Boat Won't Start or Crank: Starter Solenoid, Ignition Switch and Wiring Diagnosis

An outboard starter solenoid is a heavy-duty relay that connects the battery to the starter motor when you turn the key to start. If your boat won't crank, test it after the battery, cables, start fuse, lanyard and neutral safety switch, because those cause far more no-crank problems than a failed solenoid does.

Most of the no-crank calls I take end with a corroded terminal, a loose engine ground or a safety interlock, and the owner has often bought parts he didn't need. The sound the engine makes when you turn the key tells you where to look first:

• Nothing at all: the starting signal isn't reaching the solenoid. Check the lanyard, the start-in-gear interlock, the fuse, the ignition and the battery.
• One click and nothing else: burned solenoid contacts, a weak battery, or high resistance in a cable, ground or battery switch.
• Rapid clicking: a battery or connection that collapses the instant the starter draws current.
• Slow cranking: battery, cables or ground, unless the engine is mechanically tight.
• Starter spins but doesn't engage: low voltage at the starter or a sticky Bendix drive.
• Cranks but won't start: a fuel, spark or lanyard issue. Open neutral start interlock circuit. The starting system is doing its job.

How a Boat Engine's Starter Circuit Works

The starter motor draws a couple hundred amps on an outboard engine, so the ignition can't switch it directly. Turning the key sends a small current through the start-in-gear interlock in the remote control, then through a fused wire to the solenoid's coil. The coil closes heavy contacts that connect the main cable to the starter.

ABYC's color code uses yellow with a red stripe from the starting switch to the solenoid (E-11, Table 12), and Mercury and OMC engines follow it. Yamaha runs the starting signal on a brown wire and calls the part a starter relay. MerCruiser sterndrives add a slave solenoid ahead of the one on the starter, plus a 90 A fuse and a 50 A breaker on the engine.

Cranking motor cables are the one place ABYC doesn't require a fuse (E-11, 11.10.1.1.1.1), and their size should follow the engine maker's recommendation (11.15.1.2.7.2). If you swap the solenoid on a gasoline engine, use a marine, ignition-protected part (11.5.1.4.1), never an automotive one.

Step 1: Check the Kill Switch, Neutral and Battery

Make sure the shifter is in neutral and the lanyard clip is properly seated. Whether a pulled lanyard stops the engine from cranking depends on the model. Mercury's manual for its V6 and V8 digital outboards lists "Lanyard stop switch not in RUN position" first under starter will not crank. On many older mechanical engines, the lanyard only kills ignition, so the engine turns over but never fires.

Next, check the battery with a meter at the posts, not the dash gauge. A charged one reads 12.6 V or more at rest. A reading right off the charger can fool you: one iBoats owner saw 13.66 V of surface charge on one that settled to 12.6 V (iBoats). While cranking, James Hebert of Continuous Wave says it "should not sag (drop) below 10-volts" (Continuous Wave). A proper load test pulls half the rated CCA for 15 seconds and should hold above 9.6 V.

Size matters too. ABYC requires a cranking power source that meets the engine maker's rating (E-11, 11.6.1.1.1), and the Mercury 250 in one Boating Magazine repower called for an AGM battery with 975 CCA (Boating Magazine). Then look at the terminals for white powder, loose wing nuts and green cable ends, and clean them before you test.

Step 2: Find Loose or Bad Connections With a Voltage Drop Test

A voltage drop test finds the resistance anywhere in the system that a parts swap misses, but it only works while the starter is cranking. Put one meter lead on the positive battery post and the other on the starter's big stud, and have someone turn the key. Fluke puts acceptable loss at about 0.3 V on the positive side and 0.2 V or less on the ground side (Fluke). Then check from the engine block back to battery negative.

Measure across the battery switch and the solenoid's large studs the same way. Nigel Calder says each should lose no more than "a tenth or two" of a volt, and that the likeliest problem spot is "where the negative cable bolts to the engine block" (SAIL). A RIBnet owner whose Mercury F150 clicked but would not turn over, even with new batteries, found a "corroded master battery switch and loose connections due to corrosion" (RIBnet).

Step 3: How to Test an Outboard Starter Solenoid

With the shifter in neutral, have a helper turn the key while you check the small terminal for 12 V. On most outboard solenoids, the coil runs between two small terminals, so make sure the second one has a good ground.

• Power at the small terminal, no click: the coil has failed or lost its ground. Replace the unit.
• A click, but the starter doesn't turn: measure across the two large studs while cranking the engine. More than a few tenths of a volt means burned contacts.
• No power at the small terminal: the problem is upstream. Go to Step 4.

On the bench, 12 V across the small terminals should pull it in with a click, and an ohmmeter across the big studs should read near zero while energized. One iBoats owner with a 2005 Yamaha 150 had power going into his starter relay and none coming out: "I replaced the relay and that fixed it" (iBoats).

What burns out a solenoid? Low voltage makes it chatter, which arcs and pits the contacts. Salt spray and bilge water corrode it, and holding the key too long overheats the coil. If the starter won't stop after you let go of the key, the contacts have welded shut, so disconnect the battery.

Jumping the big studs will tell you whether the starter works, but it bypasses the start-in-gear protection, so do it only with everyone clear of the prop. Federal rules require that protection on outboards with 115 pounds or more of static thrust (33 CFR 183.710).

Step 4: Ignition Switch, Safety Interlock and Start Wire

When the solenoid never sees 12 V, trace the starting signal toward the ignition. The ignition switch's S terminal should show 12 V while you hold the key in START. From there, check the interlock in the remote for continuity, then the fuse, the harness plug and the wire to the solenoid.

The safety interlock is the most common no-click problem I find. A Bass Boat Central owner's 1997 Evinrude 150 would "not even click" until he replaced the safety switch (BBC Boards). A Parker owner with a 2005 Yamaha F250 found the one at his back helm station had come loose after control work; a boat with two stations has one at each (Hull Truth).

Wiring is next. A Mercury Tracker owner found 12.43 V going into his solenoid but nothing on the starting side, because "the yellow/red stripe was not connected" (iBoats). A MerCruiser owner with no lights or cranking found "the pin in the connector was burnt" in the helm harness (iBoats). Some engines also have a 20 A fuse in the starting circuit, so check the one your manual lists. A bad ignition can also fail only in START, and sometimes only when it is hot.

Engine Cranks but Won't Start: Check the Fuel System, Spark Plugs and Water

If the engine cranks at normal speed, leave the solenoid alone. Check the lanyard, then the fuel system:

• Fuel supply: the tank vent open, the fuel valve on and enough fuel in the tank. A kinked fuel line or a stuck anti-siphon valve will starve the engine.
• Primer bulb: squeeze it until it is firm before you crank the engine. A bulb that never firms up points to an air leak or a bad fuel line fitting.
• Water in the fuel: drain the water separator bowl and replace the filter. Ethanol fuel that sits for months can absorb water and separate.
• Old fuel: stale fuel is a common issue on boats that sit through the winter. If the boat sits, use a stabilizer and fresh fuel each season to prevent it.
• Spark: check each cylinder with a spark tester, then inspect the spark plugs for fouling.

On an inboard or sterndrive, run the blower for 4 minutes before cranking, as the federal label requires (33 CFR 183.610). Gasoline fumes in the engine compartment and a starting system don't mix.

Case Study: One Click and a Weak Engine Ground on a Bay Boat

Last summer a customer with a 23-foot bay boat and a Yamaha F150 called because the engine gave one click and wouldn't turn over. He had already replaced the starter relay and the battery.

His battery read 12.7 V at rest and held 11.2 V while cranking the engine, so it was fine. The positive side lost only 0.2 V, but from the engine block to battery negative I measured 1.4 V. The ground cable's lug at the engine was green inside its heat shrink, and the starter was seeing about 9.6 V.

I cut back the cable, installed a new tinned lug with adhesive heat shrink, and cleaned the mounting point. The ground side now loses under 0.1 V, the starter sees about 10.9 V while cranking, and the engine starts the first time he turns the key. The relay he bought is now his spare.

How to Prevent Dead Battery and Starting Problems

Once a season, before the boat goes in, clean and tighten the battery terminals, the battery switch studs and the engine ground, and inspect the solenoid for corrosion. Keep the battery charged and the charging system working, swap out cables that run warm after cranking, and use the lanyard every time you run the boat. A wrench and a few minutes of time prevent most of the starting problems I see.