Important Pcm Notes; Test Equipment - Mercury V6 175 Diagnostic Manual

• Fuel injectors
• Ignition coils
• Power steering
• Trim up
• Trim down
• Start relay
• Fuel pump
• Diagnostics
• Engine Guardian
• Tachometer link (analog tachometer output)
• Advanced sound control (V8 Verado engines only)
• CAN communications with helm controller
• Moving propeller (MP) alert (optional)
The PCM operates in four modes: power off, stall, crank, and run. The PCM also provides for a smooth throttle response
between varying throttle positions and engine loads. A warm‑up mode is integrated with the run mode and disengaged after the
engine accumulates a given amount of power.
Power off mode ‑ With the key switch OFF, the PCM function does not exist; however, trim will remain available for 15 minutes
after key off on DTS engines. On mechanical engines, pushing the trim up or down button will turn on the PCM and allow the
engine to trim. 12 VDC power is available from the battery at the starter battery terminal, fuses, positive cable terminal, and at
the alternator.
Stall mode ‑ With the key switch in the RUN position, the PCM is energized. The PCM powers the main power relay, the
sensors, and the fuel pump relay (for five seconds at key‑up). The PCM records barometric pressure from the manifold
absolute pressure (MAP) sensor, intake air temperature from the intake air temperature (IAT) sensor, and coolant temperature
from the coolant temperature sensors. The PCM uses this information to establish a warm‑up strategy that controls fuel delivery
and engine speed during warm‑up and idle.
Crank mode ‑ When the key switch is turned to the START position, the start relay and the starter solenoid are energized and
battery power turns the starter motor. With the engine rotating, a pulse is generated at the crankshaft sensor, which provides
the PCM with engine speed information. The PCM then supplies power to the fuel pump and ignition coils through the main
power relay and the fuel pump relay.
Run mode ‑ At approximately 500 RPM, the PCM transitions to the run mode.
• The warm‑up strategy continues to adjust engine speed with the fuel injector pulse width and engine spark advance until
the engine accumulates a given amount of power.
• The camshaft position sensor provides cylinder compression phasing information to the PCM for sequential fuel injection.
• The intake air temperature (IAT), throttle position, and manifold absolute pressure (MAP) are monitored to determine
proper spark timing and the fuel needs necessary to develop the amount of power asked for by the operator.

Important PCM Notes

• PCMs have a low failure rate. Often the problem may be a corroded connection, and simply removing and reinserting the
connector was enough to clean the pins and restore the connections. In general, assume the PCM 112 is working correctly
unless you have reason to believe otherwise.
• If possible, familiarize yourself with the data values displayed by the engine when it is running well. This will make it easier
to spot out‑of‑range values later on.

Test Equipment

In diagnosing Mercury PCM 112 engines, the following test equipment will prove to be invaluable:
•
The Mercury CDS G3 System. CDS G3 is designed to work specifically with Mercury engines. It includes live data
monitoring, active tests, and calibrations for many Mercury engines. Often, a calibration update can resolve an issue.
CDS G3 Interface Kit
•
A digital multimeter. An analog multimeter will not provide the accuracy required to test the equipment involved.
DMT 2004 Digital Multimeter
•
Spark gap tester. Two of these, one for each bank, allows you to see if a spark is being produced.
Spark Gap Tester
90-8M0146617 eng JULY 2018
PCM 112 Important Information and Pinouts
8M0138392
91‑892647A01
91‑850439T 1
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