Test Sequence - Mercury 75 Service Manual

TROUBLESHOOTING TIPS:
4 CYLINDER MODELS –
1. Intermittent, weak, or no spark output at two
spark plugs usually indicates a bad TRIGGER.
2. Intermittent, weak, or no spark output at all four
spark plugs usually indicates a bad STATOR,
SWITCH BOX or STOP CIRCUIT.
3. Intermittent, weak, or no spark at any one spark
plug usually indicates a bad GROUND, SPARK
PLUG, COIL, or SWITCH BOX.

Test Sequence

1-A) Check primary input voltage to coils. (See
Test Chart).
1. If voltage readings to coil(s) are BELOW specifi-
cation, proceed with Step 2-A.
2. If voltage readings to coil(s) are WITHIN specifi-
cations, proceed with Step 1-B.
1-B) Check coils for spark. [Connect Spark Gap
Tester (91-63998A1) between coil high voltage
tower and spark plug.]
1. No spark or weak spark. COIL is bad.
2. Spark is OK, proceed with Step 1-C.
1-C) If Step 1-A and 1-B check OK, replace spark
plugs.
If problem exists after replacing spark plugs, proceed
with Step 1-D.
1-D) If Steps 1-A, 1-B, and 1-C check OK, check ig-
nition timing.
1. If ignition timing does not check to specification
(or a sudden or unexplained timing change oc-
curs) check trigger advance linkage for loose
and/or broken parts and check trigger magnet
ring (on flywheel hub) for looseness and/or a shift
in position.
2. If ignition checks to specification and engine
does not run or runs poorly, trouble exists with
fuel system or engine mechanical.
90-830234R3 DECEMBER 1997
2-A) Check switch box "stop" circuit. (See Test
Chart).
1. If reading is BELOW specifications, proceed with
Step 2-B.
2. If 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).
3. If reading is WITHIN specifications, proceed with
Step 3-A.
2-B) Check ignition switch/wiring, as follows:
CAUTION
To prevent engine from starting, remove spark
plug leads from ALL spark plugs and ground
leads to engine.
1. Disconnect ignition switch and stop switch
leads from switch box and isolate the leads.
2. Repeat check in Step 2-A.
3. If reading is still BELOW specification, proceed
with Step 3-A.
4. If reading is WITHIN specification, either the ig-
nition switch, stop switch, or wiring is bad.
3-A) Check stator low speed and high speed in-
put to switch box. (See Test Chart).
1. If either the low speed or high speed reading to
switch box is BELOW specification, Stator or
Switch Box is bad (test stator as outlined in this
service manual section; if stator checks to speci-
fication replace switch box and repeat check).
2. If both the low speed and high speed reading are
WITHIN specification, replace switch box and re-
peat test.
ELECTRICAL - 2A-13