1986+ Mercury 6/8/9.9/10/15HP 2-stroke Factory Service Manual, Page 208Get this manual
POWER TRIM
Power Trim Relay Test Procedure
The trim motor relay system used on permanent magnet trim systems, connects each of the two wires from the trim motor to either ground, or positive, in order to allow the motor to run in both directionsIf the motor will not run in the UP direction, it could be either the UP relay is not making contact to 12 volts OR the DOWN relay is not making contact to groundThe opposite is true if the system will not run DOWNWhen the system is not energized, both relays should connect the heavy motor leads to groundTo test which relay is faulty if the trim system does not operate in one direction: 1Disconnect the heavy gauge pump wires from the trim control relay2Check for continuity between the heavy leads from the trim relays to groundOhmmeter Leads Between Resistance (Ohms) Scale Reading (R 1)