Component Location
General Description
Main relay operates when Ignition ON signal is inputted to terminal 28 of ECM CUD-K connector and it supplies power to ECM, fuel pump relay, CMP, EGR actuator, throttle flap actuator, PTC heater relay and brake switch, etc. Especially, the power supplied to ECM is used as a main power of injectors, rail pressure control valve and ECM operation.
When IG KEY ON signal is shut off during turning engine OFF, ECM stops injector operation then engine turns OFF. Approx. after 5 sec., ECM shuts OFF main relay and system turns OFF.
Main relay is very important for engine operation, so careful inspection is strongly required.
DTC Description
When IG KEY OFF signal is imputted to ECM. ECM stops injector operation andn engine turns OFF. Then, After-run(turning off process of ECM intermal system) is performed in order to shut system down. During this process, if main relay power supply is cut too late or power supply is cut too early before After-run process completes, ECM detect main relay error and P0685 is set.
DTC Detecting Condition
Item | Detecting Condition | Possible Cause |
DTC Strategy | Voltage monitoring
| Main relay circuit
Main relay component
|
Enable Conditions | IG KEY "ON"
|
Threshold Value | When main relay respond to IG KEY - ON/OFF signal too fast or slow (After-run finished and it requires more than 2 sec. for Main relay to be deactivated. Main relay deactivated before After-run finished more than 3 times.)
|
Diagnostic Time | Immediately
|
Fail Safe | Fuel Cut | NO | |
EGR Off | NO |
Fuel Limit | NO |
Check Lamp | NO |
Specification
Main Relay Coil Resistance | 73±10 O (20?) |
Diagnostic Circuit Diagram
Signal Waveform
Fig.1) Main relay operates when IG KEY "ON" (CUD-K terminal 28 ON signal), it turns "OFF" in 12 sec. after IG KEY "OFF".