Power Trim Only Goes Up or Only Down: Relays, Switches and Wiring

Support the engine with the tilt lock or an engine support if it's up, chock the trailer, and keep everyone clear of the transom. Then press each switch in each direction and write down what you see and hear: does the motor hum, does a relay click, does the engine move, and does it behave the same from every switch?

BoatUS's first check is the simplest: make sure the battery is fully charged and the battery switch is on. A weak battery can still click a relay but not turn the trim motor under load, especially trimming up with the boat on the trailer. Check that the trim motor's power feed (often from the starting battery or the engine's starter terminal) is tight and clean.

Many trim systems have a fuse or breaker for the switch circuit, and some have a large one for the motor feed. Look near the battery, at the helm and on the engine harness. Mercury's general rule applies: replace a blown fuse with the correct size and test, and if it blows again, have the dealer find the short. Carry spares.

Most outboards, and many sterndrives, have a second trim switch on the engine cowl or bracket (sometimes labeled for trailering). BoatUS points out that if trim works normally from the engine switch, the trim system itself is fine and the problem is in the helm switch or its wiring.

• Works from the cowl, not from the helm: look at the switch in the throttle handle, the wiring through the control box and the harness plug. Worn switches and broken wires inside the control handle are common on older boats.
• Fails the same way from both switches: the problem is downstream, at the relays, wiring or motor. Keep going.

On some sterndrives there's also a trim limit (or trailer) switch that restricts trim-up beyond a certain point while running. If "up" stops partway but the trailer button still lifts the drive, ask your dealer about the limit switch and trim sender.

Have a helper press up, then down, while you listen and hold a finger on each relay. BoatUS: if you hear nothing, the problem is typically the switch, the relay, or the wires between them (or a fuse). If you hear a click, the switch side is working and the problem is in the heavy-current side: relay contacts, connections or the motor.

On outboards the relays are usually under the cowl, often near the starter or in a plastic relay box. On sterndrives they're often on or near the trim pump in the engine compartment. Your owner's or service manual shows where.

Turn the battery switch off. Photograph the wiring, then unplug the relays and the trim motor's quick-disconnect plug. BoatUS lists failed relays and corrosion at connections, relay sockets and quick-disconnect plugs as some of the most common causes of trim failure.

Look for green or white crust, burned or melted plastic, and loose or pushed-back pins. Clean with electrical contact cleaner and a small brush, let dry, apply a thin film of dielectric grease if your manual allows, and reconnect firmly. Yamaha suggests wiggling wires and connections to find loose ones; corrosion inside the insulation isn't always visible from outside.

If your up and down relays are identical plug-in units with the same part number, swap them and test again:

• The fault changes direction (now it goes down but not up, instead of the other way round): you've found a bad relay. Replace it with the same part. Many owners replace both, since they've had the same years of heat and moisture.
• The fault stays exactly the same: the relays are probably fine. Look at the switch, the wiring and the motor.

Never substitute a relay that only looks similar. Contact layouts and ratings differ, and the wrong one can burn out or leave the motor running.

If you're comfortable with a multimeter, you can confirm the switch side. With the relay unplugged and the battery switch on, have a helper press the trim switch in the failing direction while you measure between the relay's coil signal terminal in the socket and a good ground. Your wiring diagram shows which terminal that is.

• About battery voltage only while the switch is pressed: the switch and its wiring are good.
• Nothing: the problem is upstream, in the switch, fuse, harness plug or wire.

If you're not sure which terminal is which, skip this step and use the direct motor test below, or let a technician do it.

This is the BoatUS method for proving whether the motor works:

1. Turn the battery switch off. Unplug the motor's blue and green wires at the quick-disconnect plug (or disconnect them from the relays).
2. Make sure the area is well ventilated with no fuel vapors.
3. Using a fused jumper, apply 12 volts directly to the correct motor wire: blue to raise if the engine is down, green to lower if it's up. How the motor gets its ground varies: through the motor housing, through a separate black wire on older motors, or (on some reversing motors) through the other colored wire, which then has to be connected to ground for the test. Check your service manual's motor test before you connect anything.
4. Turn the battery switch on only for the test, keep everyone clear, and touch the jumper briefly.

Motor runs: the motor is good and the problem is power not reaching it (relay, connection or wire). Motor doesn't run: the problem is the motor, its plug or its ground. A trim motor that's taken on water is often corroded inside and usually gets replaced.

A motor that runs slowly, groans or works only when you tap it often has a poor ground or a corroded heavy connection rather than a bad motor. Follow the ground from the motor or pump to the engine block or battery negative, and the heavy feed from the battery or starter to the relays. Clean to bare metal and tighten. Yamaha's guidance is to clean connections down to bright metal on both sides, then protect them with a corrosion spray.

BoatUS: if the trim motor works fine but the engine won't raise or lower, leaks, settles down when raised or won't stay trimmed, the problem is likely the hydraulic pump or valve body. Check these, in order:

• The manual release (tilt relief) valve. If it's even partly open, the engine won't go up, or drifts down. Close it as your manual describes.
• Fluid level. Yamaha says to check the fluid and top up with the specified trim fluid if the engine won't hold its tilt. A low level means a leak somewhere, since hydraulic fluid doesn't evaporate (see the trim fluid guide). Mercury's quick tip for its outboards has you tilt the engine all the way up to relieve pressure and engage the tilt lock before opening the reservoir; a little top-up now and then is normal, but an engine that keeps needing fluid, or shows signs of a leak, belongs with the dealer.
• Leaks at the rams, hoses or fittings (see the trim hose and cylinder leaks guide).
• A motor that runs freely but the pump doesn't build pressure usually means internal pump or valve trouble, which is a shop job.

If the trim keeps running after you let go of the switch, turn the battery switch off right away. The usual cause is a relay whose contacts have welded together, or a helm switch that has shorted. Running against the end stops can burn out the motor or damage the hydraulics. Swap or replace the suspect relay before using the trim again, and check the helm switch for water inside.

Reconnect everything, turn the battery switch on, and run the trim fully up and down from every switch, listening for normal relay clicks and smooth motor sound. Then:

• Spray connectors with a corrosion protectant and keep relay boxes closed and dry.
• Yamaha recommends rinsing the trim unit after each trip, greasing the tops of the trim rams, and storing the engine with the rams retracted (engine supported on the tilt lock or an engine support) so the chrome stays protected by fluid.
• Write down which relay you replaced and when in your BoatMend maintenance log.