CAP 4693104 - Caterpillar

The 1656-2 diagnostic code is generated when the two start/run/stop inputs to the Engine Control Module (ECM) are not complementary. The status for the Engine Control Switch will display "Abnormal/Incorrect Data" while the fault is active. The Electronic Modular Control Panel 4 (EMCP 4) controls the starting and stopping of the engine. The engine parameters for the total number of crank cycles and the engine cooldown must be programmed to zero (0).The following inputs for the engine control are located on the panel for the EMCP 4: "RUN", "AUTO" and "STOP". These switches control a primary input and a secondary input to the ECM. The primary input to the ECM is over the J1939 data link. The secondary input is a set of relay contacts in the EMCP 4 that are normally open to digital return. If the primary input is lost, the ECM will look to the secondary input. The engine parameter for the secondary input must be set to the enabled state in order for the secondary input to be recognized. When the "RUN" switch is pushed, the panel for the EMCP 4 the start/run/stop inputs 1 and 2 to the ECM are pulled low. If the signals to the start/run/stop input of the ECM are in opposite states, the ECM determines that one of the signals is faulty. When one of the signals is faulty, the ECM will keep the last good input. When the diagnostic code activates and the switch is in the "STOP" position, the engine will not start. When the diagnostic activates and the switch is in the "RUN" position, the engine will continue to run.
Illustration 1 g02736056
Schematic for the ECS with EMCP 4
Test Step 1. Inspect the Electrical Connectors and the Wiring
Set the EMCP 4 to the STOP position.
Turn the main disconnect switch to the OFF position.
Illustration 2 g02721290
Location of the J1/P1 and the J2/P2 ECM connectors (typical engine view)
(1) J1/P1 ECM connector
(2) J2/P2 ECM connector
Thoroughly inspect the ECM connectors J1/P1 and J2/P2. Inspect all of the other connectors. Refer to the diagnostic functional test Troubleshooting, "Electrical Connectors - Inspect" for details.
Illustration 3 g02737060
Terminal locations at the P2 ECM connector
(P2-43) Left starter enable -
(P2-51) Left starter enable +
(P2-92) Digital return
Illustration 4 g02737080
P1 ECM connector
(P1-40) Start/Run/Stop 1
(P1-54) Start/Run/Stop 2
(P1-70) Keyswitch
Illustration 5 g02737097
Engine interface connector
(Terminal 46) Start/Run/Stop 1
(Terminal 47) Start/Run/Stop 2
(Terminal 70) ( P1-70) Keyswitch
Illustration 6 g02737104
Power interface connector
(Terminal 30) Left starter enable -
(Terminal 29) Left starter enable +
Perform a 45 N (10 lb) pull test on each of the wires in the ECM connector that are associated with the circuit.
Check the allen head screw on each ECM connector for the proper torque. Refer to Troubleshooting, "Electrical Connectors - Inspect" for the correct torque values.
Check the harness and the wiring for abrasion and for pinch points. Expected Result:All connectors, pins, and sockets are coupled and/or inserted, and the harness and wiring are free of corrosion, of abrasion and of pinch points.Results:
OK - The connectors and wiring are okay. Proceed to Test Step 2.
Not OK - The connectors and/or wiring