Now that how an alternator produces electricity with the interaction of the stator and rotor has been discussed, how alternating current produced by the stator and rotor is corrected to direct current will be examined.
The device that changes alternating current to direct current has various names, such as diode trio, rectifier, or simply diodes. A diode is a one way electrical door that allows electrical energy to flow in one direction only. The rectifier is the device that contains as many as eight diodes, but usually only six, half of which are positive and the other half are negative. The positive diodes are insulated from the alternator. The negative diodes are not insulated. Because there is such a great deal electrical energy flow through the diodes causing excessive heat buildup, the rectifier may have a heat sink also. Since the stator has three separate windings, three phases of sine wave current or alternating current are produced. To rectify the sine wave or alternating current to direct current each individual winding has a positive diode connected on one end and a negative diode connected on the other end. Once the alternating current has been rectified to direct current through the diodes, or rectifier bridge, the current, or electrical energy, leaves the alternator through the battery term to the rest of the electrical demand on the vehicle. This process is known as full wave rectification.
Diodes can fail to function properly. A diode can become shorted or open. An open diode will cause one phase or winding of the stator to not produce current reducing the alternator output by one third. Alternators are rated by the maximum amount of amperage produced by the stator, for example, 15, 30, 45, 60, 75, 90, or 105 amps. The strength of the amperage is determined by the amount of wire windings existing on the stator, i.e., for example, fifteen loops versus thirty loops. Therefore if one phase or wire in the stator, or one diode were open, not creating a complete circuit, then, for example, a 90 amp alternator would only produce 60 amps max. A shorted diode would allow alternating current to enter the vehicle electrical circuits and cause damage to electronic components. If a positive diode is shorted it could cause the battery to drain when the vehicle is not running. If a positive and a negative are shorted then high current from the battery could cause the fusible link to blow or burn.
The rectifier bridge or diode trios are located inside the rear housing of the alternator and can be accessed by disassembling the alternator and removing the diode assembly. Once the rectifier bridge or diode trio has been removed, the digital volt ohm meter or multi-meter can be used to test the diodes. Using the ohms scale on the meter the diode can be tested for continuity in one direction and no continuity in the opposite direction. If the diode has continuity in both directions, then it is shorted. If there is no continuity in both directions, then the diode is open. If either one of these conditions exist, then the rectifier bridge or diode trio will have to be replaced.
As always, if any procedure in this series of articles appears to be beyond the capabilities of the vehicle owner or driver, then testing and servicing the alternator should be performed by a professional or ASE Master Certified mechanic. The vehicle would have to be taken to a repair shop that employs these types of mechanics such as A & M Alternator Services located at 2419 E. Jackson St. in Phoenix, Auto Electric Specialists located at 5216 W. Lamar Rd. in Glendale, Village Auto Electric Service located at 19 N. Miller St. in Mesa, All Start Electric located at 13501 E. Chandler Blvd. in Chandler, Jordan’s Automotive Specialists located at 8718 E. McDowell Rd. #3 in Scottsdale, or Rob’s Quality Automotive located at 11801 N. Cave Creek Rd. in Phoenix