Parts for your 2017 Subaru Impreza-Alternator

2017 Subaru Impreza alternator: what it does and how to look after it

For the 2017 Subaru Impreza, an alternator is absolutely fitted and relevant. Technical sources including the Subaru 2017MY Impreza Service Manual (Charging System/ACG), the Subaru Genuine Parts Catalogue for the FB20 engine (listing the alternator/ACG assembly), and the 2017 Owner’s Manual (charging system and Auto Stop-Start notes) all confirm the vehicle uses an engine-driven alternator with an internal regulator and ECU-managed “smart charging”.

Its job is straightforward: keep the 12‑volt battery charged and power everything electrical while the engine runs. On the 2017 Impreza, charging is actively managed by the engine control module, working with a battery current/temperature sensor to optimise voltage for battery life and fuel economy. On models with Auto Stop-Start, the system demands an OE-spec alternator and an EFB or AGM battery so it can cope with frequent restarts and higher electrical loads.

As part of routine servicing, a quick charging-system check under the bonnet goes a long way. Look over the serpentine belt for cracks, glazing, or frayed edges, and listen for chirps that could hint at pulley or tensioner wear. A healthy battery is crucial to alternator life, so test state of charge and conductance, and clean and tighten the terminals, especially the battery sensor on the negative post.

When replacement is due, stick with the correct output rating for the FB20 engine and stop‑start spec. Disconnect the negative terminal first, protect vehicle memory if needed, and avoid straining the battery sensor loom. After installation, many workshops perform a battery/charging system relearn using a scan tool so the ECU’s smart charging targets and the stop‑start system behave properly. Routing the B+ cable neatly and confirming all mounting points are clean and tight helps prevent voltage drop and noise.

A basic test with the engine idling should show roughly mid‑13s to mid‑14s volts, but don’t panic if the reading dips during light cruise—smart charging can lower voltage to reduce load on the engine. If the battery lamp comes on, or lights dim at idle with the rear demister and blower running, it’s time for a proper diagnostic: belt, battery, grounds, and alternator output under load.

• Service tip: test belt, battery, and charging voltage at every service interval.
• For stop‑start cars, use the correct EFB/AGM battery and perform system relearn after battery or alternator work.

Popular questions

What are the common signs the Impreza’s alternator is failing?
Flickering headlights, a glowing battery/charging light, weak cranking, or electrical gremlins at idle are classic clues. On smart‑charging cars, a low dash‑voltage reading isn’t always a fault—look for symptoms under load (lights, demister, blower) and check belt, battery health, and grounds before condemning the alternator.

What voltage should a 2017 Impreza charge at?
Typically somewhere around 13.5–14.8 V when the system is replenishing the battery, temperature‑dependent. During light load or deceleration, the ECU may command lower voltage as part of fuel‑saving strategy. Consistently low 12‑volt readings with dimming or warnings under load warrant a proper test.

Does the stop‑start system change alternator or battery requirements?
Yes. Stop‑start variants rely on an OE‑spec alternator and an EFB or AGM battery designed for frequent cycling. After battery or alternator replacement, many workshops reset or relearn the charging strategy so the Auto Stop‑Start works smoothly and battery life is protected.