Trim help needed on installing new Mercury control box to older motor

Jaybird73

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Joined
May 16, 2020
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10
Short story: I need help diagnosing why my trim won't operate with a new control box on an older outboard.

Long story: I've got a c.1984 Mercury 115 tower of power with the original Quicksilver control. Boat is a late 80's 17' bowrider. Had to replace the control box and had to upgrade to a Mercury 4000 SMC box. The original box had the 4 wire trim bundle leaving the box completely separately from the 8 pin control harness. Then a few feet from the outboard, the 4 wires changed over to a 3 prong plug, etc. Then after the plug, a new trim harness began and ran all the way to the trim solenoids.

The new box has the trim wire embedded in the harness all the way to end then 4 wires emerge next to the 8 pin motor control end. The 4 wires are blue, green, and brown with male bullet connector ends. The fourth wire is tan with a female bullet end. I tried using the old original trim wire out of the old Quicksilver box and putting corresponding bullet ends on it then keeping the plug assembly in place with my new "reverse harness" plugging to the new ends next to 8 pin plug then running back out to the 3 prong plug. The Solenoids clicked when up or down is pushed in each direction but no action of actually making the trim motor actually engage.

I then went and cut through the old harness coming from the solenoids (left myself plenty of wire to play with) to try to connect directly with the new harness. The old harness was three wires: green, blue, red. Blue for the up solenoid, green for down and red for hot. I have some alligator clip leads to test with. My guess was the brown or the tan that one was hot and one was a ground. I connected green to green and blue to blue. Then I tried each of the other two on the new harness (brown and tan) alternatively with one to red and the other grounded. Also tried each one separately to red with no ground. All results were the same: solenoids remained clicking strongly but no other action.

Other possibly relevant details:

8 pin motor control part is working fine. Motor fires up and runs well in both forward and reverse. I tested it with a garden hose and "ears."

The trim is either fully down or nearly fully down. I cannot examine anything with the trim motor or pump at this point. If I need to, then I need a pointer on how to tilt the motor up manually.

I have checked the inline fuse connected to the trim solenoids. It is good.

The motor control was replaced due to the ignition switch breaking then I messed it up trying to repair the switch. Totally separate story which led me to purchasing the new unit. Besides, I want to get this boat in reliable enough condition that my 21 year old daughter or 17 year old son can take it out on their own without me worrying about its reliability.

Before the motor control being replaced, there was an issue with the trim, particularly when raising (going up). Sometimes the trim would engage the first time you mashed the switch, sometimes it took as many as 4 or 5 times before it would engage. As now, the solenoids would click but no action, and as I said it was much more of an issue in moving up than down. But now, no matter how many times the up or down switch is pushed or how long it is held, there is no engaging in either direction.

Thanks in advance for any help! I live in an isolated small town. The only nearby marine repair shop is 6 weeks out on work and I'd really like to get on the water a lot sooner than August.
 
Last edited:

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,036
Your description is a bit confusing. However, I am pretty familiar with the wiring on your motor. Wire colors should be the new scheme, but brown is not used in the new scheme, so not sure what that is.

The parts list on that motor shows it has the power trim unit between the transom clamps. If you have something different, it can affect the following.

Power trim wires are (as you guessed), red for power, blue for up and green for down. The tan wire is likely for overheat warning. Check the brown wire, as it may be tach signal. The newer tach signal wire color is grey. Grey ran the choke in the old color scheme. So that is confusing.

All of these wires (PTT, overheat, tach signal) would normally run to the front of the boat.

If the trim up function is intermittent, I would check the up solenoid wiring.

Simply connect the trim wires to the new control. I assume it has a toggle switch for trim? Wire colors should match.

The tan wire (if overheat) would connect to a buzzer. Some controls have a built in buzzer, some are external. The other side of the buzzer gets power from the ign accessory wire, normally purple in color.

Brown wire (if it is tach signal from rectifier), would connect to tachometer. Older Merc Controls had a three prong plug for exactly that purpose. It supplied ground, ign accessory power and tach signal.

hope that helps.
 

Jaybird73

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May 16, 2020
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Thanks for the suggestions. I still don't understand why that might keep the trim from engaging. It was intermittent before the whole motor control swap out. Now there is nothing from it except the solenoids clicking.

So, if neither the brown nor the tan is the power wire and the original trim harness was a 3 wire (green, blue, red), would I need to get power from somewhere else to fully power up the solenoids? I admit I don't understand solenoids well enough. I thought if you could hear them click then they were functioning?
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,036
Red is the power to the solenoid secondary (energizer coil). The solenoids would also need power (heavier wire) on their primary, from the starter solenoid primary battery cable.

The solenoids would also be grounded. Your power trim motor should be 3 wire with the black wire grounded. Your power trim motor could fail, such that it only works in one direction, although that is rare.

BTW your PTT solenoids are likely the same as your starter solenoid. You could swap them for testing purposes.
 

Jaybird73

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Joined
May 16, 2020
Messages
10
Thanks again for the input. Turns out my wiring was correct all along. I think there may have been some air trapped in the hydraulic reservoir. Had my son help me manually lift the outboard to the full up position. A few taps on the trim motor got it turning. Checked the fluid level and lowered it back to midway and no luck. Then I remember to retighten the valve that I had to loosen for lifting, and it started working better than before. Wired it up and went on the water late yesterday.
 
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