1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 21Get this manual
Test Sequence 1-A) Check primary input voltage to coils(See Test Chart)1If voltage readings to coil(s) are BELOW specification, proceed with Step 2-A2If voltage readings to coil(s) are WITHIN specifications, proceed with Step 1-B1-B) Check coils for spark[Connect Spark Gap Tester (91-63998A1) between coil high voltage tower and spark plug 1No spark or weak sparkCOIL is bad2Spark is OK, proceed with Step 1-C1-C) If Step 1-A and 1-B check OK, replace spark plugsIf problem exists after replacing spark plugs, proceed with Step 1-D1-D) If Steps 1-A, 1-B, and 1-C check OK, check ignition timing1If ignition timing does not check to specification (or sudden or unexplained timing change occurs) check trigger advance linkage for loose andor broken parts and check trigger magnet ring (on flywheel hub) for looseness andor shift in position2If ignition checks to specification and engine does not run or runs poorly, trouble exists with fuel system or engine mechanical2-A) Check switch box "stop" circuit(See Test Chart)1If reading is BELOW specifications, proceed with Step 2-B2If reading is ABOVE specifications, the Trigger or Switch Box is bad (test trigger as outlined in this service manual section; if trigger checks OK, replace switch box and repeat check)