The car owner found it hard to believe that all the faults were caused by this part.

A driving school’s Volkswagen Santana pulled into the shop. As the coach stepped out, he told the mechanic, “The car ran perfectly fine this morning, but something’s wrong this afternoon. The AC isn’t blowing cold air like it was earlier, lots of warning lights lit up on the dashboard, and the steering wheel even locked up just now—it’s really strange.”

The mechanic asked the coach to turn on the AC while he inspected the engine bay. The low-pressure AC pipe felt cold, meaning the compressor worked fine. But the cooling fan spun too slowly; it should run at high speed to dissipate heat when the AC is on. The engine also shook violently.

Glancing at the dashboard, the mechanic saw plenty of yellow and red warning lights illuminated.

Connect the diagnostic computer, and the trouble codes shown in the picture appear. The mechanic explained all the faults indicate low voltage. The vehicle’s power supply comes from the battery and the alternator, and the battery gets its charge from the alternator. It is highly likely that the alternator is defective.

The mechanic grabbed a multimeter right away. With the engine running, he connected its probes to the positive and negative terminals of the battery to test the charging voltage of the alternator. The normal reading should be above 13 volts, yet the measured voltage was only 10 volts. What does this mean?

It must be the faulty alternator!” the mechanic said. He then decided to remove the alternator, as shown in the picture below.

The mechanic said he would take the alternator apart to show the driving coach exactly where the fault lay. The picture below shows him removing the rear plastic cover of the alternator.

After taking off the rear plastic cover, the faulty component came into view, marked in the green area in the picture below—the contact point between the carbon brushes and the rotor. The mechanic pointed out abnormal wear on both the rotor shaft and the carbon brushes.

The carbon brushes suffered abnormal wear, as seen in the picture below. The one on the left is almost completely worn down.

Check the contact area between the rotor and carbon brushes: deep grooves have formed from wear. The mechanic explained that if this rotor surface were less worn, only replacing the carbon brushes would suffice, but now the entire assembly needs to be changed.

“Does that mean a brand-new alternator has to be fitted?” the coach asked.

The mechanic then started the engine and cleared all the trouble codes, and everything worked perfectly afterward. Seeing this result, the coach said he never realized how vital the alternator was and that insufficient charging could trigger so many malfunctions.

The mechanic explained: "When your car first arrived, the alternator had already stopped generating electricity even though it looked intact. The cooling fan, blower motor, ignition system and dashboard lights all rely on power from the alternator. Back then they ran solely on power stored in the battery, which was far from enough. As the battery drained, numerous warning faults popped up. Now that the alternator charges properly, all issues are resolved."