from the case where it meet the stator pins. Set the voltmeter to DC volts. Insert red meter pin into one of the female regulator plug terminals, place black lead on good ground i.e. the engine cases. Do the same to both regulator pins.

If you get any reading above a couple of volts, the regulator diodes are buggered and allowing the battery voltage, and no doubt plenty of alternator output when running, to leak to earth.....last year I had a regulator-plug reading of 12 volts!!

The same test can be done with the regulator ground wire (if equipped) or with meter pins placed between unbolted regulator and ground.

For some reason, this leak-to-ground fault does NOT always show with a test light. Be warned. It cost me £120 to discover that. Use the voltmeter.


The alternator puts out (hey!) Alternating Current, which the regulator/rectifier, to give it its full name, rectifies to the more useful Direct Current .
First do an AC voltage output test.

UNPLUG the regulator at the crankcase where it connects to the stator pins.

Set the multimeter to the AC voltage setting just higher than the figures we need and start the bike, which you can do now it has a known good battery. Place each meter lead into each stator plug peeking out of the crankcase. Unless you have long arms, you might need a friend here...

You should be getting around 20-26 AC volts at 1,000revs. The exact amount depends on the amperage of your bike's charging system (Read The Manual!) :)

But the critical thing is that if you have around 20 AC volts at 1000rpm, then you must get 40 or thereabouts at 2000rpm, something like 60 at 3000rpm......get the picture? If there is weak output or no output, you're gong to be RTFM a whole lot more...

But be warned - even a good reading is no guarantee it's working. Read on.

Turn the engine off. Next check to see if the stator wiring is OK.

We are measuring resistance now. First thing you need to do is check your meter.

Turn the multimeter to the Ohms scale at its lowest reading (usually 20), and touch the Positive and Negative probes together. You may well get a resistance reading. For example, if it reads .4 ohms when you touch the probes together, you need to remember that. For when you put your meter probes across the stator winding, you might get a reading of say .6 ohms, but remember there's .4 ohms ''built in'' to the meter, so your stator is actually .2 ohms.

With motor off and regulator obviously unplugged from stator, connect meter's red lead to one stator pin in the insulation peeking out of the cases, and the black lead to a good ground on the bike (cases is good).

If you get a large ''1'' or ''I'' then a decimal point on the meter this is INFINITE resistance or no path to ground i.e. an open circuit. This is good.

If you get any other reading then you have a grounded stator winding and it will not produce good electricity. You will lose the ability to charge your battery. There may be enough voltage to keep the bike running with the lights off, but turn the lights on and the voltage disappears. The bike then begins to discharge the battery, slowly killing the power until it dies.

Do the same on the other pin. Same reading = good.

Now place each meter lead into each stator plug. There should be continuity (less than one ohm NET resistance, usually .2 to .4 ohms NET. Anything above that you have a open circuit. Anything below that spec. you have a short.

NOTE! A shorted out (or grounded) stator will produce the recommended AC voltage at given RPM's. However, it produces nearly zero amperage - the good stuff. So the resistance tests with meter set to Ohms are vital....


An intermittent arcing connection between the regulator plug and the stator female plugs is a common problem on pre-97 bikes. Make sure your bits fit tight. Watch for blackening on regulator pins. If the fit seems sloppy use a dielectric grease or spray to ensure a good connection.


...can be a mystery. They can fail in many ways, some inexplicable and invisible to the eye or meter. But knowing the other bits which feed it are in good shape is valuable. Carry a voltmeter.

NEVER use a battery sold to you by a shop which promises at 10am that it will be ''charged and ready'' by 3pm. If it's not TRICKLE charged for at least 24 hours initially, it will never be 100% on the bike and its life will shorten. Take that voltmeter with you to the shop and test the battery before you pay for it...if it's below 12.7DCV, trickle charge it overnight at home before fitting it to bike. Trust no one as to its state of charge.

In recent years the MoCo has switched to sealed batteries. Jap shops sell them a whole lot cheaper. The beauty of these is there is no overflow tube to spew acid on to your frame, etc if things go haywire. Recommended.

Also, if you are running a kick-only bike, you can get away with a really small battery because few cranking amps are called for. you can get one from the Jap shops.....and using a block of wood and a bit of bicycle tyre tube to fill out the space in the tray!

(Thanks to KiwiDave)
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