Mercury V6 175 Diagnostic Manual page 42

PCM 112 and Engine Control Overview
PCM 112 Operation, Faults, and Data Fields
PCM Operation, Faults, and Data Fields
When the engine is started, the PCM begins checking the various engine sensors and actuators for proper operation. During
this time, the system is said to be in closed‑loop control. If for some reason, the PCM cannot enter closed‑loop control, it will
remain in open‑loop control and revert to default look‑up tables to control the engine.
In response to changing inputs, it sends control information to the various systems, such as to control the pulse width of the fuel
injectors in response to a throttle change by the operator.
Faults
A fault indicates that the PCM has either sensed that the circuit in question has recorded a sensor value outside of its
acceptable window, meaning that the sensor circuit has failed, or that a sensor value has gone outside its normal range, as
when the engine is running too hot.
Mercury fault codes are explained in Section 4A.
Not every engine issue triggers a fault; however, the Live Data feature in CDS G3 can provide important clues to the engine's
operation.
Live Data is available after the CDS G3 system has been correctly connected to the engine, the correct electronic bill of
materials (eBOM) has been selected, the key switch is in the run position, and the engine module has been selected under the
Module Data screen in CDS G3.
CDS G3's Live Data Screen provides real‑time feedback from the engine, identifying the values reported from the various
sensors, how the emissions control system is working, how much spark and fuel is being added or subtracted, and many other
data items. Live data takes time and experience to interpret. With experience, you will be able to determine which values are
out of range or inconsistent with other data. For an explanation of the data included in the live data stream, refer to Section 3C
‑ CDS G3—Live Data.
System Diagnostics Overview
Comprehensive Component Diagnostics
Upon engine start‑up, the PCM will conduct a comprehensive component diagnostics routine. This includes open‑ and
short‑circuit checks of circuits and sensors, as well as rationality checks to ensure that the sensors are returning expected
values.
While a short or open circuit will show a value that is at the extreme of its range and is fairly easy to detect, a rationality check
checks the value of the sensor against an expected value. For example, the barometric pressure reading shows 61.2 kPa,
which equates to approximately 14,000 feet. This would be the altitude of Pikes Peak, in the US. Since there are no lakes that
high where engines will be run, it will not pass the rationality check for barometer.
The system must be able to identify components that are not functioning properly. These components include:
• Manifold absolute pressure (MAP) sensor
• Camshaft position sensor
• Crankshaft position sensor (CPS)
• Throttle position sensor (TPS)
• O2 sensor
• Injectors
• Ignition coils
• Temperature sensors
• Pressure sensors
If the diagnostic routine finds an issue with a device or circuit, it will return a fault code that can be viewed on CDS G3. It may
indicate a faulty component or device, or that a value is out of range. Refer to Section 4A ‑ Fault Codes. It may then revert to
open‑loop operation, and depending on the severity, may enact Engine Guardian to limit engine speed.
Closed‑Loop Control Status
The term O2ControlState on the Engine data tab in CDS G3 displays the current status of closed‑loop control. The statuses
that can appear in this field are:
• Open‑loop means the engine has not yet met the requirements to go into the closed‑loop mode, the engine is undergoing
a change in demand or gear, or it is running a diagnostic routine.
• Closed‑loop means that the engine is in the closed‑loop fuel control mode and responding to inputs from the O2 sensor to
adjust fueling.
Page 2B-2 © 2018 Mercury Marine 90-8M0146617 eng JULY 2018