Typical TRIM Circuit

Electrical and electronic topics for small boats
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Typical TRIM Circuit

Postby jimh » Sat Aug 11, 2018 10:04 am

Most outboard engines have a power trim-tilt feature. The electrical circuit shown below is typical:

Typical power trim-tilt circuit for outboard engine
typicalTrimCircuitDwg685x564.png (33.38 KiB) Viewed 16 times


The TRIM MOTOR M is a reversible DC motor connected by BLUE and GREEN conductors to the common pole of two SPDT relays, K1 and K2. If neither relay is energized, both leads to the trim motor are connected to 0-Volts (or battery negative or chassis ground) via the normally-closed contacts of the relays, and the motor is not energized.

The relay K1 is the UP direction relay; when K1 is energized, the BLUE conductor has 12-Volts via the normally-open contact from the RED conductor, and the trim motor turns in the direction to raise the engine.

The relay K2 is the DOWN direction relay; when K2 is energized, the GREEN conductor has 12-Volts via the normally-open relay contact from the RED conductor, and the trim motor turns in the direction to lower the engine.

The coils of relays K1 and K2 are independently operated by remote control switch S1, located typically on the handle of the remote shift-throttle control lever. Not shown is a second remote control switch, wired in parallel with the first, typically located on the engine cowling. The switches are usually combined into a three-position ON(momentary)-OFF-ON(momentary) arrangement in a single rocker switch. Depressing the UP switch energizes the UP relay and causes the engine to tilt up; depressing the DOWN switch energizes the DOWN relay and causes the engine to tilt down. The circuit for the coil for the UP relay is usually provided on a wire with BLUE with WHITE insulation; the circuit for the coil of the DOWN relay is usually provided on a wire with GREEN with WHITE insulation.

The 12-Volt power to the relay contacts or the high-current circuit generally comes from a heavy-gauge RED conductor that is connected to the battery positive; in some engines there may be a circuit breaker (not shown above) to protect the circuit from excessive current. The 0-Volt power to the relay contacts generally comes from a heavy-gauge BLACK conductor that is connected to the battery negative. It is typical that in most outboards these circuits are made directly to the main battery cable connections to the engine from the starting battery. This conductor is usually bonded to the engine chassis. The relay circuit BLACK conductor is also bonded to the engine chassis.

The 12-Volt power for the relay coils comes from the remote switches; this is the low-current or control circuit. The switches at the remote throttle lever often get their 12-Volts from the ignition key circuit, so the ignition key switch must be in the ON or RUN position for those switches to work. The switches on the cowling often get their 12-Volts directly from the battery connection, so they will work when the engine is not running and the ignition key can be in the OFF position. The UP circuit wiring usually has insulation of BLUE with WHITE; the DOWN circuit usually has insulation of GREEN with WHITE. A simple way to remember the color coding is to associate blue with the sky or up, and green with the grass or down. These circuits are also typically protected by a fuse F1.


Although not shown in the simplified circuit diagram above, there are typically several multi-pin connectors for the wiring in the remote control or low-current part of the circuit. Any discontinuity in the electrical connection in those connectors will cause a failure of the control. This is often seen at the remote shift-throttle lever, which is always being manipulated; the wiring to the switches can and does fail due to the flexing.

The circuit to the motor is also often run through a connector; again, any contact problems in this connector will affect the operation.

The process of tilting the engine up to a fully-raised position also causes all the electrical conductors in the engine control cable to be flexed. Abrasion of the conductor's insulation can occur and cause problems.

The remote switches themselves are often exposed to water from spray, splashing, or rain. The switches can fail, particularly ones on the cowling.

The relay contacts are handling high current and an inductive load, conditions which cause arcing of the contacts. Relay contacts can fail due to build up of carbon from arcing or pitting. They can also weld together.

The load of the trim motor when running can cause voltage drops in the battery and associated power distribution wiring; low voltage at the relay coils can cause the relay coil to have insufficient magnetic force to hold the relay contacts closed. When the contacts open the load is removed, the voltage returns to normal, and the contacts close again. This creates a cycle of relay contact chattering open and closed. Such operation is certain to damage the contacts of the relay in a short time.

The relays are sometimes arranged into an assembly, and replacement of the assembly will replace both relays. If the relays are provided separately, they can be replaced individually.

Outside of the relays, the control switches, and the wiring and connectors associated with them, the trim motor itself can also fail.