Outboard Electrical FAQs

Should my boat be rigged with a starting or a deep cycle battery, and why?

There is a difference between an ordinary starting battery and a deep cycle battery. Starting requires a high amount of energy for a short period of time (about 30 seconds maximum). Only a small amount of the battery's capacity is used. Once the engine starts running, the battery is recharged quickly by the charging system.

A deep cycle battery supplies a relatively low amount of current for a long duration. Deep cycle batteries, unlike starting batteries, can be run down and recharged repeatedly with minimum loss of capacity. Deep cycle batteries are designed to power electric fishing motors, and other electrical accessories in boats and recreational vehicles such as radios, TVs, fans, etc.

Since starting batteries and deep cycle batteries are designed for different purposes, they are constructed differently inside. Starting batteries have porous active material. The plates are thin and are designed so the high-amp energy can be quickly delivered for maximum starting power. Repeated cycling, which involves lower capacity drains and recharges, weakens the positive plate. The active material drops from the grid, thus, in repeated deep discharge/recharge applications, the capacity of the starting battery drops below desired levels in about 50 cycles.

The deep cycle battery has a denser active material and thicker plates to withstand deep discharge/re-charge service.

I'm seeing a small spark when I connect the battery cables. Is this ok?

Yes, newer engines require a small reference signal.

How do I charge two batteries with my outboard motor?

Using an outboard to charge two batteries is only effective if the outboard charging system is capable of delivering more than 15 to 20 amps. One point to remember: most systems are rated at wide-open throttle and charge substantially lower amounts at slower engine speeds.

Engines equipped with belt-driven alternators and some smaller outboards can use a battery isolator to direct current to each battery. The isolator allows one battery to be discharged separately from the other and still allows both to charge from a single source. Carbureted and EFI 2.5L outboards are equipped with a dual regulated charging system. This system can be split, allowing half of the system to charge both batteries.

Other options may be available, contact your local Mercury dealer for additional information.

How do I hook up a tachometer or other gauge to my outboard motor?

Each Mercury Precision Part gauge contains instructions on how to connect the instrument. Our Service manuals contain similar information and are available for purchase through any authorized Mercury dealer or online.

What do the warning horns indicate?

The warning horns vary model to model. Generally a solid tone indicates a problem that can cause immediate engine damage and the engine should be stopped. An intermittent tone indicates a problem that will not cause immediate damage but the operator should inspect the engine as soon as possible*. Refer to your Operation, Maintenance & Warranty manual for detailed information concerning the warning systems for your engine.

*On 135 HP (2L) through 200 hp (2.5L) V-6 models, an intermittent warning horn may indicate a problem with the oil injection system. The engine should be stopped and inspected as outlined in your Operation, Maintenance & Warranty manual.

What is the correct battery for my outboard motor?

Note: These are minimum engine specifications. Additional electrical loads from the boat will require larger batteries.

FourStroke Models

9.9/15 & 25

465 Marine Cranking Amps (MCA) or 350 Cold Cranking Amps (CCA)

30/40, 45/50

Operation Above 32°F (0°C) 465 Marine Cranking Amps (MCA) or 350 Cold Cranking AMPS (CCA) Operation Below 32°F (0°C)

1000 Marine Cranking Amps (MCA) or 750 Cold Cranking AMPS (CCA)

75/90

Operation Above 32°F (0°C) 465 Marine Cranking Amps (MCA) or 350 Cold Cranking AMPS (CCA) Operation Below 32°F (0°C)

1000 Marine Cranking Amps (MCA) or 750 Cold Cranking AMPS (CCA) Ampere-Hours (Ah) 70 - 100

2-Stroke Models

6 - 15, 20/25, 30/40, 40/50 & 60

465 Marine Cranking Amps (MCA) or 350 Cold Cranking Amps (CCA)

75 - 125

525 Marine Cranking Amps MCA) or 450 Cold Cranking Amps (CCA)

V-6 Carb/EFI Models

135/200 & 225/250

630 Marine Cranking Amps (MCA) or 490 Cold Cranking Amps (CCA)

OptiMax Models

2.5L/3.0L

1000 Marine Cranking Amps (MCA) or 750 Cold Cranking AMPS (CCA)

What is the correct battery for my OptiMax?

1000 Marine Cranking Amps (MCA) or 750-800 Cold Cranking AMPS (CCA)

What are the general wiring colors used on Mercury outboard engines?

Mercury/Mariner Standard Outboard Wiring Color Code

Color

Where Used / Function

Black

All Ground

Brown

Reference Electrode MerCathode System

Orange

Anode Electrode MerCathode System

Lt. Blue/White Stripe

Trim UP Switch

Lt. Green/White Stripe

Trim DOWN Switch

Brown/White Stripe

Trim Sender to Trim Gauge

Purple/White Stripe

Trim "Trailer" Switch

Gray

Tachometer Signal

Black/Yellow Stripe

Shorting or Stop Circuit

Pink

Fuel Sender to Gauge

Red

Unprotected Wire from Battery

Red/Purple Stripe

Protected (Fused) Wire from Battery

Red/Purple Stripe

Protected (12 Volt +) to Trim Panel Control

Purple

Ignition (Switch) to 12 Volt Positive

Tan

Temperature Switch to Warning Horn

Tan

Temperature Sender to Temperature Gauge

Tan/Blue Stripe

Temperature Switch to Warning Horn

Yellow

Starter Solenoid to Starter Motor

Yellow

Starter to Regulator (Charging Circuit)

Yellow/Red Stripe

Start Switch to Start Solenoid to Neutral Start Switch

Yellow/Black Stripe

Choke (Enrichener System)

Green with Color Stripe

Switch Box to Coil - Striped and Numbered

White/Black Stripe

Bias Circuit (Switch Boxes)

The ABYC Recommended Colors

Color

Where Used / Function

Yellow/Red

Starting Circuits

Brown/Yellow or Yellow

Bilge Blowers

Dark Grey

Navigation Lights & Tach Signals

Orange

Accessory Feeds

Brown

Pumps

Purple

Instrument Feeds

Dark Blue

Cabin and instrument Lights

Light Blue

Oil Pressure

Tan

Water Temperature

Pink

Fuel Gauge Sender

What is the correct tachometer setting for my outboard motor?

 

Models

Positive Pulses per Crank Revolution

Poles

Switch Setting

Models 35 and 40

2

4

4P or 3

S/N Range U.S.A. - 5823918 and Above

Canada - 7169337 and Above

Australia - 8070639 and Above

Belgium - 9298955 and Above

Mercury/Mariner 6 through 25, 35, and 40 (2 cylinder) models

All other Mercury/Mariner Models

6

12

6P or 4

Force 40 and Above with Mercury Ignition

6

12

6P

FourStroke Models

8/9.9 hp

3

6C or 2

6

9.9/15 hp

3

6C or 3

6

25 hp

6

6P or 4

12

40/30

3

6C or 2

6

50 hp

3

6C or 2

6

50/60

3

6C or 2

6

75/90*

3

6C or 2

6

*Use QSI tachometer, other models require engine wiring harness changes.