Operation,

When the ignition key is turned to the ON position, electricity flows from the ignition switch to the CHARGE light and then to the D+ terminal of the voltage regulator. (The CHARGE lamp internal resistance acts to limit the current and to give a visual indication that the lamp is OK). This small current then flows through the voltage regulator from the  DF terminal on the voltage regulator to the DF terminal at the brush holder at the rotor, then into the rotor via the carbon brush and slip ring. It then flows out the rotor via the other brush and slip ring to ground. This makes a complete electrical circuit, and the small amount of current flowing through the rotor induces a small magnetic field in the rotor. The CHARGE light will be brightly lit.

When the engine is started and the rotor is rotating  the rotor magnetic field is transformed into AC  in the non-moving stator windings. Six small diodes on the diode board change a small amount of the produced electricity into DC, and this is then supplied back to the voltage regulator. As engine RPM increase to the point that the small diodes can supply 12V to the regulator, it reaches the point where the voltage regulator has 12v on both sides. The voltage drop across the voltage regulator becomes zero and the CHARGE light will extinguish.The small diodes continue to produce more current as engine RPM increase and fully power the rotor . From this point on the system no longer requires excitation and it continues to supply itself. 

If the rpm are high enough (or system needs low enough), the alternator could produce too high a voltage to the battery and the rest of the bike. The voltage regulator therefore has an internal voltage reference and compares this to the output of the six small diodes, previously described. As the voltage rises above the reference point the voltage regualtor reduces the current flow into the rotor which then reduces the current flow into the rotor and in turn reduces stator output

There are also six high power diodes mounted on the diode board. They are connected to the stator main output windings (same as the 6 smaller diodes). As the rpm, and therefore the voltage in the alternator stator rises higher than the lamp-extinguishing point, these large six diodes begin to pass current directly to the battery. The current is rectified from AC to DC by these 6 large diodes, which consist of 3 positive and 3 negative diodes. The system relies on the high resistance of the diodes to prevent reverse flows from the battery and discharging the battery when the the engine is not running.

Common problems

There are three common failure modes of the CJ750 alternator system.The alternator charging light (or lack of it) can provide a diagnosis of the system problem.

If the alternator light does not come on when the bike is switched on with the engine not running

The most common alternator failure mode on a 12v CJ750 is caused by an open circuit in the excitor system for the charging system. The easiest way to determine if an open circuit exists is to check if your alternator light comes on. If the light does not come on when you switch on the bike, with the engine not running then it is safe to assume that your charging system is not working

1) Check that the Alternator light bulb is good.Wiring color codes and even alternator warning light color can differ from bike to bike, so to identify the charge light check which of the bulbs in the headlight shell has one side connected to the power distribution bar and the other side is coonected to the voltage regulator. If the alternator charge light bulb is blown the alternator will not charge.

2) Remove the front engine cover and trace the wire that runs from the voltage regulator down to the front of the alternator (DF). Ground this wire and check if the alternator light comes on. If it does come on, proceed to step 3. If it doesn’t, then go back and check again for 12v+ feed in series from the ignition switch to the alternator bulb holder,then to the D+ terminal on the voltage regulator and then from the DF terminal on the voltage regulator to the DF terminal on the stator housing.if the circuit is not broken somewhere the light must illuminate when the DF terminal is grounded.

3)   If the Alternator light comes on when the warning light wire is grounded, check the rotor for open circuit. To do this first lift the brushes and hold them away from the brass slip rings. Using an Ohmmeter, check if there is continuity between the two slips rings. The meter should read a resistance of approx 4 ohms. If there is no continuity the rotor needs to be replaced.

If the alternator light comes on but does not go off when engine is running

1)With the brushes lifted away from the sliip rings test the rotor for short circuit by checking with an ohm meter for a short circuit between the rotor slips rings and the body of the rotor. There should be no electrical connection  beween the slips rings and ground – if there is your rotor is faulty

2) Stator failures are uncommon, however to test your stator measure between the “N” terminal on the stator and each of the three phases individually. The resistance of each phase should be approx 0.6 ohms.

3) To test the voltage regulator simply bridge the D+ and DF wires. If the light goes off then the voltage regulator is faulty.

If the alternator light glows dimly when increasing engine RPM.

The most common cause of this symptom, is a poor earth or battery connection, problems with the brushes  or the loss of one of the 3 phases from the alternator. It is recommended that before replacing the diode board the voltage regulator and brushes be checked  as Diode board failure is not common.

Alternator system & troubleshooting

Circuitry

The CJ750 is fitted with alternator  type MJF-280B.This is a 3 phase negatively grounded AC generator, whose rated power is 280w at 14volts. See below for charging system schematic

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