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Testing Your Charging System

Your equipment needs for this are very simple. You need a "floating ball" battery tester to check the specific gravity of your battery's fluid (assuming you do not have a sealed battery), and an inexpensive multimeter. It should measure volts in both DC and AC, and resistance in ohms. If it has a setting to test continuity, that's nice too. You can pick up a decent one for under $25 at Radio Shack or Wal-Mart or almost any automotive supply place. No point spending big bucks on it either. Make sure that the set you buy has both needle ends and clamps on the cables - you'll need both. You might have to buy a set of adapters, or even another set of cables, but they are pretty cheap.

Now, a moment's basic theory. Your engine spins the alternator, which is up front, inside your primary cover. The alternator puts out Alternating Current that fluctuates with the engine rpm at a rate of ABOUT 20 volts per thousand rpm. That current goes to the voltage regulator, which turns it into Direct Current that the battery can use, and cuts the voltage to a steady (more or less) 13.5 volts. The current that it eliminates is turned into heat, which is why the regulator has fins - to dissipate the heat. And voltage regulators must be grounded to the frame or they won't pass voltage. This last point is important to remember - if you ever get your frame painted, make sure the mounting points are clear of paint.

Here's how you test your system.

First read the instructions for both tools, especially to see how to set up the meter with its cables and selector switch. It's really simple, but the biggest source of going crazy when trying to track down electrical problems is misuse of the meter. For instance, if you are trying to measure your alternator's output, but forgot to set the meter to measure AC volts instead of DC, the meter is gonna read wrong and drive you crazy. The FIRST thing to check when you don't get the answers you expect, is that you have your tool set up right.

Second step is to check the battery fluid's specific gravity. The battery's SG is a good indication of its capacity to turn out current, that is, amps. 'course you can't do that if you have a sealed battery.

Third step requires no measurements. Go to the voltage regulator and find the single wire that leads back to the battery. Trace that wire by hand every damned inch of the way from one end to other and make sure that is has not broken or chafed somewhere. If you skip an inch, sure as hell, that's where the break will be. If it is chafed through somewhere, the electrons it carries are probably bleeding into the frame or whatever chafed it, not getting to the battery, so the battery is not getting its charge. If it is a big chafe, with steady contact to the frame, you'll get a dead short from the battery back through the wire. That will result in melted insulation, smoke, bad smells, and maybe fire.

Next, with the motor off, put the red meter cable on the positive battery terminal and the black one on the negative terminal. You should see at least 12.5 volts DC on the meter if the battery is freshly charged. And before you start all this, make sure it is freshly charged. A Harley starter typically won't engage at all below 10 volts, and it'll give you a hard time below 11.5. You can skip the test if you did the specific gravity check and it came out ok.

Many Harleys, especially Evos run that wire from the voltage regulator to the main circuit breaker, where it is clamped on the same post (the copper one) as the wire from the positive battery terminal to that circuit breaker. Don't be fooled by the routing though; the circuit breaker plays no part in controlling that current, it is only a wire junction. Some guys, wanting to avoid the possibility of a dead short will put in a dedicated circuit breaker, or even an inline fuse, for the voltage regulator wire. It's a good idea.

Now start the bike and let it run at 1500 to 2000 rpm. The meter cables should be attached as before. You should see at least 13.5 volts DC at the battery with no accessories or lights on, but not more than about 15 or your voltage regulator is allowing an overcharge that can boil the battery over time.

Sometimes you'll run into a "lazy" voltage regulator that responds slowly, and it'll run up into the high teens for a while. If so, just let the motor run a while to see if it comes down. If a battery is really drawn down you may also see 14 or 15 volts for a while, but it ought to come down as the battery charges. This is especially true if you've just had to use the starter hard. A starter draws an incredible amount of current.


If you aren't getting 13 to 13.5 VDC at the battery you need to work back into the system to figure out if it's the regulator, or the alternator.

You make that decision by elimination of the alternator as a problem. If it's ok, then the regulator must be bad.

To test the alternator you are first going to measure ALTERNATING CURRENT output directly from the alternator. It's less than 100 volts (actually, somewhere around 45 to 60 or so), so set your meter up for that level. It's easy; just follow me.


Unplug the voltage regulator from the hole in the front of the engine. Check the plugs for crud and corrosion, or even being burned away. If a plug gets loose, but doesn't come clear out, it can arc internally and eat up the prongs, so you lose contact. That can lead to a burned up alternator since it is pumping out electrons that have no place to go, so they sit inside getting hotter and hotter, and finally cause an alternator failure. If you find a problem with the plug, we'll go ahead and check the alternator output now, as long as we're here anyway, but you'll want to fix the plug afterwards. It may only require cleaning, or you may have to obtain and solder on a new one.

Set the meter up to measure AC Volts. If you leave it set to DC you are not gonna get the right reading. Insert the needle probes into each of the small holes you'll find in the hole the plug came out of. Since this is AC current there is no polarity - it doesn't matter which probe goes in which hole.

If you didn't get cables with probes on 'em, then you can cheat by taking finishing nails or stiff wire and putting one in each hole, then attaching the clamps to them, but DO NOT LET THE NAILS OR CLAMPS TOUCH EACH OTHER!!! It REALLY matters.

At this point the alternator is not spinning, (engine off, right?) so you'll see no current indicated. Check your meter AGAIN and make sure it is really set up to measure AC current less than 100 volts.

Now fire up the bike and let it idle. You should see somewhere around 20 volts, more or less; the exact amount's not important.

Roll the throttle on slowly up to about 3000 rpm. The meter should follow, giving you approximately 20 volts per 1000 rpm. The exact amount is not so important; you just want to see the numbers get reliably bigger at about that rate. It should top out somewhere around 60 to 75 volts.

If that's what you get, turn the motor off; your alternator is good, and the problem is with your voltage regulator. Before you toss it, make sure it is well grounded, that the bolts are tight; that there are star washers under the bolt head and nuts, and that there's no paint in the way. If all that's correct, then toss it; it's a non-repairable item. Buy a new one and install it, then check the battery with the motor running for 13.5 volts. If you have it, good. If not, swear hard and set about doing some complicated checking which I won't go into here.

If the alternator does NOT put out, then it's bad. Check to see why, not that it really matters, but it just makes you sound smart when you go into buy a new one and say "Got a shorted stator" or whatever to the parts guy, 'cause that's probably what you are going to have to do.

ENGINE OFF NOW. Disconnect the battery ground cable from the battery.

What you are gonna do now is check to see if one of the windings in the alternator is broken, or if something external to it has grounded it out. You'll do that by sending an electron from your meter into one of the holes. Since all those windings on the stator are really just one long wire, with an end in each hole in the plug, it should come right back out through the other one and be "seen" by the meter. If it doesn't come out, the wire's broken or grounded internally.

Switch your meter to the lowest OHMS setting. Take your two cables and touch 'em together; watch what the meter does. It should indicate zero resistance, for there's nothing in the way.

Now put one pin into each of the small holes, like you did for the last test. You should get a reading that is essentially zero, no resistance - that means that electrons from the red wire of your meter are flowing through the one long wire that is wound hundreds of times in the stator and coming back to your meter through the black cable. The alternator's winding is intact.

If it shows any resistance your wiring is broken or shorted internally.

Now the final stator check.

Pull the black cable out of its hole and touch the probe to the engine case. Your needle should show infinite resistance - there should be no current flow from the windings (where you are putting current from the meter, through the red wire) into the case (where you may it up again with the black probe). If you get a reading at all there is a current flow of some sort, and that means that your stator is shorted - something is touching both the windings and the case, and it should not be that way.

Any stator problem usually requires replacement, and that means pull the outer primary case, primary drive (clutch, chain, and compensator sprocket), and alternator rotor. It's really not a complicated job if you have book and it's a good one to learn on.

Now, a caution. There are other, more complicated problems that can arise and also drive you crazy. This explanation has been pure "bonehead" electrics, nothing complicated. If you can't straighten things out with this diagnosis, you got a big job ahead. It took me a year of starter problems, for instance, to discover that my problem was due to a brand-new starter that had its armature machined a few thousandths too long, so it was binding up as it heated from use. No way can I cover all of those possibilities here.


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