SENSOR-SPEED 4646781 - Caterpillar

Use this procedure to troubleshoot any suspect problems with the electrical power supply.This procedure covers the following diagnostic codes:
168-00 System Voltage High
168-01 System Voltage Low
168-02 System Voltage intermittent/erraticThis procedure tests whether proper voltage is being supplied to the Electronic Control Module (ECM).Unswitched battery voltage is supplied through the customer connector to the ECM at P1-48, P1-52, and P1-53. P1-61, P1-63, and P1-65 is connected to engine ground. The ECM receives the input from the keyswitch at P1-70 when the keyswitch is in the ON position or in the START position. When the ECM detects battery voltage at this input, the ECM will power up. When battery voltage is removed from this input, the ECM will power down.The cause of an intermittent power supply to the ECM can occur on either the positive side or the negative side of the battery circuit. The three connections for the unswitched +Battery must be routed through a dedicated protective device (circuit breaker).The engine ECM requires the keyswitch to be in the ON position in order to maintain communications with Caterpillar Electronic Technician (ET).
Illustration 1 g01330881
Schematic diagram for the electrical power supply
Test Step 1. Inspect the Electrical Connectors and the Wiring
Turn the keyswitch to the OFF position.
Illustration 2 g01183529
Location of the P1 ECM connector (typical left side engine view)
Thoroughly inspect the J1/P1 ECM connector, the battery connections, the ground connections and the connections to the keyswitch. Refer