Marine engine electrical system

Boat owners who grew up sailing boats without engines often treat the idea of boat engine maintenance with a fair amount of trepidation, whether inboard or outboard varieties. This trepidation has a tendency to go up a notch when the subject of maintaining an engine’s electrical system is contemplated.

As with all things boating, it helps a boat owner to gain some understanding of the basics of all the systems found on a boat – including the electrical system of their engine – in order to avoid running into a problem at sea that could either have been avoided by regular maintenance or even be dealt with by a competent skipper without having to call the rescue services. This is particularly useful when tracing and rectifying a fault that may be as simple as a loose wiring connection that costs nothing to fix except some time and a little patience.

The typical basic electrical system associated with a marine engine includes a dedicated engine starting battery, a starter motor, a charger in the form of an alternator, a solenoid and some engine sensors and instruments. For the engine to remain operational and healthy these few components and the wiring that connects everything together need to be kept in good working order. This system is self-contained with its own battery and wiring that is separate from the rest of the boat’s electrics.

Alternator belt tension

Most small to medium sized inboard engines are fitted with a single belt to drive the battery charging alternator and fresh water circulating pump. If the belt is slipping or fails then there will be reduced electrical output from the alternator and the batteries will discharge. Belt tension needs to be checked regularly through the year when the boat is being used and adjusted where necessary. It is also a good idea to check the belt when the boat is ashore and decide whether it needs replacing or not. To adjust belt tension:

There are usually two bolts that first need to be slackened, a link adjust bolt and a support bolt on which the alternator pivots.
With both bolts slackened, swing the alternator outwards to tension the belt by pivoting it on the support bolt. Tighten the link adjust bolt.
Check the tension of the belt by pressing downwards on it with your thumb. The belt should not depress more than 12mm (0.5in).
Re-tighten the bolts.
Take care not to over tighten the belt because this will put too much load on the alternator and water pump bearings and cause them to fail.

Black dust

Black dust on and around the alternator is very unsightly but more importantly it indicates the sides of the belt are wearing significantly. This can be caused by corrosion of the pulleys, or could mean the pulleys are out of alignment or the engine compartment temperature is too high.

Rusty pulleys can become very abrasive and quickly chew their way through an alternator belt. To remedy this, remove the belt and and sand smooth the pulleys with emery paper. If they are very badly corroded then consider replacing the pulleys completely.

Check the pulleys are all aligned correctly as if they are out of alignment the belt will wear through very quickly.

If the engine compartment temperature is very high then improving the compartment ventilation should help. Your engine manufacturer may also advise replacing the belt with one with a higher temperature tolerance.

Wiring checks

Your engine manual should include a wiring schematic of your engine wiring. This will help to identify the wiring and which wire belongs to which component. A telescopic inspection mirror can help with these checks:

Bearing in mind the cramped conditions of many engine bays, check the wiring as best as you can, for signs of chafe or melting of the wire insulation.
Check for corrosion on the connections.
With so much vibration caused by the engine, it is wise to check the integrity of connections to the instruments, ensuring the engine wiring loom and any stray cables are properly clipped in place.
Check that the engine instruments and warning lights are all working. It will help to refer to the engine manual fault finding section as helpful information is usually provided.

Note: Battery and alternator care will be featured in forthcoming blog posts.

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