Smart alternators – avoid misdiagnosis

Posted on January 4th, 2019 by Rob Marshall

Smart alternators - avoid misdiagnosis

Connections required to test a smart alternator with an oscilloscope. Image provided courtesy of Pico Technology 


While the internal combustion engine continues to receive a hammering from environmentalists, it is easy to forget that the proliferation of electronics in modern cars has made them far less polluting and considerably more efficient. The humble alternator, which charges the battery, is one such item that has evolved. Should you experience battery problems and you suspect that the alternator is not recharging it properly, be aware that ‘traditional’ testing methods may lead you to think that the expensive part is faulty, when it might not be to blame after all.


Old vs. New

So that they do not overcharge the battery, ‘traditional’ type alternators possess an internal regulator that monitors battery voltage. Should the voltage dip, the alternator will generate more current. The usual way of testing these units is to apply a multimeter across the battery terminals to check that the voltage does not dip above, or below, 13 volts. This should be checked with and without the headlights, heated rear windscreen and interior fan switched on – the reading should remain fairly consistent.


Yet, newer and more sophisticated cars, typically those that are fitted with stop-start and regenerative braking systems, tend to have ‘smart’ alternators that do not rely on battery voltage feedback alone but consider other parameters. For example, as batteries can tolerate a higher rate of charge when cold, a smart alternator can detect this and produce a higher voltage. Similarly, in certain conditions, they can produce a lower voltage, too.


In any case, the usual method of testing for a fixed voltage is inappropriate for a smart alternator and a professional oscilloscope is needed to check alternator output, as well as the signals that it is receiving and sending back to the controlling ECU. Ensure that your garage has used the correct equipment to diagnose your smart alternator, before you request that it is replaced.