HARNESS AS-WIRING 1888589 - Caterpillar

System Operation Description:This procedure tests whether proper voltage is supplied by vehicle wiring. Use this procedure if any one of the following diagnostic codes are logged several times:
168-02 Intermittent Battery (51)
43-02 Ignition Keyswitch Fault (71)Also, use this procedure if you suspect that the ECM is not receiving the battery supply voltage.Note: This code can be generated by rapidly cycling the ignition keyswitch. Some control modules on the vehicle require this action in order to prompt flash codes. If this occurs, clear the logged diagnostic codes in order to prevent future confusion or an incorrect diagnosis.The ECM receives electrical power (battery voltage) through the wiring that is supplied by the vehicle manufacturer. The ECM input at connector P1 terminal 70 (ignition keyswitch) receives battery voltage from the ignition keyswitch when the ignition keyswitch is in the ON position or 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 (unswitched +battery) or the negative side (−battery). Both sides are routed from the ECM to the battery. The two connections for the unswitched +Battery must be routed through a dedicated protection circuit.Some vehicles may be equipped with an engine protection shutdown system or an idle timer shutdown system that interrupts electrical power to the ECM in order to shut down the engine. Also, some vehicles may be equipped with an engine protection shutdown system or an idle timer shutdown system that interrupts electrical power to the ignition keyswitch. The engine protection shutdown system can be an aftermarket device and the idle timer shutdown system can be external to the ECM. Some of these systems will not supply power to the ECM until one of the following conditions is met:
The engine is cranked.
The engine oil pressure achieves acceptable limits.
An override button is pressed.Keep in mind that these devices may be the cause of intermittent power to the ECM.Usually, battery power to the cab data link connector is available and the battery power to the cab data link connector is independent of the ignition keyswitch. Therefore, you will be able to power up ET, but you may not be able to communicate with the engine ECM. The engine ECM requires the ignition keyswitch to be in the ON position in order to maintain communications. The ECM may power down a short time after connecting ET if the ignition keyswitch is in the OFF position. This is normal. Refer to Troubleshooting, "ATA (SAE J1587 / J1708) Data Link Circuit - Test".For intermittent problems such as intermittent shutdowns that could be caused by vehicle wiring, temporarily bypassing the vehicle wiring may be an effective means of determining the root cause. If the symptoms disappear with the bypass wiring, the vehicle wiring is the cause of the problem. A means of bypassing vehicle wiring is explained in this test