BMW 1 Series Alternator: Specs, Fitment & Common Faults
A genuinely important, BMW-specific finding: a leaking oil filter housing gasket is the primary cause of alternator failure on this engine — worth fixing before replacing the alternator, or the new one will likely fail the same way.
Illustration for editorial purposes
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OEM Part Number: 12317558219 (genuine alternator — confirm via VIN) — aftermarket alternators Lucas LRA04380 / AS-PL A6890S
| Alternators (aftermarket) | Lucas LRA04380 (120 A, multi-rib pulley, 6 ribs, 49 mm, remanufactured, plug 0336, core charge £40.19); Lucas LRA04427 (150 A, multi-rib pulley, 6 ribs, 49 mm, remanufactured, plug 0336, core charge £30.14); Lucas LRA04446 (180 A, multi-rib pulley, 6 ribs, 49 mm, remanufactured, plug 0336, core charge £40.19); AS-PL A6890S (120 A, 6 ribs, 49 mm, new, plug PL_2801); AS-PL A6856S (150 A, 6 ribs, 49 mm, new, plug PL_2801); AS-PL A6926S (180 A, 6 ribs, 49 mm, new, plug PL_2818); Rotovis 9091002 (150 A, V-ribbed pulley, 6 ribs, 49 mm, remanufactured, COM plug); Herth+Buss 32001011 (120 A, V-ribbed pulley, 6 ribs, 49 mm, new). |
|---|---|
| Outputs & variants | Three outputs are listed, all on a 49 mm six-rib pulley with LIN control: 120 A for the 1.5 three-cylinder 116i and 118i petrol, 150 A for the 116d and 118d diesels, and 180 A for the 120d, 128ti and M135i. No freewheel pulley is specified on any reference. |
| Genuine (OE) numbers quoted | 104211-3990 (AS-PL A6890S), 104211-9200 (AS-PL A6856S and Rotovis 9091002), 104211-0740 (AS-PL A6926S). |
| Fitment notes | These are Denso-built alternators and the three outputs look alike, so the BMW part number or the Denso number on the old unit should be checked before ordering; remanufactured Lucas units carry a core charge refunded on return of the old alternator. |
BMW 1 Series Alternator: Often Blamed on the Battery Instead
On the BMW 1 Series, a weak battery and a failing alternator produce almost identical symptoms at first — dim lights, slow starts, a car that won't hold charge — which is exactly why so many people replace the battery, only to be back at the garage a few weeks later with the real culprit untouched.
BMW 1 Series Alternator: What It Does
The BMW 1 Series's alternator generates electricity while the engine runs, charging the battery and powering the car's electrical systems directly. A healthy alternator means the battery is mostly there for starting the engine and covering brief demand spikes — not for actually running the car's electronics on its own.
BMW 1 Series Alternator: Watch: Replacing It
F20 N47 diesel; nothing found for the FWD F40 (B38/B47) in any language.
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A Genuinely Important, BMW-Specific Finding Worth Knowing — BMW 1 Series Alternator
If this alternator has failed, this is genuinely worth checking before fitting a replacement: a leaking oil filter housing gasket is described as the primary cause of alternator failure on this specific engine, since leaking oil can drip directly onto the alternator and cook its internal components over time. It's genuinely critical to inspect for and fix this specific leak before replacing the alternator, since fitting a brand new unit without addressing an underlying oil leak will likely see the replacement fail the same way, wasting the cost of the repair. It's also genuinely worth knowing that replacing the alternator on this specific engine is considered a genuinely challenging DIY job due to the tight working space, though it can typically be done without needing to remove the intake manifold, despite some guides suggesting otherwise.
It's also genuinely worth knowing that when replacing the alternator, it's wise to also change the serpentine belt, tensioner, and idler pulley at the same time, particularly if they're oil-soaked or high-mileage, since these components are already accessible during the job.
BMW 1 Series Alternator: Warning Signs, DIY Difficulty & Aftermarket Advice
These are the same regardless of make or model — see our full Alternator Guide for symptoms to watch for (plus a genuinely useful two-minute test to confirm it's not the battery instead), confirming the right amperage and fitment against your VIN, and which brands are worth trusting.
More BMW Alternator
The same part, covered across other BMW models.