CTS V8-5.7L VIN P (2004)
Main Relay (Computer/Fuel System): Testing and Inspection
MAIN RELAY CIRCUIT DIAGNOSIS
CIRCUIT DESCRIPTION
The main relay is a normally open relay. The relay armature is held in the open position by spring tension. Battery positive voltage is supplied directly
to the relay coil and the armature contact at all times. The powertrain control module (PCM) supplies the ground path to the relay coil control circuit,
via an internal integrated circuit called an output driver module (ODM). When the PCM commands the relay ON, the relay coil creates an
electromagnetic field. This electromagnetic field overcomes the spring tension and pulls the armature contact into the stationary contact of the relay
load circuit. The closing of the relay contacts allows current to flow from the battery to the following fuses and relays:
-
TOS fuse
-
PCM fuse
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ODD INJ/Coil fuse
-
Pre O2/CAM fuse
-
Post 02 fuse
-
Even INJ/Coil fuse
-
Low speed fan relay
-
S/P fan relay
When the ignition switch is turned to the OFF position, the main relay will continue to be energized for 20 seconds. Once the power is interrupted to
the output driver module in the PCM, the relay electromagnetic field collapses. This allows the spring tension to separate the relay armature contact
from the relay load circuit contact, which interrupts current flow to the fuses.
If the main relay fails to close the engine will crank, but will not run. The PCM class 2 communications will be available with the use of a scan tool.
The main relay system diagnosis table assumes that the vehicle battery if fully charged. Refer to Battery Inspection/Test in Starting and Charging.
See: Starting and Charging/Testing and Inspection/Component Tests and General Diagnostics/Battery Inspection/Test
TEST