Ignition Key Switch Circuit and Battery Supply Circuit

288

TM 9-2320-312-24-2

Troubleshooting Section

Some vehicles may be equipped with an engine

protection shutdown system or an idle timer

shutdown system that interrupts electrical power

to the ECM in order to shut down the engine.

Also, some vehicles may be equipped with an

engine protection shutdown system or an idle timer

shutdown system that interrupts electrical power

to the ignition key switch. The engine protection

shutdown system can be an aftermarket device and

the idle timer shutdown system can be external

to the ECM. Some of these systems will not

supply power to the ECM until one of the following

conditions is met:

The engine is cranked.

The engine oil pressure achieves acceptable

limits.

An override button is pressed.

Keep in mind that these devices may be the cause

of intermittent power to the ECM.

Usually, battery power to the cab data link connector

is available and the battery power to the cab data

link connector is independent of the ignition key

switch. Therefore, you will be able to power up ET,

but you may not be able to communicate with the

engine ECM. The engine ECM requires the ignition

key switch to be in the ON position in order to

maintain communications. The ECM may power

down a short time after connecting ET if the ignition

key switch is in the OFF position. This is normal.

Refer to Troubleshooting, "ATA (SAE J1587 / J1708)

Data Link Circuit - Test".

For intermittent problems such as intermittent

shutdowns that could be caused by vehicle wiring,

temporarily bypassing the vehicle wiring may be

an effective means of determining the root cause.

If the symptoms disappear with the bypass wiring,

the vehicle wiring is the cause of the problem. A

means of bypassing vehicle wiring is explained in

this test procedure. This is especially important for

vehicles that do not provide dedicated circuits for

the unswitched battery and the ignition key switch

connections.