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Cummins ISB CM550 (1990-94) Fault Code: 442 PID: 168 SPN: 168 FMI: 0 Unswitched Battery Supply Circuit

Circuit Description
The ECM receives unswitched battery input through the engine harness. There are two in-line
10-amp fuses and three 7.5-amp fuses in the unswitched battery wire to protect the engine
harness from overheating. The battery return wires are connected directly to the negative (-)
battery post.

Component Location
The ECM is connected to the battery by the engine harness. This direct link provides a
constant power supply for the ECM. The location of the battery will vary with the original
equipment manufacturer (OEM). Refer to the OEM manual for battery location.

Shop Talk
Disconnect all aftermarket devices from the battery supply circuit. Make sure the proper size
fuses are being used (10-amp fuse or 7.5-amp fuse).

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