Every year around the end of April we start getting calls about marine generators that won’t start, won’t hold a load, or shut down on high-temperature alarms within the first hour of use. The genset worked fine in October, it sat all winter, and now it’s the first thing on the boat to give a problem. That pattern is predictable enough that it’s worth explaining what’s actually happening and what to check before it ruins a weekend.
Marine generators — Kohler, Onan, Westerbeke, and the others we’re authorized on — are diesel engines driving an alternator, and they have all the same weaknesses as a marine propulsion diesel. What makes them fail first in spring is how they get used and how they get stored.
Generators sit harder than propulsion engines
A propulsion engine on a typical recreational boat runs regularly during the season. Even low-use boats see their main engines fire up for a few hours most weekends. Generators often don’t. On a boat that runs air conditioning at the dock or anchor a few times a season, the genset might accumulate twenty or thirty hours over a summer and then sit for five months with almost no run time before that.
Engines that sit hard develop problems that engines that run regularly don’t. Fuel in the tank oxidizes. Condensation collects. Seals dry out. Batteries lose capacity. Glow plugs age in place. By the time the first hot weekend of the season rolls around and the owner tries to start the generator to run the air conditioning, the system is working through five months of accumulated issues all at once.
The fuel problem is usually the first one
Most marine generators share a fuel tank with the main engines, and the same fuel issues that affect propulsion diesels after winter storage affect generators — often worse, because the generator’s fuel pickup sits in a different part of the tank. Water, sediment, and biological growth that settle out of suspension over winter are the first thing a cold generator pump pulls when it fires up in April. The primary fuel filter, which probably wasn’t changed when the boat was winterized, clogs fast and the generator dies.
Routine practice on any generator that’s been in storage more than three months should include draining the primary fuel filter bowl before first start, replacing the primary and secondary fuel filters, and if the fuel has been sitting longer than six months, running a biocide through the system before putting the generator under load.
Cooling system failures on generators follow the same pattern as propulsion engines
Generators have the same two-circuit cooling architecture as the main engines and suffer the same raw water side failures. The impeller in a generator raw water pump takes a set over the winter just like the one in the main engine. The heat exchanger scales up over years of service. The exhaust mixing elbow corrodes from the inside.
What’s different is that generator impellers often get less attention than main engine impellers, because the generator isn’t the first thing owners think about. An impeller that failed years ago and was ignored because “the generator was still running” often finally takes out the heat exchanger in the first hour of spring use. The genset overheats, shuts down on alarm, and now the repair is two parts instead of one.
If it’s been more than two years since the generator raw water pump was serviced, spring is the right time to do it. The same architecture logic that applies to propulsion engine cooling applies here, and the fix is the same: inspect the impeller, check the strainer, look at the heat exchanger, and replace anything questionable before the system is under load.
Batteries and starting circuits
A generator that hasn’t been started in five months is asking a battery that also hasn’t been maintained in five months to crank a cold diesel engine. The math usually works out badly. Marine batteries lose capacity when they sit at partial charge, and a generator starter draws significant current during a cold start. An older battery that would start the generator in October frequently won’t in April.
Before the first spring start, it’s worth putting a load tester on the generator’s dedicated battery, checking the terminal connections for corrosion, and verifying that the charging circuit from the main engine alternator or shore power is actually topping up the battery when the boat is at the dock. A weak starting battery is an easy fix. A starter motor burned out by repeated failed cranking attempts is not.
The engine room itself is part of the problem
Generators live in engine rooms that have been closed up all winter. Moisture, salt air, and in some cases water intrusion from a hatch that didn’t seal well all affect the electrical components on the generator — the voltage regulator, the control board, the wiring harness terminals. The first start of the season can expose problems that weren’t there in October.
A proper spring generator check includes a look at the engine room for water staining, a check of the generator’s exterior wiring for corrosion at terminals, and attention to any circuit boards or electronics that are mounted in exposed locations. Salt air is hard on electronics, and an engine room that wasn’t ventilated through the winter is worse.
What a spring generator service actually covers
The short version of all of this is that a marine generator needs the same seasonal attention a propulsion engine needs, and usually gets less. A proper spring service covers the fuel system end to end, the cooling system raw water side, the battery and starting circuit, the oil and oil filter, the air filter, and a proper test run under load to verify that the unit will hold rated output when it’s needed.
We service Kohler, Onan, Westerbeke, Yanmar, and other marine generator brands through our marine generator service, and we can do most spring generator work as part of a mobile visit. If your genset didn’t get serviced over the winter and you want it checked before the first weekend you actually need it, call us at (609) 242-8448. Finding the problems in April is considerably less disruptive than finding them on a Saturday afternoon in July.