178 KENR9126
Troubleshooting Section
The cause of an intermittent power supply to the
ECM can occur on
either the positive side or on the
negative side of the battery circuit. The connections
for the unswitched battery+ may be routed through a
dedicated pro
tection device (circuit breaker).
Some applications may be equipped with an
engine prote
ction shutdown system or an idle timer
shutdown system that interrupts electrical power
to the keyswitch. The engine protection shutdown
system can b
e 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 on
e of the following conditions is met:
•
The engine is cranked.
•
The engine oil pressure achieves acceptable limits.
•
An overrid
e button is pres sed.
Keep in mind that these devices may be the cause of
intermit
tent power to the ECM. These devices may
also shut down the engine.
Usually,
battery power to the diagnostic connector
is available and the battery power to the data link
connector is independent of the keyswitch. Therefore,
you will
be able to power up the electronic service
tool, but you may not be able to communicate with
the engine ECM. The engine ECM requires the
keyswit
ch to be in the ON position in order to maintain
communications. The ECM may power down a short
time after connecting the electronic service tool if the
keyswi
tch is in the OFF position. This is normal.
For intermittent faults such as intermittent shutdowns
that c
ould be caused by the application wiring,
temporarily bypassing the application wiring may be
an effective means of determin ing the root cause. If
the sy
mptoms disappear with the bypass wiring, the
application wiring is the cause of the fault. A means
of bypassing the application wiring is explained in
this
test procedure. This is especially important for
applications that do not provide dedicated circuits
for the unswitched battery and the connections for
the k
eyswitch.
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