Re: 1976 Merc 850 Power Trim wiring diagram
Current systems, which may resemble the older systems but are different as one pump cylinder is used whereas on the older ones a cylinder was on each side of the engine and I don't remember where the pump was....maybe attached to inside of the transom.
The trim is nothing more than a bi directional (pair of) hydraulic cylinder(s) with a couple of popoff valves to satisfy things like trim vs the lower pressure tilt, and trim and tilt limits. Trim only runs about -4 to +20 degrees of engine angle and the tilt part (low pressure) runs it on up to 40 or thereabouts. If you have it in the tilt position and goose it the tilt popoff will activate and the engine will be forced down to the upper limit of trim. This for your and my safety.
The motor that generates the hydraulic pressure is a dc motor which can be reversed by reversing the voltage to it. The motor pumps hydraulic fluid OUT of a sump, located wherever the pump is mounted, to raise, and pumps it BACK to lower the engine. You have to have positive control of the fluid due to trim being bi-directional. Tilt could be just a single valve raising the engine with pressure and letting it off to allow the engine to fall via gravity.
So you have two solenoids wired with the + and - of the motor connected to + and - battery voltage at different times which determine if the pump is going to run cw or ccw and run the piston(s) out or in.
The separate solenoids don't use as much current as the starter, but more than is normally carried by regular electrical wiring so they will have larger wires to carry the drive currents as compared be the solenoid control input wiring from the TT switch.
The control for the solenoids will be ground (for both control returns at the relays) and on current systems it is the blue (blue/white stripe maybe) wire to energize the UP solenoid (relay for short), and the DOWN relay is via the green or green/wh. The voltage that is sent down the respective wire comes from the trim switch where it also has a red wire (3 wires total to the switch) which is the 12v power input, fused from the battery via the engine. The switch just determines which relay gets energized.
looking at the business end of the relays, following the wiring you should note that 12v and ground go to both relays, but the outputs are wired opposite so that (as stated) one will drive the motor cw and the other ccw.
On fluid, the rule from Merc is: If red use ATF, if clear use Merc/QS PTT fluid. Just match what is already in the sump. Full is normally even with the bottom of the fill plug threads.....in short if you had the plug out to fill it and put any more fluid in the sump it would run out the fill hole.
Ok, now you have the inputs for a general understanding. Go for it and if you have a problem ask.
Mark
