Question 1

When an alternator needs replacing

Accepted Answer

The alternator supplies power to all the car's systems and charges the battery while the engine runs. With the engine running, the battery should receive 13.8–14.4 V of charging voltage. When this voltage drops or fluctuates, the symptoms are obvious: a battery warning light on the instrument panel, lights dimming at idle and then flickering, the car stalling in traffic without warning, and the battery sitting dead in the morning — even if it's new.

Alternator failure is usually gradual. First a bearing issue starts, heard as a whining noise from the engine bay. Soon after, the voltage regulator starts to misbehave — the battery won't fully charge on short drives, or overcharges and boils the battery electrolyte. In the final stage the copper windings burn out and the alternator stops producing current altogether. With a multimeter the alternator can be checked at any time: across the battery terminals with the engine running it should read 13.8–14.4 V, and the battery's behaviour with the alternator disconnected tells you whether charging is happening.

Question 2

How an alternator is replaced

Accepted Answer

The battery negative terminal is disconnected first for safety. The alternator is located in the engine bay — usually at the front of the engine, driven by an auxiliary belt from the crankshaft pulley. The belt tensioner is pressed down with a dedicated tool and the belt is slipped off the alternator pulley. The alternator wiring (B+ charging lead and multi-wire control plug) is disconnected, the mounting bolts are undone and the old alternator is lifted out.

The new alternator goes in and the bolts are torqued to specification. The wiring is reconnected — the charging cable is tightened especially firmly, because a loose connection heats up and can cause a fire. The drive belt is refitted and the tensioner released smoothly. The battery negative is reconnected and the engine is started. The diagnostic tool measures charging voltage and monitors that it remains stable under various loads (air conditioning, lights, rear demist). On modern cars the alternator is also coded into the system so the engine ECU correctly manages the charge profile.

Question 3

Why do alternators fail?

Accepted Answer

Alternator service life is typically 150 000–250 000 km, but operating conditions can shorten it significantly. The most common cause of premature failure is water or salt intrusion — especially on alternators mounted low in the engine bay, where road salt reaches the bearings quickly in winter. Another common fault is overloading: if the battery hasn't been coded correctly into the BMS or the auxiliary battery is poor, the alternator has to push maximum current continuously and overheats. A third is belt tension — an over-tight belt stresses the bearings, a loose one slips and wears the contact surface. Regular belt inspection and battery coding at replacement noticeably extend alternator life.

Alternator Replacement in Vantaa