What is a Regulator/Rectifier
The regulator rectifier is a combined unit. It does the rectifying part as well the regulation part. It is part of a battery charging system. It usually gets an AC power feed from the stator coil of the generator (alternator). Most modern regulator rectifiers and Stators form a three phase system, so there are three wires coming from the stator feeding into the regulator rectifier. The regulator rectifier then rectifies the voltage; that is it turns the voltage from AC into an undulating DC. The voltage is then regulated; the voltage being limited to a maximum of about 14.5 volts and feeds this regulated DC out to the battery. There are some single phase, regulator rectifiers as well which perform a similar function but there are only two wires coming from the stator into the regulator rectifier which is then fed to the battery in the same way as the three phase system. Why are there two different systems? Three phase is more efficient and single phase are cheaper to produce.
Types of Regulator/Rectifiers
There are two main types of alternator that are fitted to most bikes, each requiring a different type of regulator rectifier.
1) Permanent Magnet Rotor alternator (PMR) - This has permanent magnets that revolve with the engine (rotor), either inside or around a set of wound coils (stator) to produce power. These come in various shapes and sizes. Some have two output wires (single phase), but most have three (three phase).
2) Field Control Type (FCT) - This has a 'field' or 'exciter' coil that is in place of the fixed magnets. When supplied with power from the regulator this becomes magnetised. Some types have this coil spinning inside the output 'phase' coils to give power, and will have carbon brush connections. Others have a stationary field coil, stationary phase coils and have a metal rotor spinning between the two, and need no brushes. The amount of power supplied to the field coil from the regulator decides how much output the alternator will give.
How to test the Regulator
Check the battery voltage, with the engine not running. Start the bike (increase the rpm's up a little), the voltage should now be a couple of volts more than the original battery voltage. Check both voltages (running and not running) at battery terminals.
What can go wrong with it
If yours does fail. Before going to the time and bother and expense of replacing one, it is prudent to consider that there are usually contributory factors to a failure. You should conduct a thorough inspection of all other parts of the electrical system and verify that all components are in good working order and that the regulator was correctly mounted to allow dissipation of heat produced. You could also perform some basic post-mortem checks on the regulator/rectifier itself, and attempt to determine what has failed, internally if you have the necessary test equipment.
Total failure does not usually mean that every part inside the unit died at the same time. All of the parts share a common ground or hot connection; if the unit tests out totally dead, then this internal connection could have failed. This is typically due to either a manufacturing defect, overheating or metal fatigue from too many heating and cooling cycles. A failed connection can cause any of the observed failure modes, so keep that in mind: just because the device doesn't test out totally dead, doesn't mean that it wasn't defective or simply overheated one too many times.
If this happens, your battery will stop being charged, the lights become progressively more dim, and eventually the engine will stop. First look for a short or bad connection to the alternator stator coils. A bad connection can cause some serious voltage spikes, which can destroy diodes. Check also for a bad battery connection and any oxide build up on the terminals and connectors. A shorted battery or reversed terminals could cause the diodes to draw too much current and burn out. These symptoms could also point to stator failure so check the coil resistances and or output voltage if your meter has an AC range, to eliminate this.
Failed Shunt Regulator
If this happens, your headlight may become very bright and then blow. Your battery may have boiled dry also.
If the regulator burned out, check your battery connections, if they are loose or corroded the regulator has nowhere to route the output and so must get rid of the power produced in the form of heat. Also, make sure that all of your running lights are working; remember, the regulator sinks excess power, and generates a lot of heat in the process. If all of the lights aren't working, that's more heat for the regulator to get rid of.