880891T14 Mercury ECU, Engine Control Unit

Grounding
Typically in marine applications, electrical ground, whether AC or DC, is not connected to the hull or chassis.Desired Speed Signals
There are three desired speed signals that are transferred into each MCM panel from Cat signal converters. One of the signal converters is located in the User Interface Box and converts the local speed pot signal to a PWM for the MCMs. The other two converters are In the MCM Panel and convert the two propulsion desired speed signals to PWM for the MCMs.The signal converters output a high-level PWM signal, which means that a high signal is considered positive pulse width. For example, a 75 percent duty cycle will have a voltage close to Batt+ for 75 percent of the signal period.The ECMs, however, read low-level PWM signals, which means that a high signal is considered negative pulse width. For example, a 75 percent duty cycle will have a voltage close to Batt- (normally ground) for 75 percent of the signal period.The MCM software is programmed to make this difference transparent using a software inversion so that the MCM converts the low-level reading to match the high-level output of the signal converters. For this reason, under normal operation, both the signal converters and the MCMs behave as a user would expect, and interpretations of duty cycle readings with a volt meter should be intuitive. However, due to the software inversion that takes place, wire breaks, and shorts can be deceiving and are explained in the following figure and table. The converter output PWM would be read with a multimeter is assumed.Note: The values for an open circuit vary in Cat ET based on which portion of the circuit (Fault Point A or Fault Point B) is faulted open.
Illustration 1 g03285756
Desired speed signal troubleshooting connections
Table 1
Desired Speed Signal Troubleshooting Values in Cat ET    
Fault    Fault Point    Converter Out PWM    Cat ET PWM    
Open     A     0 Percent     0 Percent    
B N/A     100 Percent    
Short Batt +     A     100 Percent     100 Percent    
B N/A     100 Percent    
Short Batt -     A     0 Percent     0 Percent    
B N/A     0 Percent    CAN Communications
The MCM panel relies on communication with the engine panels over the J1939 CAN datalink. Additionally, the Master MCM and the Slave MCM communicate with each other over the same datalinks. When either of the MCMs detects a communication fault over either of the channels, the MCM will throw an appropriate diagnostic, and using these diagnostics, determine which segment of the CAN network is likely to be compromised. Consider the diagram shown in Illustration 2 depicting one of the CAN channels.
Illustration 2 g03285758
CAN network troubleshooting branchesChannel A would be connected to Engine 1 ECM while Channel B would be connected to Engine 2 ECM:
X represents the segment connecting the Master MCM to the network
Y represents the segment connecting the Slave MCM to the network
Z represents the segment connecting the Engine ECM to the networkTable 2 can be used to determine which segment is likely to be compromised using the event codes shown in Cat ET. An example from the table is: If Cat ET shows codes 247-9 from the Master MCM and 3625-9 from the Master MCM, it is likely that segment X is broken on CAN channel A.
Table 2
CAN Network Troubleshooting Codes    
Compromised Segment    Event/Diagnostic Code    
CAN A (Engine 1 ECM) CAN B (Engine 2 ECM)    
X     247-9 from Master     2348-9 from Master    
3625-9 from Master 3626-9 from Master    
Y     247-9 from Master     2348-9 from Master    
3625-9 from Master 3626-9 from Master    
Z     3625-9 from Master     3626-9 from Master    
3625-9 from Master 3626-9 from Master