Electronic Control System Signature, ISX and QSX15 Engines – FAULT CODE 434

Unswitched Battery Supply Circuit


Fault Code: 434
PID: S251
SPN: 627
FMI: 2/2
LAMP: Yellow

Supply voltage to the electronic control module (ECM) fell below (+) 6.2-VDC for a fraction of a second or the ECM was
not allowed to power down correctly (retain battery voltage for 30 seconds after keyswitch is turned off).

Possible no noticeable performance effects or engine dying or hard starting. Fault information, trip information, and maintenance monitor data can be inaccurate.

Unswitched Battery Supply

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 two in-line 15-ampere fuses in the unswitched battery wires to protect the engine harness from overheating. The ECM receives switched battery input through the vehicle keyswitch wire, and one 5-ampere fuse when the vehicle keyswitch is turned ON. The battery return wires are connected directly to the negative (-) battery post.

Component Location

The ECM is connected to the battery by the OEM harness. This direct link provides a constant power supply for the ECM. The location of the battery will vary with the OEM. Use the OEM service manual.

Shop Talk

If the ECM unswitched battery supply is taken from the starter, check for low voltage during cranking. Low voltage during cranking can cause the ECM power supply to drop below specification and log Fault Code 434.

Refer to Troubleshooting Fault Code t05-434.

Last Modified:  26-Oct-2010