Electronic Control System Signature, ISX and QSX15 Engines – 019-087   Unswitched Battery Supply Circuit

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-amp fuses in the unswitched battery wires to protect the ECM. The ECM receives switched battery input through the vehicle keyswitch wire when the vehicle keyswitch is turned on. The battery return wires are connected directly to the (-) negative battery post.

The unswitched battery wires and the battery return wires are in the OEM harness.

Always check the unswitched battery supply fuses when troubleshooting the ECM and power supply circuits.

Check the battery voltage. Refer to

Procedure 019-008.

Disconnect the OEM harness connector from the ECM.

Adjust the multimeter to measure resistance.

Insert a test lead into pin 29 of the OEM harness connector. Attach it to a multimeter probe. Touch the other multimeter probe to the engine block ground and measure the resistance. The multimeter

must show 10 ohms or less.

Test pins 30, 39, 40, and 50 in the same manner.

If the resistance value is

not correct, check the batteries, cables, and cable connections. Repair or replace the parts as required.

When the checks have been completed, connect the OEM harness connector.

Check the battery return wires in the OEM harness for proper grounding. Disconnect the harness from the ECM. Check for damaged pins in the ECM and the harness.

Repair or replace any damaged pins.

Check the battery voltage supply at the 50-pin OEM harness connector.

Turn the keyswitch to the OFF position. Disconnect the 50-pin OEM harness connector from the ECM. Set the multimeter to measure VDC.

Measure the voltage from the OEM harness connector pin 7 to ground. Repeat the process at pin 8, 17, 18, and 28.

The voltage

must read battery voltage at pin 7, 8, 17, 18, and 28.