By Mike Grage, Customer and Technical Support at The Bonneville Shop
Table of Contents
12 Volt Charging System – Quite often we get questions about electrical diagnostics, most common is the charging system and how to test rectifiers. Whether it is a solid state or the factory diode pack, they work the same. The first bit of advice I give less experienced technicians is to think of electricity like water, electricity flows the same way. The battery is the storage tank, the wires are the pipes, the stator is the pump, the key switch is the tap, the zener diode is a pressure relief valve, and the diodes in the rectifier are check valves/back flow preventers. I’m going to keep this article about the 12V systems used in British motorcycles in an effort to keep as simple as possible. The technical name for these charging systems are full wave AC generator (aka-single phase) and 3-phase AC generator. With all that being said, it’s time to grab your multimeter and read on.
How it Produces Electricity
The Lucas service manuals show 6 coils that represent the 6 poles in the single-phase alternator, wired in parallel pairs. Each time a magnetic pole passes across the coils the magnetism pushes electrons, also known as EMF (electromotive force). When the north pole passes by it pushes the electrons one direction, and the diodes direct the current where it needs to go.
The same thing occurs in the other direction when a south pole passes by. The diodes direct the current again, turning the negative AC wave into a DC positive wave.
In other words, the rectifier corrects (rectifies) the direction (DC) of the alternating (AC) current.
Stator Testing
The simplest test on a charging system is the stator, these tests are good for full wave (single phase) and three phase stators. There are two tests. Performing these tests at the rectifier will check the stator and the integrity of the wires coming from the stator, perform the test at the stator connections if the tests fail at the rectifier. The yellow wires on solid state rectifiers connect to the stator. First test is continuity between all stator wires, a good stator will have under 10 ohms between all wires. Second test is checking for a grounded stator. Turn your ohmmeter all the way up, put one lead in a connector, the other to ground, and repeat for all stator wires. There should be no continuity (infinite) to ground. A grounded stator can cause a stator fire, Moto Guzzi’s are well know for this so it’s not just our beloved Lucas prince of darkness that turns into the god of hellfire. There is a third test on the stator to check the integrity of the rotor magnets, the AC voltage output of the stator. Most service manuals do not provide a specification for this test. A couple decades of experience has taught me that at idle 25 VAC is good bottom number on a full wave system, and 40VAC on a 3-phase system.
Rectifier Testing
The next component to test is the rectifier. The purpose of a rectifier is to turn the AC electricity produced by the stator into DC electricity, this is done through diodes (electric check valves). Solid state rectifiers function the same way the Lucas diode pack rectifier functions, the main difference is a few components (capacitors, resistors, etc.) added to smooth out the flow of electricity. The good news is the continuity test procedure for a solid state is the same, but the readings will be a little different due to the extra components. This image from the workshop manual below will make explaining the continuity test easier.
The image above is good for analog multi-meters, digital multi-meter continuity tests flow the opposite direction.
All of our full bridge (single phase) rectifiers use similar designs and standard color codes. Yellow (stator) wires are #1 and #3, black wire #2, and the red wire is #4. A big thing to point out is that this test is for analog multi-meters, a digital multimeters continuity tests flow the opposite direction (black +, red -). The forward resistance checks should have 1-20 ohms of resistance. Your back leakage test should have no (infinite) continuity with an external regulator. Solid state rectifiers with an internal zener diode regulator will have no less than 5K ohm readings.
These same tests can be performed on the 6 diodes in 3-phase rectifiers too. The color codes are the same, only difference is two more diodes to test.
Regulator Testing
Now I have some bad news for people who buy solid state rectifiers to get away from zener diode regulators. Solid state rectifiers/regulators for AC generators use zener diodes for voltage regulation. A zener diode is the pressure relief valve for the charging system. Failures are similar in nature to oil system failures. When they fail in the open position the battery drains and charging voltage is low, similar in nature to leaking check valves in an oil pump. When they fail in the closed position there will be over 14V, much like an oil pressure regulator that is stuck closed will cause a variety of engine problems. There are two simple and effective tests that can be done without an oscilloscope. The first one is a charging voltage test at the battery, a good zener diode should not go over 14V. Next is an amperage draw test from the regulator (open zener diode check). With the key off and all regulator/rectifier wires connected, connect the ammeter (or multimeter with 10A capability) between the regulator/rectifier wire and the battery. A good zener diode will read 0 amps.
I would also like to point out that these diagnostic processes are not limited to vintage British motorcycles. Most of the modern (Hinckley) Triumph motorcycles use a 3-phase AC generator (exceptions are the liquid cooled triples that use a Nippon-Denso alternator). American and Japanese motorcycle manufacturers use these two charging systems on a variety of models too.
Thank you for your time, and I hope this guide helps.