Parts for your 2017 Subaru Legacy-Alternator

2017 Subaru Legacy alternator — what it does and when to service it

Based on the 2017 Subaru Legacy factory service manual (Charging System section) and Subaru’s genuine parts catalogue, this model uses a 12‑volt Denso alternator (Subaru labels it a “generator”): 110 A on 2.5i FB25 models and 130 A on 3.6R EZ36 models. So yes—an alternator is very much part of the vehicle and central to everyday reliability.

The alternator’s job is to keep the battery topped up and power everything electrical once the engine’s running—headlights, heater fan, wipers, infotainment, EyeSight support systems where fitted, and more. On the 2017 Legacy it’s a smart‑charging unit, the ECU can vary output to suit loads and battery temperature, which helps fuel economy and battery life. If it underperforms, owners might see a battery light on the dash, dim lights, slow cranking, or random electrical gremlins.

As part of regular servicing, it pays to give the charging system a quick once‑over. A healthy alternator will typically show around 13.8–14.6 V across the battery with the engine idling, lights and A/C on. Because this model’s regulator is ECU‑controlled, voltage can float a bit—so testing under different loads is smart. If voltage sags below the mid‑13s under load, or spikes well above the high‑14s, it’s time to dig deeper.

• Listen for bearing whine or grinding from the alternator area under the bonnet.
• Inspect the drive belt for cracking, glazing, or frayed edges, check the automatic tensioner tracks smoothly.
• Test battery health—weak batteries overwork alternators and mask faults.
• Keep terminals clean and tight, poor grounds can mimic alternator failure.

When replacement is needed, quality matters. Denso‑spec units match the factory output and LIN/ECU control. For 2.5i owners, look for ~110 A spec, for 3.6R, ~130 A. Always disconnect the negative battery terminal, protect the ECU plug on the alternator, and torque mounting hardware correctly. After fitting, clear any stored charging codes and verify output with the engine fully warm, lights, demister, and blower on. Many workshops will also load‑test the battery and run a parasitic draw check to make sure a sneaky drain isn’t what cooked the previous alternator.

With sensible maintenance—belt checks every service, battery tests annually or before long kilometres—the 2017 Legacy’s alternator should deliver years of hassle‑free motoring.

Popular questions about the 2017 Subaru Legacy alternator

What are the common signs the alternator is failing?
Drivers often notice the battery warning light, dim or flickering headlights, a flat battery after short trips, or a whining/grinding noise from the front of the engine. On smart‑charge Subarus, erratic voltage can also show up as touchy idle or random warning lamps. A quick voltage test at the battery with accessories on will usually point the way.

What voltage should it produce?
Typically between about 13.8 and 14.6 volts at the battery with the engine idling and electrical loads on. Because the Legacy uses ECU‑controlled charging, seeing it float a little lower during light load isn’t unusual. If it can’t hold mid‑13s with lights, A/C, and demister on, it’s worth scheduling a charging system test.

How long do these alternators last?
On well‑maintained cars they commonly run well past 150,000 km. Heat, a tired battery, or a slipping belt can shorten lifespan. Keeping the belt and tensioner in good nick and replacing a weak battery early are the easiest ways to help the alternator go the distance.