Mercury 3-Wire Trim Pump Wiring Diagram
- Mike Hill
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- Nov 24, 2023
- 2 min read
Updated: Jan 24
Mercury 3-Wire Trim Pump Wiring Diagram (Quick Guide)
If you’re rigging or chasing a trim issue on a Mercury outboard, the 3-wire trim pump wiring is one of the most common setups you’ll run into on legacy and performance rigs. Understanding the Mercury 3-wire trim pump wiring diagram makes it easier to diagnose “no up,” “no down,” clicking relays, blown fuses, or a pump that runs the wrong direction.
This short guide explains what the three wires typically do, how the solenoids (trim relays) reverse the motor, and what to check when the trim system won’t cooperate.
How a Mercury 3-Wire Trim Pump Motor Works
A typical Mercury trim pump motor uses three wires because it runs in two directions (UP and DOWN). One wire is usually the motor’s common feed, and the other two wires are used to control direction by energizing the correct relay/solenoid.
When you hit Trim Up, the “UP” relay sends power in one direction. When you hit Trim Down, the “DOWN” relay reverses the polarity (or switches the feed path) so the motor spins the opposite way.
That directional control is why most systems use two trim solenoids near the pump.
Mercury 3-Wire Trim Pump Wiring Diagram
While colors can vary by year and harness, the functional layout is usually consistent:
Battery (Red) positive feeds the trim relays/solenoids (often through a fuse or breaker).
The UP (Blue) relay energizes when you press Trim Up and sends power to the motor’s UP lead.
The DOWN (Green) relay energizes when you press Trim Down and sends power to the motor’s DOWN lead.
Ground (Black) the pump and the solenoids.
If your pump runs backward (UP makes it go down), the directional motor leads are typically swapped at the relay outputs.
Common Symptoms and What They Usually Mean
If you hear a click but the pump doesn’t run, the relay may be working but the motor, ground, or high-current connection is weak. If nothing happens at all, start with power feed, fuse/breaker, and switch signal to the relay coil. If the motor runs but trim won’t move, that usually points toward hydraulic issues (fluid level, stuck valve, air in system) rather than wiring.
A trim system that works only one direction often indicates a failed relay/solenoid, a bad trigger signal on one coil, or a broken lead on the motor output side.
Fast Checks Before You Replace Parts
Confirm you have a strong battery and clean connections first—trim pumps draw serious current. Then verify power is present at the relay input studs, and that each relay coil is receiving signal when you press UP or DOWN.
If one relay never energizes, trace the switch signal and harness. If both relays energize but the motor doesn’t run, test motor power and ground directly to confirm the motor condition.
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