One of the most common issues that cause people to buy unneeded parts is a dead battery. Whether it’s replacing the dead battery with a new one or throwing it on the charger and replacing the voltage regulator. If you go about it this way, there’s a 66% chance your going to end up with another dead battery which really puts a dampener on a solid day of riding.
Even though this sounds cliche, to be able to diagnose a charging problem you first need to understand how it works.
There are three components in the charging system. The battery, voltage regulator/rectifier, and the stator.
Harley’s use a permanent-magnet alternator to keep the battery fully charged. This is located inside the primary on the left side of your engine block. It consists of a stator (a group of field coils) mounted to the engine block and a rotor which is splined to the crankshaft. Magnets are fixed to the rotor and as the crank rotates, the magnets move over the coils producing AC current. As your RPM gets higher, so should your voltage.
Before you can use this power to charge the battery, you first need to convert it to DC power using a rectifier. I will go into more detail about how this works in another article. To reduce space and cost, most motorcycles including Harley Davidson’s have incorporated them into the voltage regulator, which does exactly what you would think. It regulates how much voltage is being sent to the battery.
How to diagnose Your charging systemWarning: These tasks should be performed by an experienced proffesional. What you need: Multi-meter
Before you start running any tests, you first need to make sure that your battery is fully charged (12.6V +). It is also a good idea to check the year the battery was manufactured. Personally, I recommend replacing them every 4 or 5 years even though I’ve seen batteries over twice that age being used.
Voltage Output Test
Once you know that the battery is in good condition, the next step is to run a voltage output test. The purpose of this is to see what voltage is at your battery while it is running. Start your bike, set your multi-meter to DC Volts and connect it to your battery (+/+ and -/-) . With the bike running at 3000 rpm you should have a constant 13.0v or more. As you raise the rpm the voltage should rise but never go over 15.0v. If everything seems to be working, go on to the milliamp draw test. If not, perform a stator check.
To check your stator, locate your voltage regulator (usually mounted on the lowest part of the frame in front of the engine) and trace the group of wires that lead into the left side of your engine block. Find the connection between these two points and disconnect it. Set your multi-meter to Ohms. Check the resistance between the pins on the stator side. The reading should be less than 0.5 ohms. If higher, replace. Next check the resistance between one of the pins and a primary bolt. There shouldn’t be any continuity between these points. If there is, your stator is grounding out and needs to be replaced.
AC Output Check
With the stator still disconnected, connect the multi-meter to two of the pins on the stator side with it set to AC Volts. With the bike running, it read about 18.0v ± 2 per 1000 rpm for a 38 amp charging system and 24.0v ± 2 per 1000 rpm for a 45 amp. If your readings are low, the stator should be replaced.
Although there is no official test for your voltage regulator, if the battery is in good shape and the tests performed on the stator all check out perfect; In most cases this means that the regulator should be replaced.
Milliamp Draw Test
If your voltage output test passes but your battery still goes dead, there is a good chance that there is a draw somewhere in motorcycle. Disconnect the negative battery cable and connect your multi-meter in series from the cable to the battery negative. With the bike off, set the meter to mille Amps. Readings will vary from bike to bike depending on how many accessories it has. Generally you need to be concerned if your bike has a draw of more than 10.0mA. The easiest way to find a draw is to unplug each accessory one at a time and redo the test until you noticed a significant drop.