Alternator – Fits Mercury/Mariner Outboard 225–300 HP (Replaces 821663A1)
Alternator – Fits Mercury/Mariner Outboard 225–300 HP (Replaces 821663A1) is backordered and will ship as soon as it is back in stock.
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Worry-free returns
You may return most new, unopened items within 7 days of delivery for a full refund. We'll also pay the return shipping costs if the return is a result of our error (you received an incorrect or defective item, etc.).
You should expect to receive your refund within four weeks of giving your package to the return shipper, however, in many cases you will receive a refund more quickly. This time period includes the transit time for us to receive your return from the shipper (5 to 10 business days), the time it takes us to process your return once we receive it (3 to 5 business days), and the time it takes your bank to process our refund request (5 to 10 business days).
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Delivery and Shipping
Delivery and Shipping
Free standard shipping on all orders under 1 KG
Alternator – Fits Mercury/Mariner Outboard 225–300 HP (Replaces 821...
Product Details
This is a direct replacement Alternator for Mercury/Mariner Outboard 225–300 HP engines, specifically designed for 3.0L 2-Stroke Carb & EFI models manufactured between 1994 and 2000. It provides a reliable power source for your marine electrical system, ensuring your battery stays charged and accessories run smoothly.
It interchanges with multiple OEM and aftermarket part numbers, including Mercury 821663A1, Denso 101211-3020, and Sierra 18-6449.
- Engineered for direct bolt-on installation.
- Provides 12 Volts / 50 Amps of power.
- Features an internal regulator and fan for a compact design.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Mercury | 821663A1 |
| Mercury | 821663-1 |
| Mercury | 821663 |
| Denso | 101211-3020 |
| Denso | 101211-3021 |
| Sierra | 18-6449 |
| API Marine | 20300 |
| Arrowhead | AND0171 |
| J&N | 400-52198 |
| WAI | 12358N |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
This alternator fits a range of Mercury and Mariner V6 3.0L outboards. Please verify your specific model and year for an exact match.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Mercury | 225 (Carb) | 225 | 2-Stroke | 1994–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225 (EFI) | 225 | 2-Stroke | 1995–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225 SEAPRO (Carb) | 225 | 2-Stroke | 1996–1998 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225CXL | 225 | 2-Stroke | 1994–1999 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225CXL EFI | 225 | 2-Stroke | 1995–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225CXL SEAPRO | 225 | 2-Stroke | 1996–1998 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225CXXL | 225 | 2-Stroke | 1994–1999 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225CXXL EFI | 225 | 2-Stroke | 1995–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225L | 225 | 2-Stroke | 1994–1999 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225L EFI | 225 | 2-Stroke | 1996–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225XL | 225 | 2-Stroke | 1994–1999 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225XL EFI | 225 | 2-Stroke | 1995–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225XXL | 225 | 2-Stroke | 1994–1999 | 821663A1 | 3.0L 185.0ci |
| Mercury | 225XXL EFI | 225 | 2-Stroke | 1995–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 250CXL EFI | 250 | 2-Stroke | 1995–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 250CXXL EFI | 250 | 2-Stroke | 1995–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 250XL EFI | 250 | 2-Stroke | 1995–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 250XXL EFI | 250 | 2-Stroke | 1995–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 300CL PROMAX RACING | 300 | 2-Stroke | 1998–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 300CXL PROMAX RACING | 300 | 2-Stroke | 1998–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 300L PROMAX RACING | 300 | 2-Stroke | 1998–2000 | 821663A1 | 3.0L 185.0ci |
| Mercury | 300XL PROMAX RACING | 300 | 2-Stroke | 1998–2000 | 821663A1 | 3.0L 185.0ci |
| Mariner | 225 (Carb) | 225 | 2-Stroke | 1994–2000 | 821663A1 | GM 3.0L 183ci 6cyl |
| Mariner | 225 (EFI) | 225 | 2-Stroke | 1995–2000 | 821663A1 | GM 3.0L 183ci 6cyl |
| Mariner | 225 SEA PRO (Carb) | 225 | 2-Stroke | 1996–1998 | 821663A1 | GM 3.0L 183ci 6cyl |
| Mariner | 250 (EFI) | 250 | 2-Stroke | 1995–2000 | 821663A1 | GM 3.0L 183ci 6cyl |
Technical Specs
- Product Type
- Alternator
- Installation
- Direct OEM replacement
- Voltage
- 12 Volts
- Amperage
- 50 Amps
- Polarity
- Negative ground (Earth return)
- Pulley Type
- Single V-groove
- Fan Location
- Internal
- Regulator Location
- Internal
- Regulator Clock Position
- 3:00
- Stud Size
- M6-1.0
- Mounting
- Direct bolt-on
- Height
- 20.5
- Width
- 19.0
- Length
- 18.0
- Warranty
- 12 months (varies by vendor)
What’s in the Box
- 1 x Alternator (821663A1)
Installation Notes
- Disconnect the battery terminals (negative first) to prevent electrical shorts.
- Loosen the alternator belt tensioner and remove the serpentine belt from the alternator pulley.
- Disconnect the wiring harness and main power lead from the back of the old alternator.
- Remove the mounting bolts securing the old alternator and carefully lift it out of the engine bay.
- Position the new alternator and secure it with the original mounting bolts. Do not fully tighten yet.
- Reconnect the wiring harness and main power lead.
- Reinstall the serpentine belt and apply correct tension using the tensioner.
- Tighten all mounting bolts to the manufacturer's specified torque.
- Reconnect the battery terminals (positive first) and start the engine to verify proper operation.
Troubleshooting & FAQ
How do I confirm this alternator fits my specific engine?
The most reliable method is to match the part number from your old alternator to the numbers in our Part Number Matrix. Additionally, check your engine's model, HP, and year against our compatibility table. If you are unsure, please contact us with your engine's serial number.
What are common signs of a failing alternator?
Symptoms include dimming headlights or dash lights, a warning light on the dashboard (often shaped like a battery), a weak or dead battery, slow-cranking engine, or a whining/grinding noise coming from the engine bay.
Can I install this alternator myself?
While installation is possible for those with mechanical experience, we recommend professional installation for safety and to ensure proper function. The process involves handling electrical connections and correctly tensioning the drive belt, which are critical for performance.
What's the difference between this and an OEM part?
This is an aftermarket part designed to meet or exceed OEM specifications for fit, form, and function. It offers a cost-effective solution without compromising on quality or performance. The primary difference is the manufacturer and price.
How often should a marine alternator be inspected or replaced?
There is no fixed replacement interval, but regular inspection is key. Check for belt tension, corrosion on connections, and listen for unusual noises at least once per season. Replace it when it no longer provides a consistent charge or shows signs of failure.
OEM vs. Aftermarket?
This listing is an aftermarket replacement (not OEM). Built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, batch-inspected for consistency.
Shipping speed & cost
We ship worldwide quickly—typical transit 5–15 days. Standard shipping is free; expedited options appear at checkout where available.
Warranty & returns
Covered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact support with your order details. Returns are accepted within our standard window (unused & in original condition). See our policy page or reach out for instructions.
Safety & Important Info
- Verify images/specifications vs your original part. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement, not an OEM component.
- Professional installation is recommended for safety and performance.
About Sea Sierra
- Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.
- Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
- Manufacturer-Direct Value — Fair pricing without compromising on materials or QC.
Payment & Security
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