How to test and troubleshoot your charging system


There comes a time in most vintage motorcycle owners' lives when they suspect their charging system may have bitten the dust.

Luckily, these systems and not complex and fixing them is not terribly difficult. The process can be time consuming and identifying exactly which component is at fault is likely to take a few hours (especially for the first timer), but even the novice mechanic is capable of these procedures.

Before we begin, lets list out the three main components in your charging system and cover their basic function:
  • Alternator - An alternator (aka dynamo aka generator) is the device which actually produces the electricity for your bike. The alternator is made up of two parts, the rotor and the stator. The rotor is basically a giant magnet (which can be a permanent magnet or an electromagnet). The rotor generally spins in a concentric circle inside the stator. The stator is a series of many, many loops of copper wire, wound around iron cores. As the poles of the rotor's magnet pass by each coil, a voltage is induced along the coil and it travels to the rest of the charging system. It's important to note that an alternator produces AC, and not DC, voltage. This is because the magnets induce voltage in the coils as they approach and as they pass, so the voltage alternates its direction.
  • Rectifier - Most everything on your bike is designed to only use DC voltages and so the rectifier exists to convert the AC voltage from the alternator into DC. The standard design is a bridge rectifier. A bridge rectifier is a series of diodes arranged into a loop with an AC wire located in between each diode. The properties of diodes are such that current can only pass through them in one direction. So if you wire diodes into a big circle and connect AC wires in between each diode, you can guarantee that at any reading point in the loop, you're only getting current coming one way. It's very common to see rectifiers paired in the same physical unit as the next item on the list.
  • Regulator - The regulator's responsibility within your charging system is to ensure the voltages generated by your charging system are not s great that you start damaging circuitry further down the line. This can be done in a variety of ways, with the more common being a methods including bleeding the excess voltage out as heat, shunting the voltage directly back to the alternator, and/or adjusting the strength of the electromagnet within the rotor.

Testing Your Charging System
Testing your charging system is a simple process, but can take a while, especially if the necessary components are hidden behind air filters, gas tanks, seats, etc. For this test you will require a multimeter than can read both AC and DC voltage as well as resistance.

One of the first tests you should perform is a straight up test of the entire system. Hook up your multimeter across the terminals of your battery. You should see somewhere between 12.1V and 12.5V, DC. Now, start the bike and let it idle. This number is not likely to change. After the bike has warmed up a bit, rev the engine halfway to the redline and hold it there while you check voltages again. Voltages should now be reading well into the 13V range and be approaching 14V. If so, congratulations. Your charging system is working fine and you can skip the rest of this. :)

Assuming your voltages have climbed far over the 14V mark, it's time to make a judgment call. Give it a bit more revs and see if the number continues to climb. If it doesn't you're probably OK, but if you start topping 16V with more room left on the tach, you probably have a regulator issue. A faulty regulator will usually cause such symptoms as blown headlights and turn signals on a fairly regular basis.

If your voltages haven't climbed at all (or only a very little), it's time to dig deeper. It's possible that the regulator is still at fault, but we need to rule out some other things before we can determine for sure.

The next step is to test the voltages at the rectifier. If you have a combined regulator/rectifier, skip this step. With your bike idling, measure the DC voltage, and then the AC voltage coming from the rectifier, to the regulator or battery; usually a red wire or red/white wire. Depending on your setup you may see anywhere from around 12V to 40V DC at idle. There should be no reading when you measure AC. If you are reading any AC voltage, it's time for a new rectifier. Assuming no AC voltage, but you got in excess of 20V DC at this step, you're good to go. Your problem is likely your regulator. If you got essentially the same voltage reading you got when you measured your battery (around 12V DC), keep reading...

Now it comes time to test your stator. Our initial test will be for continuity, we're checking to make sure all the coils are still in place and there are no broken wires. Coming from your stator will be a number of wires, often with more than one of them being the same color. Refer to your service manual for this one, but there is usually one common wire and several phase wires. For Honda twins, this is usually a pink wire, several yellows, and a white. In this scenario, the pink wire is your common. Using your multimeter, measure the resistance from each wire to the common wire. You should have a very low value, usually measured as a couple of ohms. If you can't get a reading at all, you have a broken wire and likely a broken stator. If all of the circuits check out, it's time to do a live test.

Disconnect the stator from the rest of the electrical system and ensure your battery has a decent charge. Start the bike and let it idle. You may want to put on some gloves at this point, if you haven't already. Measure AC voltage between each phase wire and the common. You should see between 20V and 40V from each. Anything less (or even no voltage at all) represents a failure in the stator.

Assuming this test passes as well, it's time to really go hunting. Bad ground, shorted wire, dead battery... all of these are likely candidates for your issue, but your charging system has just been verified to be working, so that's one thing you no longer have to look at.
Nice write up!

I was taught not to unplug the stator during a live test (supposedly was risky to the Reg/Rec) but to probe the back side of the connectors to check the AC output.

I'm no EE, so I don't know for certain if this risk is real or imagined.
Yeah, probably not a good idea.

The implication is that you would unplug the alternator prior to starting the bike. Running the bike without the alternator hooked up won't cause any issues that weren't already present.
The risk depends on whether you have a permanent magnet alternator or an excited field alternator. The excited field type would definitely risk the Reg/Rec as disconnected, the output of the field coil would rise trying to keep the battery charged.

A PM alternator, disconnected, has little effect on the regulator. However, I would not connect it while running, as you may have some high voltage on the output of the alternator.

Excited field alternators are very different creatures than permanent magnet types. The PM are simpler in concept, as the regulator does not actively control the output until the output exceeds a predetermined voltage level. Excited field regulators actively, and continuously, are adjusting the rotors magnetic field to keep the output at the voltage. what is nice about excited field is that they can generate voltage at a much lower RPM than PM types.
Don't y'all use a load for testing alternator output?
Standard English practice is to connect a 1 ohm load resistor across the leads. This insures that the alternator can supply sufficient current. Voltage alone tells only half the story.
mydlyfkryzis said:
Excited field alternators are very different creatures than permanent magnet types. The PM are simpler in concept, as the regulator does not actively control the output until the output exceeds a predetermined voltage level. Excited field regulators actively, and continuously, are adjusting the rotors magnetic field to keep the output at the voltage. what is nice about excited field is that they can generate voltage at a much lower RPM than PM types.

Yes, and with excited field alternators, you also need to add checking the rotor's resistance and making sure the carbon brushes that send electricity for to it are in good shape as well. The testing is similar to testing the continuity on a stator, but rotors usually only have two connections. You check the resistance across the connection and compare it against the manual value (2-6 ohms for Hondas, normally the higher the better). No resistance means the rotor is shorted and bad, infinite resistance means that the rotor has a broken wire or two and is bad. You can check the functionality of the brushes by checking the rotor's resistance at the end of the alternator harness that plugs into the rest of the bike. The resistance readings should be similar to those you measured at the rotor itself.
Had a problem with severe misfire/ engine died, after 15-20 minutes of running. Tested everything from gas tank cap to spark plugs etc. Always started up fine when cold but started to misfire after a couple of miles. Electrics were fine, 14 V output and so. Almost given up on this, tried one last thing and changed the rectifier/regulator. Problem solved. I think one of the diodes in the rectifier quitted working properly when warm and started to emitt AC into the electrical system, causing the ignition system (Boyer) to go haywire.

Regards, Harry
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