Checking the Harley-Davidson Charging System: Quick and Easy Electrical Diagnostics

Very often, especially during winter or when we ride our motorcycle less than usual for other reasons, we hear that ominous noise coming from the starter motor shaft: "Tac-Tac-Tac-Trrr." This is when we realize the battery isn't at its best, probably for two fundamental reasons: the battery is reaching the end of its useful life or the charging system is not working properly.

The charging system
In this article, we'll focus on the motorcycle's charging system, which "produces" the electricity the vehicle needs to operate, powering its electrical systems (ignition, lighting, sensors, accessories, etc.) and creating a small "extra" current to charge the battery, which will be used later during start-up.

The fundamental components of the charging system are, apart from the battery:

• The alternator

Composed in turn of the rotor (magnets that rotate driven by the crankshaft)

and the stator (a fixed winding).

This device generates electric current when the rotor magnets rotate around the stator windings, creating an induced current (there is no physical contact between the two elements; it is generated by variations in the magnetic field). The resulting current is alternating current (like that used in homes) and will have a higher voltage the faster the motor spins.

• The regulator-rectifier

This device receives the alternating current coming from the alternator and converts it into direct current (with positive and negative).

Once rectified (this is what this process is called), the regulator sends this current to the battery, but only what it needs, discarding the excess current and not allowing the charging voltage to exceed 15V, which could damage the battery or the electronic systems.

Checking the system
When checking the charging system, it's a good idea to follow a series of basic steps to help determine if any of the components aren't working properly. To perform these tests, you'll need a multimeter .

• Battery and connections

Before performing any other tests, it's essential that the battery is fully charged (around 12.6V or more; otherwise, we'll charge it until this voltage is reached) and that the terminal connections are properly tightened, as they tend to loosen due to vibration. In addition to this test, we must also visually inspect all cables, relays, fuses, and connections for any fluid leaks from the battery itself, frayed or burned wires, etc.

• Alternator

Let's check the alternator's current output. To do this, unplug the output connector (refer to your motorcycle's workshop manual) and connect the multimeter's test leads to the alternator output.

We put the meter in AC voltage measurement mode, then start the engine.

The voltage generated should be 18-20V per 1000 rpm, again we recommend consulting the manual for your year and model to verify this information.
If the output voltage isn't correct, turn off the engine and, without disconnecting the multimeter leads, switch to resistance measurement. The reading should be 0.1–0.5 ohms. Remove one of the two leads and touch it to the engine block. In this case, the reading should be "infinity." Repeat this last test with the other connector.
If these tests are correct but the output AC current is still insufficient, the problem is likely to lie in the rotor magnets; if any of the tests fail, the cause will be in the stator.

• Regulator-rectifier

When all the alternator tests are correct, a regulator failure should be suspected. The easiest way to check this device is to start the motorcycle (remember to plug in the connector we removed in the previous step) and measure directly across the battery terminals (this time with the multimeter in continuous voltage measurement). The voltage should be around 12.8-13V at idle, and when accelerating above 2000 rpm, it should never exceed 15V.

If this test doesn't give you the correct results, you'll probably need to purchase a new regulator-rectifier. Tip: Before replacing this component, make sure it has a good ground or connection to the chassis. Rust often causes it to malfunction. If this is the case, clean the connections thoroughly and retest.

Conclusion
Although it may seem like a complex operation at first, performing these tests will only take a few minutes and will allow us to know with considerable precision whether our motorcycle's charging system is operating correctly.
It's essential to rely on the data and tests in the workshop manual for each year and model, and to understand the basic operation of the multimeter you're using. This way, you can perform a quick, easy, and accurate diagnosis.

Frank Burguera

#harleymechanics