New technology wields a double-edged sword. It’s the lament of every automotive professional when we’re faced with the diagnosis of systems we may not yet understand, and we’re quick to gripe about it, especially when we were comfortable with the previous iteration.

Then we realize – especially after we become comfortable with it – that the changing and new technology keeps our career interesting and creates perhaps the most important aspect of it: job security.

Electrical diagnosis always has been one of the more consistent challenges and is even more so with electrical systems and computer control being the hub of automotive technology, with no conceivable end. Rotating-electrical components – primarily alternators and starters – are more important than ever, playing an integral role in vehicle and electrical-system operation.

Starters no longer see the age-old abuse of extensive cranking, thanks to the dependability of modern fuel injection and ignition. Consumers and professionals alike are accustomed to quick starts, and from a service standpoint – which even extends to non-professionals – we know that if a vehicle doesn’t start within a couple seconds, it’s not going to.

The starter-frying mindset that cranking an engine non-stop until you drain the life out of a battery will mysteriously fix all ailments resulting in a magical start-up is luckily a thing of the past. Starters aren’t completely out of the woods, however, due to the popularity of the automatic start-stop system on today’s cars.

The automatic start-stop system improves fuel economy and reduces emissions, but demands more from the starter on a regular basis. The stress on the electrical system continues as the battery is asked to provide power more than it has been in the past, and, in turn, the alternator must keep up with the increased demand to keep the battery at a full state of charge.

Not only do alternators have to keep up with the demands created by the start-stop system, but vehicle-electronic systems in general also are consuming much more than in the past to power a myriad of control units and components. On top of this, battery usage with the key off is far more than ever, as many control units run tests and stay “awake” for several minutes before going into a sleep mode.

Misdiagnosing rotating-electrical components is a reality that can and will occur. The most important factor to start with is the battery. Plain and simple, if a battery cannot perform at its rated capacity – even if it can still crank and start the engine – it won’t be able to provide what the electrical system on a modern vehicle needs to operate correctly, putting additional stress on the rotating-electrical components. Starters and alternators and their related computer control may not function properly if the battery is in poor condition.

Starter diagnosis generally is cut and dry, and there are fewer misdiagnosis problems, with the most common symptom being a simple no-crank. Battery-cable connections should never be overlooked, however, and power to the solenoid wire should always be checked to confirm all circuits are good.

Alternator diagnosis is more involved than in the past – the primary reason being the computer control of alternator output, which is varied depending on the electrical consumption of the vehicle. A common cause of misdiagnosing a bad alternator is misunderstanding normal charging-system output. The old rule of thumb that we could go by was if you had 13.5 to 14.5 volts at idle, the alternator was charging properly. This is not always accurate with computer-controlled alternator output. Some systems will reduce output to as low as 12 volts if the power is not needed, and may raise it as high as 14.8 volts if many electrical-consuming components are running.

Startup charging voltage is often higher in order to replenish the battery and can drop considerably after a few minutes. This will be reflected directly on the voltmeter of so-equipped vehicles and can cause the misconception that charging-system voltage is erratic when, in reality, it is normal. Service information and understanding the normal operation of the system for every vehicle is the key to proper diagnosis.