SMART ALTERNATORS

While the real-world environmental advantages of start-stop and regenerative braking technologies are unclear, it is certain that the more sophisticated batteries and alternators are more expensive to replace, compared with conventional technologies. So, whether you are performing the work yourself, or having a garage check the system for you, be aware that misdiagnosis, or a misunderstanding of how the systems work, can result in parts being replaced unnecessarily and involve you with extra expense.

Essentially, to reduce emissions, the alternator output is not controlled by its own regulator that is reacting solely to voltage input. A more sophisticated control system, located either within the engine ECU or as a separate module that can be part of the sensor body, is mounted to the negative pole of the battery on newer cars. This makes the alternator work at maximum capacity only if certain conditions are met. As batteries can accept higher voltages when they are cold, the ECU can instruct that more current be fed to the battery without overheating it. Various battery charge and state of health algorithms are used, including estimating the battery temperature using the engine air intake temperature sensor, which provides a fairly accurate indication of underbonnet temperatures. This means that testing an alternator for a constant voltage is unsuitable, because the output fluctuates, so do not judge smart alternators by the same measurements as those taken from conventional alternators.

More involved interrogation should be carried out for an accurate diagnosis, as demonstrated on a 2017 Ford Mondeo using a Picoscope, made by Pico Technology (picotech.com).