Chart R1: Engine System Fails To Power - Mercury MerCruiser Service Manual

Chart R1: Engine System Fails to Power Up
This procedure is intended to diagnose a PCM 112 system that does not turn on. You are here because you answered no to
step 1 of the diagnostic roadmap.
The PCM 112 receives continuous battery power and ground through a special 2‑wire PCM power harness that contains a
5‑amp fuse. The helm is powered separately through the battery cables. This circuit includes the 90‑amp block fuse, the
50‑amp circuit breaker, and a 20‑amp fuse.
The remainder of the engine harness receives battery power through the battery cables, which may or may not be connected to
a battery switch. If the key switch is turned to the run position while the battery switch is off, diagnostic faults will be set.
Step Diagnostic Process
Are the battery switches set for
1
normal operation?
Check the condition and state of
2 charge of the main engine battery.
Is the battery serviceable?
Check the battery cable connections
3 at both the engine and battery.
Are the battery connections good?
Check the 5‑amp PCM power fuse,
and check for correct routing of the
4 PCM power circuit.
Is the fuse good and the harness
routing correct?
Check for battery voltage at the red
lead of the helm harness key switch
5 connector.
Is the voltage within 1.0 V of
battery?
Turn the key switch to the run
position. Check for battery voltage at
the purple lead of the helm harness
6
key switch connector.
Is the voltage within 1.0 V of
battery?
Check the battery voltage between
the black lead of the helm harness
key switch connector and the positive
7
terminal of the main engine battery.
Is the voltage within 1.0 V of
battery?
Turn the key switch to the run
position. Check for battery voltage at
8 the warning horn purple lead.
Is the voltage within 1.0 V of
battery?
90-8M0099748 eng DECEMBER 2015
Details
Not all boats will have a battery
switch. They are optional.
Refer to the engine service manual
for battery specifications.
Ensure that the ground stud is
tightened securely in the engine
block. Ensure that corrosion‑resistant
hex nuts are used instead of
wingnuts.
The PCM power harness must be
connected directly to the main engine
battery. It cannot be connected to a
battery switch. The PCM power
harness connects the battery to a
2‑pin engine harness connector.
Voltage must be present
continuously, regardless of battery
switch position. If the main engine
battery is connected, voltage must be
present.
Voltage must be present when the
key switch is in the run position, but
not when the key switch is in the off
position.
Voltage must be present
continuously. This test verifies the
ground circuits between the PCM
power harness and the helm.
–
© 2016 Mercury Marine
Troubleshooting
Yes
Reset the switches for
Go to step 2.
normal operation.
Charge or replace the
Go to step 3.
battery.
Clean and tighten all
Go to step 4.
connections.
Replace the fuse.
Reroute or reconnect the
Go to step 5. harness as necessary.
Install the harness if it is
missing.
Repair or replace circuit.
This circuit includes the
battery cable, 90‑amp
block fuse, 50‑amp circuit
breaker, and a 20‑amp
Go to step 6. fuse. It includes the red
lead in pin A of the 14‑pin
connectors on the
engine, main data
harness, and helm
harness.
Replace the key switch,
Go to step 7.
as necessary.
Repair or replace circuit.
This circuit includes the
battery, PCM power
harness black lead,
Go to step 8.
14‑pin main harness
black lead, and helm
harness to the key switch
black lead.
Repair or replace the
purple circuit between the
Go to step 9.
key switch and the
warning horn.
No
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