Charging System Diagnostics

Most people take it for granted that when you push the starter button on your motorcycle, it'll turn over and fire up. My friends and I were taken by surprise before a ride recently, when his motorcycle refused to turn over. It had been parked for a couple of weeks, but that wasn't long enough to allow the battery, which had been replaced a few months earlier, to drain. I suspected a problem with the charging system, and after a couple of tests, I was proven right.

All modern motorcycles operate on a 12-volt DC charging system, which consists of three main components: a battery, an alternator and a regulator.

The battery stores the energy needed to start the motorcycle and operate all of the electrical components, even when the motorcycle is running. The alternator provides unregulated AC (alternating current) electricity, which is eventually used to maintain the battery's charge while the motorcycle is running. The regulator, a small finned box, performs two functions: it regulates the voltage leaving the alternator, which increases with engine revs to approximately 12 volts, and it converts alternating current to direct current (DC).
Aside from a service manual and a couple of basic hand tools to access the various charging system components, the only other item you'll need to verify the charging system is a multimeter with AC/DC voltmeter and ohmmeter functions. But before performing any tests on the charging system, make sure the battery fluid is at the proper level (no need to check this on a maintenance-free battery), and that it is fully charged. A charging system, despite its name, will not charge a dead battery, and a good battery is needed to perform the various tests.

To be sure the battery is charged, attach a battery charger to it. Once the battery is fully charged, it should read more than 13 volts DC. Even though motorcycle electrical systems are rated at 12 volts, they actually operate at more than 14 volts when the engine is running. Note that a reading of 13 volts at the battery isn't necessarily an indication that is is in good condition, but it's enough power to start the motorcycle and perform charging-system tests. A proper battery test should be performed using a load tester, usually found at an car or motorcycle garage.

To test the charging system, run the engine and place the multimeter leads (reading DC volts) across the battery terminals. With the engine revving at about 3000 rpm, the voltage should read more than 13.5 volts but not more than 15 volts. On my friends motorcycle, voltage at idle was about 13 volts, which is normal, but when the engine was revved, the voltage actually dropped, reading about 12.5 volts at 3000 rpm or more. This told me that further testing was needed.

The next item to verify is the alternator. Locate the alternator connector plug – it usually has three large-guage wires coming off the stator, and most of the time they're yellow. With the ignition off, disconnect the plug and attach the multimeter leads (in the ohmmeter function) to any two of the three wires on the alternator side. The service manual will tell you what the resistance should be when you check any two of the three wires (on our motorcycle it was 0.2 to 0.55 Ohm), and they should all have similar readings. Also check between each wire and ground by connecting one lead to one wire and the other lead to a metal part on the motorcycle. Any resistance here means there is a problem; it should be an open circuit.

There's one more check to perform on the alternator. With the engine running, connect the multimeter leads (meter set to AC voltage) to any two alternator wires while revving the engine. Again, the specs here vary, but the stator should produce about 10 to 15 volts AC for every 1000 rpm. My friends motorcycle read 72 volts at 5000 rpm, so it was fine.

Since the Motorcycle of my friend only registered 12.5 volts at the battery with engine revving, the alternator passed all tests, and all wires and connectors proved intact, the only culprit left was the regulator. There is a procedure in the manual to check all the wires coming out of the regulator, but I was confident enough in my testing procedure that I skipped this step.

I ordered an aftermarket regulator unit through a international online motorcycle shop at one-third the cost of the OEM regulator, plugged it in and immediately got 14.2 volts across the battery terminals at 2000 rpm. The motorcycle starts now as new....
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