I thought I would spend a bit of my spare time while its raining outside to sit down and describe the 25 amp ND alternator as found on my 1998 Sprint Executive. Contrary to what’s been written before, motorcycle electrics are not as mysterious as people would have you believe. With a good understanding of the basic operation theory I’ll cover here, there isn’t anything complicated or mysterious about the charging system on our Triumphs. Additionally, it’s my belief that anyone capable of reading this and understanding it is capable of doing a teardown and component replacement on the ND 25 amp alternator…with one small caveat. One component, the rectifier, is a soldered in component. To replace this item you’ll need a good 40 watt soldering iron. The other commonly failed components like the regulator and brushes are easily replaceable with only a Phillips screwdriver. There are other components present that might be a bit of a challenge to service. I’ll cover those items and let you decide if you feel comfortable dealing with it, or if a trip to the alternator shop would be better.
The ND 25 amp alternator overview
Most people probably already know most of what’s this section. On the off chance that you’re completely unfamiliar with the alternator and how it works, here’s a close-up of what I’ll be describing.
The ND 25 amp alternator as used on various Triumph motorcycles is part number T1300000-T0301. The actual Nippondenso part number as shown above is 100211-4950. As you can see, it’s a compact device that can easily be removed and replaced as a complete unit.
The theory of operation is quite simple. When you turn the ignition switch on, the alternator draws a few amps of battery power to turn on its internal electromagnet, creating a magnetic field. The electromagnet is comprised of many turns of thin gauge wire wound over special shaped iron core mounted on a rotating shaft, collectively called a “rotor”. When you start the engine, the rotor and its battery powered electromagnet spins inside coils of heavy gauge wire called the “stator”. The mere action of rotating a magnetic field within a coil of heavy gauge wire induces a voltage in the stator that can be used to power devices. In essence, this is all the alternator does, however it does require a few other components to be a reliable source of power on a motorcycle.
Because of the nature of the alternators design, the voltage induced in the stator windings is of an alternating nature (AC) and it must be converted or “rectified” to a constant or direct (DC) nature for the devices on the motorcycle to use. This is done by the rectifier which converts AC to DC by letting the voltage only flow one way.
With nothing to control the alternator voltage output, the faster the engine turns, the more voltage the alternator puts out. This unregulated voltage could exceed system design voltage if left unchecked, and would cause serious overcharging issues of the battery and possible electrical damage to the igniter if not for the intervention of the voltage regulator. It is the voltage regulators job to sense voltage output of the alternator and hold the voltage as constant as possible given a varying engine speed and varying electrical demand.
Once alternator voltage output reaches the voltage regulator set point (~14.5 V), the regulator responds by reducing the amount of current passing through the rotors electromagnets (which have been using full current given to them up to this point). By giving the electromagnets less current to work with, the rotors electromagnetic field becomes weaker. With a weaker magnetic force spinning inside the stator, there is less voltage induced in the stator coils and therefore less voltage out of the rectifier. Once the system voltage drops below the voltage regulator setpoint, the regulator responds by allowing the rotor electromagnets to use more current which makes the rotating magnetic field stronger, which makes induced voltage higher. This is how the regulator regulates system voltage output and it has the ability to regulate this up and down rotor current action (and therefore alternator output voltage) within milliseconds of a detected system change.
As you turn on electrical loads like headlights, the system voltage sags or drops as that device consumes power. As described above, the voltage regulator senses this voltage drop and responds by allowing more current to pass into the rotors electromagnets making the magnetic field stronger. This causes a system increase in induced voltage, up to the regulators setpoint. In this way the regulator watches for system voltage dips as you turn on electrical devices and responds by bumping up rotor current to maintain system voltage at setpoint. As you shed load, the regulator reduces rotor current which drops system voltage to maintain set point.
On the 25 amp ND alternator, the regulator is mounted internally. Contrast this to the other type of alternator used on motorcycles that have windings mounted internally to the engine, use rotating permanent magnets mounted to the crankshaft instead of electromagnets on a rotor and require an external rectifier and regulator units. In this respect, the ND 25 amp alternator is much more like an automotive alternator, and that should come as no surprise. The ND unit as used on various Triumphs, Kawasaki’s and other motorcycles actually is a much smaller version of the ND automotive alternator. The smaller motorcycle alternator however works exactly the same way as its larger automotive cousin.
Generally speaking, the faster the engine turns, the faster the alternator turns and the more “power” the alternator is capable of producing for things like charging the battery and running essential electrics (essential electrics are the coils, the alternator itself, and the igniter). Because of the way that the alternator is constructed (the size of the wires and electrical rating of the internal components used), it is only able to produce approximately 25 amps at full output. If your electrical accessories demand more power than the alternator is capable of producing, you will slowly deplete the battery and at some point, there won’t be enough power to run essential electrics.
In cases where the electrical demand is more than the alternator is able to keep up with, it is sometimes possible to replace the stator coil and rectifiers for upgraded units for more power, but I am not aware of these upgrades to the 25 amp ND alternator. There are documented cases of fitting the larger 45 amp ND alternator in place of the 25 amp unit. With a bit of electrical wiring modification, it is possible and has been successfully done.
Electrical Block Diagram
Electrically, the 25 amp alternator is simple with only three components that typically wear. The rectifier has diodes that can short or go open, the regulator has pass transistors that can short or fail open and the brushes wear over time and lose electrical connectivity with the slip rings. Each of these components is easy to replace and the alternator does not have to be removed from the motorcycle.
25 amp ND Breakdown
You know where the alternator is and you know how to remove it if need be, but for general testing and minor component replacement, it is not necessary to remove the alternator from the motorcycle. I’ve dismounted this one as to completely strip it down and test and measure various components. Depending on what your intention is, you probably won’t have to be this severe.
Start by removing the rear cover from the alternator. It’s held on by three, 8mm nuts. Once you have the rear cover off, you can access the important components. After removing the rear cover, here is what you should see. I have labeled each component as to what it is and the picture also gives you a good clear shot of where the electrical connections go, so don’t be afraid to remove components for testing or inspection.
For general component testing, I recommend removing each component and test it off of the alternator. To do this, start removing all of the Phillips head screws visible in the photo. They will all need to come off and you really can’t mix up fasteners so there are no worries. You wont be able to completely remove the rectifier unless you either clip the wires that are soldered on the rectifier or unsolder them. Unless you suspect that the rectifier is bad, simply leave it in place and test it as described later.
Linear Mode
