Category Archives: Marine Engines

Nitro Nuisance

Boat Wiring Guys,

I need your help with a 1997 Nitro 640 LX fishing boat that I just purchased.

Nitro 640LX

When I put the red/green navigation light into the plug in base at the bow, it doesn't come on, but causes one of my gauge lights to light up and stay on.

Any thoughts on where this might be going wrong?



The ground wire for the dash panel area is no longer connected. Because of this, the dash is grounding through the filament in the red/green light.

Run a test ground wire from the battery negative to any ground at the helm (maybe a gauge) to confirm.

Hope this helps



Thanks for the quick response,

You were right about the ground wire. We found and fixed that, but we have another problem now.

There is a large wire with a plug that goes into the motor that there is apparently a short in.....when we turn the ignition on, it will not do anything sometimes until you move that large wire around.

What is that wire called that plugs into the motor and apparently goes to the switch as well?

We need to figure out what to get to replace it.

Thank you,


It sounds like it is your main engine harness plug.

Most of those are modular and can be taken apart. This means that you may be able to just replace some terminals instead of replacing the harness.


Shiver Me Trimmer


Tonight I ran into a very odd boat wiring issue.

I have a 2002 Proline Sport 20CC with a 150hp Mercury outboard. Traveling along in the fog, my motor started to trim itself up and out of the water - all by itself! One thing I noticed is when the motor would trim up the bilge pump came on as well. (No water in the bilge, by the way)

Proline 20 CC

My guess is a common short.

What should I look for in tracing this short?



It could be a ghost. It is that time of year.

BUT, it is probably a short between the trim power wire (red) and trim up wire (blue) and bilge power wire (brow) near the shifting mechanism on your helm.

It is common for wires to be pulled tight to the shift cables. They have some movement when the shifter is actuated and eventually wear through.

Hope this helps,


Haha funny you say that. With the fog rolling in, the lack of visibility, and the boats mind of its own it was like out of a horror movie! I was waiting for a pirate ship to pull up out of the fog! Lol

Anyway I did some troubleshooting this morning and found the trim switch on the motor was passing power through it on the up side even when not depressed. Luckily the problem happened while I was troubleshooting. When I press the trim up on the motor and let go it would continue to trim up for a second before stopping.

When checking with a volt meter. Trim up pressed would pass about 12.6v and when let go would go down to 12.3v and slowly trickle down to about 11.3v. Up side was fine and passed no power if not depressed. So I disconnected that switch and only used the throttle trim switch and it was responsive and stopped without delay when I let go of the button. So it looks like a bad trim switch at the motor.

But it doesn't explain why the bilge came on too, lights and fish finder flickered when that happen also. I left the motor's trim switch disconnected so I guess only time will tell if that was the only problem.

Thanks for your help! I appreciate the quick response and assistance!!

Manitou Pontoon Boat Wiring


I am working on a 2005 22" Manitou Osprey Pro. Is there any source for obtaining a boat wiring diagram for this pontoon?

I just recently purchased this and have been trying to fix some electrical issues. Some are relatively straightforward.

Manitou Pontoon Boat

The pontoon's stereo speaker color codes matched the vendors color codes directly, but the stereo's power hookup did not - both yellow and red leads from the stereo were tied to a the pontoon's solid red wire. There was also a red wire with a yellow tracer in the same (stereo) wiring bundle of the pontoon but it was just clipped. Is one of the these a direct battery connection and the other one an on/accessory connection? The stereo wiring diagram shows two positive connections one labeled battery and the other labeled ignition - I don't want the stereo on all the time. The sun has faded all the indications of position for the ignition switch and all the switch/breaker functions. I don't even know if the ignition switch has an accessory function/position.

I don't know if the 5 function switch panel is a standard wiring. Is there a standard layout for the switches? Since the stereo speakers and power seem to follow the standard color codes for front -rear and power, is it safe to assume all the lights, horn and other accessories follow some standard of wiring? If so what are those standards and where can I find a standard schematic or wiring diagram for those standards?

I can't wait to get out fishing on this pontoon - it has an installation for a trolling motor battery in the console with direct wiring to an outlet mounted at the bow for the trolling motor (no battery installed yet) - there doesn't seem like a way to charge this battery from the engine's alternator/charger. Is there a way to connect this battery to the the engine's alternator but not allow the trolling motor battery to try to start the main engine? I don't what to run the higher gauge wiring for that. I just want to use the trolling motor battery for the trolling motor and accessories when the main engine is not running if that is possible.

I would like to develop a wiring diagram for this pontoon but I would hate to have to start from scratch and trace out the circuits by trail and error.

Any help would be really appreciated.


It sounds like a previous owner was doing some wiring modifications. We know the Manitou crew very well, and it is not like them to have hidden wires clipped off in their harnesses.

The key switch accessory position question is based on the brand of the engine on your boat. The easiest way to tell is to turn the key switch. It will either have 3 positions (OFF-ON-START) or 4 position (ACC-OFF-ON-START). If the key has an accessory position, it will have an A or ACC on a tab on back of the key.

There is not a standard layout for the switches on your helm. I would consider calling Manitou in Lansing MI to see if they have replacement panels for your boat that would have the proper labeling for the panels.

Here is the common boat wiring color code used by most OEM boat builders, including Manitou. You should be able to use this to trace down any wires on the boat.

If you want to charge the trolling motor battery while the engine is running, you will need to run some larger wires between the starting battery and the trolling motor battery. This wire size is based on the size of the alternator on your engine. Most VSR smart battery switch systems are capable of handling 125+ amps but the limit is the size of your alternator. If you alternator is less than 20 amp, the VSR is almost a waste of money. You will need to run your outboard at full alternator output for nearly 3 hours to replace what the trolling motor can draw in less than 1 hour.

Hope this helps,


Reset Revival

Hi Kevin,

I screwed up my boat wiring and hope that you can help.

I have a 2004 Sylvan 18 I/O. I had to jump it the other day and in my hurry I hooked up the boat battery cables in reverse. Push to reset breaker

I have power to the bilge pump, boat horn and navigation lights, but the other side of the dash has no power at all. This includes starter switch, bilge blower, and radio. I checked the fuses but found none bad.

Did I miss a fuse down on the starter?



Hi Maurice,

There is a push-to-reset circuit breaker on the engine that supplies power to the dash. It should be a red button about 3/8″ diameter on the stbd side of the engine near where the main engine harness plugs in.

Hope this helps,


Prevent Engine Flooding with a Fuel Demand Valve

This is the second in a series of new fuel product announcements from our associates at Attwood Marine. With so many of our readers using portable fuel tanks, we believed that it would be helpful to pass along this information.

Attwood has been serving the parts and accessory needs of the marine industry for more than 100 years. As a result of recent EPA regulations, they have developed innovative Portable Fuel Systems that are EPA compliant and designed with the customer in mind.

Have you had problems with your outboard engine flooding lately? If you’ve recently upgraded or replaced your portable fuel tank this year, chances are that that’s the cause. New EPA regulations control portable fuel tank emissions, and your fuel system may not have the proper equipment necessary to keep your engine from flooding.

EPA Regulations & Fuel Tanks

As of January 1, 2011, the EPA requires that all new portable marine fuel tanks follow a zero-emissions policy. To meet these new standards and regulations, fuel tanks can no longer be built as an “open” fuel system. Prior to the change, fuel tanks were manufactured with a fuel filler cap that vented and released gas vapor into the atmosphere. Now that portable fuel tanks must be fully sealed, the gas vapor is unable to vent and has nowhere to escape except to your engine through the fuel line. This builds up pressure within the fuel tank, and puts your engine at risk of flooding.

If you purchased a portable marine fuel tank that was built in 2011, it is likely a closed system and was manufactured to follow these regulations. However, the EPA is allowing all manufacturers and retailers to sell their remaining inventories of open portable fuel tanks. This means that even though you may have bought a portable fuel tank in 2011, it doesn’t necessarily follow the new EPA regulations.

If your portable fuel tank expands and contracts noticeably with changes in temperature, it is likely that you have a closed fuel system, and the pressure within the system may result in flooding your engine. Opening and closing the fuel tank filler cap releases pressure when refueling, but if you don’t refuel regularly, there is more risk of pressure building up in your tank and flooding the engine. It’s always a good practice to relieve pressure from your fuel tank prior to attaching it to your engine. That helps reduce the chance of any fuel spraying or dripping from the connector as you attach it to the engine.

Fuel Demand Valve

Fortunately, we have developed a product to help protect engines from fuel being forced in to the system causing flooding. New from Attwood is our Fuel Demand Valve (FDV). The FDV installs onto the fuel hose between the fuel tank and primer bulb, and prevents fuel from flowing unless there is demand from the engine. This eliminates excess fuel that would otherwise be forced through the fuel line due to pressure in the tank, and avoids the related flooding and engine failure.

The EPA-certified Attwood Fuel Demand Valve is compatible with all new fuel tanks, no matter the brand. The FDVs are available both individually and as part of Attwood’s fuel line assembly, which includes EPA-compliant hose, primer bulb, and the new Universal Sprayless Connector.

With proper usage of the new closed fuel tanks, not only are you helping your boat run smoothly and helping the environment, you are keeping the fuel where it belongs: in the tank. No one wants to deal with the headache or money loss of a flooded engine!

Portable Fuel & EPA Resources

By understanding how the new EPA regulations affect your fuel tank’s performance, you can take the necessary steps to keep your engine working properly. For more information on the new EPA regulations, the potential effects of compliance, and the products that keep you safe and your engine performing, visit the Portable Fuel & EPA Resources section of our website. Help the environment and help keep your fuel where you want it… in your tank!

Prevent Fuel Spray with a Universal Sprayless Connector

Many of our readers who use portable fuel tanks will appreciate this new product information from our associates at Attwood Marine.

Attwood has been serving the parts and accessory needs of the marine industry for more than 100 years. As a result of recent EPA regulations, they have developed innovative Portable Fuel Systems that are EPA compliant and designed with the customer in mind.

At Attwood, we want to help you understand how recent EPA regulations affect marine fuel systems. Our fuel system products address the potential negative effects created by these regulations, and we can be relied upon to find innovative next-generation solutions to comply with any new requirements.

Fuel can spray when pressurized hoses are connected and disconnected from fuel tanks to allow for refueling or maintenance. Recent EPA regulations state that you must reasonably expect no fuel spit-back or spillage to occur when refueling. Attwood’s Universal Sprayless Connector is an effective and award-winning solution in preventing fuel spit-back while complying with the EPA.

The Universal Sprayless Connector replaces traditional fuel tank fittings and is designed to fit any fuel system, and its ease of use makes the refueling process safe and easy. While the Universal Sprayless Connector is engineered to withstand the harsh marine environment, it is still extremely affordable and installs within minutes.

The Universal Sprayless Connector
Features and specifications of the Universal Sprayless Connector:

  • Male and female connectors for use with all standard marine tanks
  • Female fitting compatible with 3/8” and 5/16” fuel hoses
  • Male tank fitting compatible with standard 1/4” NPT fuel tank (use thread sealant)
  • High fuel flow valves (less than .25kpa restriction at 50 liters/hour) minimize potential restriction and pressure build-up within the system

In addition to being easy to install, the Universal Sprayless Connector also has recessed check valves that prevent spray if accidentally bumped. The check valves instantly close upon disconnecting and will automatically seal if ever subjected to impact or damage.

Prevent the wasteful and unsafe loss of fuel from tanks by incorporating the Universal Sprayless Connector with your fuel tank.

Boat Wiring Size


Thank you for your answers to so many electrical questions. I plan to redo my fishing boat wiring and have a couple of questions.

My 1977 18 foot Starcraft has an outboard motor with it’s own circuit and will not be involved in the rewire.

My current marine electrical has a battery in the back of the boat, approximately 16 feet from the fused, marine electrical switch panel.Click to download Kevin's boat wiring size calculator

The old boat wiring is a rat’s nest with each circuit wired individually.

I will be using the standard small boat items: a depth finder, FM radio, bilge pump, navigation lights (bow and stern), gauges and a rear-mounted gas tank.

I would like to run a duplex cable from the rear battery to the switch panel. The total duplex run would be 32 feet and the total amp draw should be no more than 30.

I think I need a 10 AWG duplex cable for the 30 foot run and 16 AWG boat wiring for leads from the switches to the instruments (+) and bus bars (-)?

Does this sound right?

Thank you for any help you can provide.


Hi Tom,

I would recommend using my boat wiring size calculator to answer your wire size questions.

  • Leave the voltage drop percentages in column B and G alone.
  • Enter the current draw of you switch panel devices in column C and H.
  • Enter you total wire length run in inches for each device (switch to device and back to ground) in column D and I
  • Enter your total wire length run in inches for the panel feed (battery to panel and back to battery) in cell 25D – 384 is your guess.
  • Your wire sizes will be in Column E and J.

Let me know if you have any questions,


Mercury Optimax – Boat Battery Wiring

Hello Kevin,

I have a 17 foot fishing boat with a 2004 115 Mercury Optimax and a 2007 9.9 Mercury kicker. I only have a single cranking battery for both motors. Mercury 9.9 Kicker

I was told that my marine electrical setup should have two cranking batteries, one for each motor hooked up to a boat battery switch.

The guy said I could damage the computer in my Optimax if by accident one motor was started while another was running. Would the automatic smart boat battery switch (the one you designed for your sister marine electrical site) prevent that if each motor was hooked up to separate batteries with this switch? How detailed is the marine wiring for this switch?

I really like the idea of this switch. My old boat had a Perko battery switch and I always forgot to switch it back and forth.



Hi Corey,

Your system is very common and I have not heard of people damaging computers in this manner. I have, however, heard of people damaging computers by turning the battery switch to the OFF position while the engine is running.

With our smart battery switch system, when either battery is above 13.7 volts, the batteries are connected together. When it drops below 12.8 volts, the switch opens and the batteries are not connected.

If you had your kicker connected one battery and your main connected to the second battery, while charging, the batteries are connected together and they would not be isolated to prevent the situation that you are describing.

The connection is fairly simple and we supply all of the boat battery wiring required with the Smart VSR Battery Switch System.

Please let me know if you have any questions,


Wellcraft Wiring

Hi Kevin,

I’m considering a total redo of the marine electrical on my 1984 23ft Wellcraft Sportsman Walkaround.

I have a 200hp Mercury and the engine harness is fine as is the boat battery wiring and the switch wiring.

What I need to rewire is my navigation lights front and rear, anchor light, the two bilge pumps front and rear (with the option of adding one more in the rear) and docking lights also adding a few underwater lights on the transom. I also have a horn, washdown pump and trim tabs. I have one furuno 620 and two gps setups.Wellcraft Sportsman

What I want to know is if I order a boat wiring harness and switch harness from your sister marine electrical site, EzAcDc, will they cover everything I need? If not any help you could offer would be great.

Thank you


Hi Tom,

Thanks for considering some of the parts on our retail boat wiring site.

Our boat wiring harnesses contain wiring for an automatic bilge pump, a livewell pump, courtesy lights, navigation lights, docking lights, and a boat horn.

When combined with our switch panels, it is simple to connect additional electronics like your Furuno and your GPSs. Additional bilge pump wiring can be run from the pump to the switch panel.

We offer several boat battery switch systems to fit your needs. Our most popular is the smart marine battery switch system. Your 200 HP Merc will work great with this system.

Hope this helps. Thanks again.


Bot Buddies

Dear Kevin,

Last summer, I purchased a new aluminum fishing boat. It’s great, but I screwed up by “cheaping out” at the last minute. I passed on the trolling motor package and – as you’ve probably guessed – now want to add a trolling motor.

As a long time reader, I know that you are a big fan of Motorguide trolling motors and I’ll be buying one in the next couple weeks. My question concerns the boat wiring.

The Motorguide will be bow mounted, but since I didn’t get the trolling package, the only wire at the bow is a 16 gauge pair for the Attwood LED navigation light. But, according to your boat wiring size calculator, the trolling motor should be wired with 6 gauge.

Is there any way to make the smaller wire work? I hate to start rewiring a brand new boat if I can avoid it.



Hi Sean,

Since your boat is new, you may be in luck. A lot of boat builders now use Ancor’s new Nanotech Wire. As you’d guess from the name, it uses nanotechnology to solve problems just like yours.

Simply put, Nanotech Wire matches wire size to power demand by moving copper to where it is most needed – sort of a Viagra for marine electrical.

In the past, running a trolling motor on undersized wire caused voltage drops, heat buildup and embarrassing insurance claims. But now, that same temperature increase signals Nanotech Wire to deploy an army of nanobots who quickly rebuild the harness to match the power demand. While at work, these little guys make the wire looks like a python that swallowed a large rat.

Once your harness is properly sized, the same bots then go to work on its insulation so that it correctly corresponds to standard boat wiring colors.

The patented process is called Nanotech Overload Sensing Heat Induced Tranference and it works great. The only time I’ve seen it fail is when there are so many accessories turned on that there is no extra copper available. If this happens a lot, I usually recommend the addition of a copper reservoir to supply additional material as needed.

Have an excellent day.