Mercury Mercruiser Electrical System

Mercury MerCruiser Electrical System Components Resources and Information

PartsVu is happy to stock a wide selection of certified Mercury MerCruiser electrical ignition components to fit the exact specifications of your engine. Whether you’re looking for MerCruiser Starter Slave Solenoids, MerCruiser Electrical Harness Repair Kits, and other electrical system components, we have what you need to make sure your MerCruiser starts up like new every time.

For more information on Mercury MerCruiser electrical parts and how to perform DIY repairs for your entire engine, check out some of our helpful MerCruiser resources:

• MerCruiser Inboard Maintenance
• MerCruiser Maintenance Parts Library
• Mercury Outboard and Mercruiser Resources Collection

Mercury MerCruiser Electrical System FAQs

Where should I check on my Mercury MerCruiser engine electrical system first?

The first place you should look when dealing with a MerCruiser electrical system problem, start with the battery(ies). 

Generally speaking, marine batteries will last about three years with proper care and maintenance, so consider a replacement if your battery is at or older than three years. Many boaters remove or disconnect their batteries for storage during the offseason. Prior to reinstalling those batteries, visually inspect them for evidence of physical deformation (swelling or sucking in), leakage, corrosion, and age. Any fluid leak or crystal accumulation on the battery should be considered as hazardous. CRC makes a battery cleaner that indicates the presence of acid and will neutralize the acidic properties. 

For wet cell batteries, this is the best possible time to check and adjust fluid levels inside the battery to compensate for evaporation. Follow the manufacturer’s instructions and use protective gear, as you may be accidentally exposed to acid. Close the battery back up and charge in the shop or re-install and charge if your vessel is equipped with a marine-grade onboard charger.

A terminal brush is an inexpensive and very handy tool to help with battery installation. You can easily clean terminals and wire ends before reinstalling the leads. Once installed, the dissimilar metals used to create the ring terminals, battery cable ends, and terminal studs will promote corrosion, so a corrosion inhibitor is really important in this area. Don’t forget to properly tighten your battery hold-downs as vibration is one of the leading causes of premature battery failure.

How do I troubleshoot my Mercury MerCruiser’s electrical system? 

In most cases, starting your Mercury Mercruiser motor only requires tilting the engine and turning the key. 

If this isn’t the case, you may need to troubleshoot which systems aren’t operating as required for a clean start. Here’s a few suggestions on where to start:

1. It’s surprisingly common for boaters to have their kill switch disconnected when electrical systems fail. Reconnect the kill switch to close the electrical connection of the Mercury MerCruiser’s electrical system.
2. Mercury MerCruiser Trim and tilt problems are often caused by a faulty connection to the trim/tilt switch. Uninstall the switch if it’s easily accessible, checking the connection for any signs of corrosion or an open connection. If detected, replace it with a new tilt and trim switch.
3. Check the battery connection and voltage level. Consider scrubbing the battery terminals for a more reliable connection and check for any short circuits. Replace any old batteries that don’t hold a reliable charge. Also, some batteries have an on/off switch. If this is true for your battery, make sure the switch is on. 
4. Check all of your MerCruiser spark plugs. To do so, start by making sure there's an ignition spark with a spark gap tester. If you’ve got a good spark, make sure that the air gap is within specification. Adjust it if needed. If you have a good spark and the gap is within specification, the problem will likely not be found in the ignition system.Clean and adjust the spark plugs, replacing any with darkened metal contacts that no longer spark reliably. 
5. If the kill switch isn’t properly attached it could very well be the source of your motor problems. Undo the connection and put it right back into place. The kill switch may need to be replaced altogether.
6. Check your gear shift, as it is easy to knock out of place, preventing the engine from starting. Make sure that you’re in neutral and try starting your engine once more.
7. It may sound obvious, but check for any gas in your fuel tank. This is more common than most boaters think, as a closed vent prevents air from replacing fuel as the motor pulls it out, which is necessary for firing up the engine. On the other hand, an open tank vent can promote evaporation of fuel during storage. 
8. Sometimes, the issue may simply be that your MerCruiser engine propeller has something tangled around it. This may be fishing line, excessive seaweed, or some other material that’s worked its way around the blades. Carefully remove anything that may be wrapped around the propeller, and give it a once-over for any damage that may be present. If the propeller is damaged, it needs to be replaced. Debris around the propeller is the most likely culprit if your boat motor starts, but quickly dies.
9. Pay attention to any anomalies when you last operated your engine. For example, a bogged down engine can indicate failing spark plugs, obstructions in the fuel lines, water infiltration via the head gasket, and more. If you can’t source the issue, consider taking it to a certified Mercury MerCruiser dealer for a full evaluation.

What’s the most common cause of electrical system MerCruiser engines?

Corrosion is the most common cause of electrical system failure for all inboard and outboard motors. Because these engines operate in an aquatic environment with moisture from the water and the air, corrosion is nearly inevitable on all exposed surfaces. This is worse for saltwater boats, where the salts from the ocean and waterways accelerates the corrosion process.

Of course, when electrical components made from metal begin to corrode, these components can fail to make consistent connections between positive and negative terminals of each component. For example, even simple corrosion around battery terminals and spark plugs can leave your boat dead in the water. 

There are three types of corrosion boaters typically experience:

• Simple electrochemical corrosion
• Galvanic corrosion
• Electrolytic/stray current corrosion

The best way to prevent these types of corrosion is to regularly check each component to ensure that it works consistently. Depending on your model of MerCruiser engine, you may need to use a continuity tester/voltmeter to test each connection. Start with the battery, work your way through each electrical system cable, spark plugs, auxiliary systems, and more. You’ll want to look for anomalies, such as low voltage - which may indicate a short circuit or the degradation of corroded metal.

Also remember that your boat’s sacrificial anodes connected in the internal system of the engine, the trim tab, and other areas can prevent corrosion from many types of corrosion, so it’s important to make regular replacements during regular service intervals (or if you’re been docked in a marina for extended periods of time). 

How often should I change my MerCruiser engine spark plugs?

Most marine mechanics and experienced boaters agree that frequent replacement of spark plugs is absolutely essential for MerCruiser electrical ignitions . Some boaters are adamant about changing their spark plugs when they break out their boats for the season, while others change their spark plugs every 100 hours. PartsVu recommends regularly inspecting your spark plugs every 100 hours during service intervals, replacing them if they are fouled or damaged, and ALWAYS replacing them every 300 hours (or every 3 years, whichever comes first).