Parts for your 2004 Subaru Forester-Alternator

2004 Subaru Forester alternator — purpose, servicing and replacement tips

Yes, a 2004 Subaru Forester is fitted with an engine-driven alternator. That’s confirmed by the Subaru Factory Service Manual for MY2004 Forester (Charging System section), Subaru’s Technical Information System, Denso alternator specifications used on EJ25 petrol engines, and Subaru’s parts catalogue listings for OE alternator assemblies for the 2004 Forester (including higher-output units on XT turbo models). Typical output is roughly 90 A for non‑turbo models and about 110 A for XT variants.

The alternator’s job is straightforward but critical: it keeps the battery charged and powers the Forester’s electrics while the engine’s running. From headlights and the heater fan to the ECU, ABS and stereo, it maintains a healthy system voltage (generally around 13.8–14.5 V) so everything under the bonnet and in the cabin behaves as it should. Without a solid charge, the battery goes flat and all those on-board systems start playing up.

As part of regular servicing, a few quick checks go a long way:

• Inspect the alternator/AC drive belt for cracks, glazing or fraying, adjust or replace if worn.
• Check battery terminals and engine/chassis earth straps for corrosion or looseness.
• Measure charging voltage at the battery: expect roughly 13.8–14.5 V at idle with lights on. Outside that range calls for further testing.

Common signs of a tired alternator include the battery warning lamp glowing, dim or flickering headlights at idle, a whining or grinding noise from the alternator, unexplained electrical gremlins, or a battery that keeps going flat. Because a dud battery can mimic alternator issues, it’s smart to load‑test the battery and perform a charging test before ordering parts.

When replacement’s on the cards, sticking with an OE-quality Denso unit or properly remanufactured equivalent is the safe bet. Turbo XT models often need the higher-output spec, so matching the alternator to the VIN/build and the original label is wise. Disconnect the negative battery terminal, note any radio/security codes, swap the unit, fit the belt with correct tension, and recheck charging voltage. After fitment, listen for belt squeal and verify no warning lights, a quick scan for stored fault codes won’t hurt either.

With routine checks every service or 10,000–15,000 km, the Forester’s alternator tends to be set‑and‑forget, keeping weekend missions and weekday errands equally uneventful.

What’s the correct alternator output for a 2004 Forester?

Most non‑turbo X/XS models use an alternator around 90 A, while the XT turbo typically runs a higher‑output unit near 110 A. The sure-fire way is to match by VIN and the label on the original alternator, or cross-check against Subaru’s parts catalogue and the Subaru Factory Service Manual charging specs.

How can the alternator be tested at home?

With a multimeter across the battery: engine off should show roughly 12.4–12.7 V. Start the engine and expect about 13.8–14.5 V at idle. Switch on headlights and rear demister, voltage should stay above roughly 13.5 V. Readings well below spec, wildly fluctuating voltage, or anything pushing 15 V suggest charging faults that need further diagnosis.

Which belt drives the alternator, and when should it be replaced?

On the EJ25 engine the alternator runs on the alternator/AC V‑ribbed belt. Inspect it at every service and replace it if cracked, glazed or noisy. Many workshops aim for replacement somewhere between 60,000 and 100,000 km, sooner if there’s visible wear or if it slips. Always set the correct tension after refitting.