Rotating electrics 30 Looking beyond voltage in alternator diagnosis DENSO explains why modern charging systems can make alternator diagnosis less straightforward, and the checks technicians should make before deciding the alternator itself is at fault. Alternator diagnosis has become more complicated as charging systems have evolved. On many modern vehicles, the alternator regulator communicates with the engine ECU, with charging voltage deliberately varied according to driving conditions, battery state and electrical load. Some systems can reduce charging during acceleration and increase it during deceleration to improve fuel economy. As a result, charging voltage on these systems can vary rather than remaining at the relatively constant level technicians may have expected from earlier charging systems. DENSO recommends taking a systematic approach to charging system diagnosis, starting with the battery. Before investigating the charging system, the battery and its terminals should be visually inspected and the battery fully charged. Open-circuit voltage should be approximately 12.6V or above, with a battery load test carried out if the voltage is satisfactory. A weak or defective battery can produce symptoms associated with charging system faults. The wider system should also be visually inspected. This should include checking the condition, alignment and tension of the drive belt, along with wiring, connections and alternator grounding. Connections should be clean, secure and free from corrosion, while the alternator should also be checked for physical damage, contamination or signs of overheating. Voltage-drop testing can be used to identify wiring and connection faults. On the positive side, DENSO advises measuring between the alternator B+ terminal and battery positive post with the engine running at around 2,000rpm and electrical loads switched on. Voltage drop should be less than 0.2V. The negative side should also be checked by measuring between the alternator casing and battery negative post, where voltage drop should be 0.2V or less. Excessive voltage drop can indicate poor connections, damaged cables or grounding problems. Regulated voltage and alternator output current should also be measured with the engine running at approximately 2,000rpm, with major electrical loads switched on for the current measurement. The results should be compared with the vehicle manufacturer’s specifications. An alternator can also be tested on a test bench to determine whether its output is within its performance specification. DENSO says that if the test shows the alternator to be out of specification, it should be replaced. If its output is within specification, technicians should investigate the remainder of the vehicle’s charging circuit and other electrical circuits that may affect charging system performance, referring to the vehicle manufacturer’s service information for the appropriate procedures. For modern smart-charging and Stop & Start systems, DENSO says a scan tool is required to distinguish between system, ECU/regulator and alternator faults. Monitoring functions and active tests can be used to determine whether the issue lies in the alternator or the control system. An oscilloscope can also be used to examine alternator ripple. A normal ripple pattern indicates correct diode and stator operation, while abnormal patterns can identify open or shorted diodes or stator winding faults. Modern alternator diagnosis therefore involves more than checking charging voltage. DENSO’s procedure covers battery condition, the belt drive, wiring and connections, voltage drop, regulated voltage and output current, as well as scan-tool testing on smart-charging systems and alternator ripple testing. www.denso.com/uk/en
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