Cummins ISB CM550 (1990-94) Fault Code: 434 PID: SID 251 SPN: 627 FMI: 2 Unswitched Battery Supply Circuit

Circuit Description
The ECM receives constant voltage from the batteries through the unswitched battery wires
that are connected directly to the positive (+) battery post. There are three in-line 7.5-amp
fuses and one 20-amp fuse in the unswitched battery wires to protect the engine harness from
overheating. The ECM receives switched battery input through the vehicle keyswitch wire,
when the vehicle keyswitch is turned on.

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 equipment
manufacturer. See the equipment manufacturer repair manual.

Shop Talk
NOTE: If a battery disconnect switch is turned off after the keyswitch is turned off, but before
the ECM completely powers down (up to 20 seconds) this fault may register.

Make sure that ECM unswitched power is coming from the batteries and not the starter or other device. Check for low voltage during cranking. Low voltage during cranking can cause the ECM power supply to drop below specifications and log Fault Code 434.

Corrupted voltage (short interruptions or spikes that may only be seen with an oscilloscope) on
keyswitch line can cause this fault. If the voltage at pin 5 of engine connector is measured as
negative with oscilloscope at key-off, Fault Code 434 can be logged.