Mercruiser Alternator 61 Amp – Fits Mercruiser IL4, V6, V8 (Replaces 78403A2)

$199.95 $319.00

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Mercruiser Alternator 61 Amp – Fits Mercruiser IL4, V6, V8 (Replaces 78403A2) is backordered and will ship as soon as it is back in stock.

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If you have any questions, feel free to email us at info@sierramarines.com, and we’ll get back to you within 20–36 hours.

Product Details

This is a direct-replacement Mercruiser Alternator 61 Amp designed for a wide range of Mercruiser IL4, V6, and V8 gasoline stern drive engines from the 1970s and 1980s. It serves as a reliable aftermarket alternative to the original Delco-style 10SI unit, ensuring proper charging system function.

It provides a direct interchange for several part numbers, including MES Marine 78403A2, Mercury/Mercruiser 78403A2 and 78477, Sierra 18-5951 and 18-5957, Arco 20104, and API Marine 20053.

Designed for direct OEM replacement installation.
Features an internal voltage regulator.
Ignition Protected per SAE J1171 standards for marine use.

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket.

Brand	Part Number
MES Marine	78403A2
Mercury/Mercruiser	78403A2
Mercury/Mercruiser	78477
Sierra	18-5951
Sierra	18-5957
Arco	20104
API Marine	20053
OMC	982364
OMC	983424
OMC	983836
OMC	988247

Engine & Model Compatibility

Confirm your engine model/year in the table below.

This alternator fits a variety of Mercruiser engines from the 1960s through the 1980s. Please verify your specific model and OEM part number.

Also compatible with some OMC and Volvo Penta applications that use an equivalent alternator design.

Brand	Model	Stroke	Years	OEM Part No.	Notes
Mercruiser	110	4-Stroke	1963–1966	78403A2	GM 2.5L (153ci) 4-cylinder
Mercruiser	120	4-Stroke	1964–1989	78403A2	GM 2.5L (153ci) 4-cylinder
Mercruiser	140	4-Stroke	1968–1986	78403A2	GM 3.0L (181ci) 4-cylinder
Mercruiser	140	4-Stroke	1963	78403A2	GM 3.2L (194ci) 6-cylinder
Mercruiser	150	4-Stroke	1964–1966	78403A2	GM 3.8L (230ci) 6-cylinder
Mercruiser	165	4-Stroke	1970–1982	78403A2	GM 4.1L (250ci) 6-cylinder
Mercruiser	190	4-Stroke	1963–1964	78403A2	GM 4.6L (283ci) 8-cylinder
Mercruiser	2.5L	4-Stroke	1987–1989	78403A2	GM 2.5L (153ci) 4-cylinder
Mercruiser	228	4-Stroke	1977–1981	78403A2	GM 5.0L (305ci) 8-cylinder
Mercruiser	233	4-Stroke	1975–1977	78403A2	Ford 5.8L (351ci) 8-cylinder
Mercruiser	250	4-Stroke	1977	78403A2	GM 5.7L (350ci) 8-cylinder
Mercruiser	260	4-Stroke	1978–1981	78403A2	GM 5.7L (350ci) 8-cylinder
Mercruiser	270	4-Stroke	1969–1972	78403A2	GM 5.7L (350ci) 8-cylinder
Mercruiser	3.0/3.0LX	4-Stroke	1987–1989	78403A2	GM 3.0L (181ci) 4-cylinder
Mercruiser	330	4-Stroke	1977–1981	78403A2	GM 7.4L (454ci) 8-cylinder
Mercruiser	898	4-Stroke	1977–1984	78403A2	GM 5.0L (305ci) 8-cylinder
OMC	Inboard & V-Drive	4-Stroke	1981–1987	982364, 983424, 983836, 988247	Various engines

Technical Specs

Product Type: Marine Alternator
Installation: Direct OEM replacement
Fuel Type: Gasoline
Connection: 2-wire or 3-wire (varies by model)
Housing Material: Cast aluminum
Housing Color: Silver
Dimensions: 21.0cm L x 19.0cm W x 19.0cm H
Fitting Type: V-belt pulley
Output: 61-78 Amp (varies by source)
Voltage: 12V
Regulator: Internal
Other Attributes: Delco-style 10SI alternator design, Ignition Protected SAEJ1171

What’s in the Box

1 x Alternator (78403A2)

Installation Notes

Disconnect the battery's negative terminal to prevent electrical shorts.
Identify and label all wires connected to the old alternator before removal.
Loosen the alternator mounting bolts and pivot bolt to relieve tension on the drive belt. Remove the belt from the pulley.
Remove the mounting bolts and carefully lift the old alternator out of the engine bay.
Compare the new alternator with the old one to ensure the mounting points and pulley are identical.
Position the new alternator and loosely install the mounting bolts.
Place the drive belt over the pulley and apply appropriate tension by levering the alternator. Tighten the pivot and adjustment bolts.
Reconnect the electrical wires to the correct terminals on the new alternator.
Reconnect the battery's negative terminal.
Start the engine and verify the charging voltage at the battery is within the correct range (typically 13.8V-14.5V).

Troubleshooting & FAQ

Why are different amp ratings (61, 68, 78 Amp) listed for the same part number?

Different aftermarket manufacturers produce this alternator with slight variations in output, though they all serve as functional replacements for the original. The performance difference is generally negligible for stock applications. We list it as "61-78 Amp" to cover these known variations.

Is this a 2-wire or 3-wire alternator?

This Delco 10SI style alternator can be configured for either 2-wire or 3-wire systems, depending on the specific engine model and wiring harness. It is compatible with both setups, but you must ensure it is wired correctly for your application.

How do I know if my alternator is failing?

Common signs include a weak or dead battery, a "Check Battery" or "ALT" warning light on the dash, dimming headlights and interior lights, or a whining/grinding noise coming from the alternator itself.

Can I install this part myself?

While installation is straightforward for those with mechanical experience, we recommend professional installation. Incorrect wiring or improper belt tension can cause damage to the part or your engine's electrical system.

What is the difference between this and the original Mercruiser part?

This part is functionally equivalent to the original. It is made from similar materials to meet or exceed OEM specifications for fit and performance in the marine environment. The primary difference is that it is produced by a third-party manufacturer.

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.

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