The ECM internally steps down 24V to 12V for sensor references. However, injector drivers may produce 90-110V spikes. Never probe injector wires with a standard multimeter.
| Pin | Function | Wire Color | Notes | |------|-----------------------------|------------|---------------------------| | 1 | Keyswitch ON (Battery +) | Red | 24V switched power | | 2 | ECM Ground (-) | Black | To battery negative | | 3 | ECM Ground (-) | Black | | | 4 | ECM Ground (-) | Black | | | 5 | ECM Power (+) | Red | Unswitched +24V | | 6 | ECM Power (+) | Red | Unswitched +24V | | 7 | ECM Power (+) | Red | Unswitched +24V | | 8 | Cat Data Link (+) | Yellow | To service tool | | 9 | Cat Data Link (-) | Green | | | 10 | Throttle signal (+) | Brown/White | 0 to 5V DC | | 11 | Throttle return (-) | White | Ground reference | | 18 | Engine cooling fan relay | Purple | ECM ground output | | 29 | Start aid (ether) relay | Orange | ECM ground output | | 40 | J1939 CAN High | Blue | | | 41 | J1939 CAN Low | White/Blue | | caterpillar 3412 ecm wiring diagram