Mercury 305 CID, Service Manual, Page: 546 / 967

ELECTRONIC FUEL INJECTION (MULTI-PORT AND THROTTLE BODY) - 5E-5 90-823225--1 1096
ECM Self-Diagnostics
The ECM performs a continual self-diagnosis on cer-
tain control functions. This diagnostic capability is
complemented by the diagnostic procedures con-
tained in this manual. The ECM’s language for com-
municating the source of a malfunction is a system of
diagnostic codes. The codes are two digit numbers
that can range from 12 to 51. When a malfunction is
detected by the ECM, a code is set and the Malfunc-
tion Indicator Lamp is illuminated.
Diagnostic Code Tool With
Malfunction Indicator Lamp
There are various manufacturers of Diagnostic Code
Tools. Most Tools are equipped with a Malfunction In-
dicator Lamp (MIL).
•
It informs the service technician that a problem
has occurred and that the vessel is in need of ser-
vice as soon as reasonably possible.
•
It displays Codes stored by the ECM which help
the technician diagnose system problems.
As a bulb and system check, the lamp will come ON
with the key on and the engine not running. When the
engine is started, the light will turn OFF. If the lamp
remains ON, the self-diagnostic system has detected
a problem. If the problem goes away, the light will go
out in most cases after ten seconds, but a code will
remain stored in the ECM.
When the lamp remains ON while the engine is run-
ning, or when a malfunction is suspected due to a
driveability problem, “EFI Diagnostic Circuit Check”
must be performed. These checks will expose mal-
functions which may not be detected if other diagnos-
tics are performed prematurely.
Intermittent Malfunction Indicator
Lamp
In the case of an intermittent problem, the Malfunc-
tion Indicator Lamp will light for ten seconds and then
will go out. However, the corresponding code will be
stored in the memory of the ECM. When unexpected
codes appear during the code reading process, one
can assume that these codes were set by an intermit-
tent malfunction and could be helpful in diagnosing
the system.
An intermittent code may or may not reset. IF IT IS
AN INTERMITTENT FAILURE, A DIAGNOSTIC
CODE CHART IS NOT USED. Consult the “Diagnos-
tic Aids” on the same page as the diagnostic code
chart. “Troubleshooting” also covers the topic of “In-
termittents.” A physical inspection of the applicable
sub-system most often will resolve the problem.
Reading Codes
The provision for communicating with the ECM is the
Data Link Connector (DLC) connector. It is part of the
EFI engine wiring harness, and is a 10-pin connector,
which is electrically connected to the ECM. It is used
in the assembly plant to receive information in check-
ing that the engine is operating properly before it
leaves the plant. The code(s) stored in the ECM’s
memory can be read either through a scan tool, (a
diagnostic scanner that plugs into the DLC connec-
tor), or by counting the number of flashes of the Mal-
function Indicator Lamp when the diagnostic code
tool is installed and SERVICE mode is selected.
73053
DLC Connector
Once the diagnostic code tool has been connected,
the ignition switch must be moved to the ON position,
with the engine not running. At this point, the Mal-
function Indicator Lamp should flash Code 12 three
times consecutively. This would be the following flash
sequence: flash, pause, flash-flash, long pause,
flash, pause, flash-flash, long pause, flash, pause,
flash-flash. Code 12 indicates that the ECM’s diag-
nostic system is operating. If Code 12 is not indi-
cated, a problem is present within the diagnostic sys-
tem itself, and should be addressed by consulting the
appropriate diagnostic chart in “Diagnostics.”
Following the output of Code 12, the Malfunction Indi-
cator Lamp will indicate a diagnostic code three times
if a code is present, or it will simply continue to output
Code 12. If more than one diagnostic code has been
stored in the ECM’s memory, the codes will be output
from the lowest to the highest, with each code being
displayed three times.