Main Relay (Computer/Fuel System): Description and Operation

Main Relay

The Main Relay provides power for the EMS2000 and other system components. It has an additional function of protecting system components from reverse battery polarity. (Main Relay will not energize with reverse polarity.) When the ignition switch is placed in the KL15 or KL50 positions, Fuse 34 is provided with power. Fuse 34 supplies the wake up or on signal to the EMS2000. Upon receipt of the "ON" signal EMS2000 supplies a ground signal from Pin 97 to the Main Relay. The ground signal energizes the Main Relay, supplying operating power to the following fuses:

- F02 - EMS2000, Fuel injectors, Crankshaft sensor, Ignition coils.
- F03 - Camshaft sensor, 02 Heaters, Engine Cooling Fan, A/C Compressor Relay, Purge System.
- F04 - Automatic Transmission controls.
- F05 - Engine Coolant Fan.

The Main Relay supplies power to the following components:

- EMS2000.
- LDP.
- O2 Heaters.
- Fuel Injectors.
- Auxiliary Fan.
- Crankshaft Sensor.
- Camshaft Sensor.
- Ignition Coils.
- ECVT/GIU.


Generator
A dedicated output from the Generator is provided to the EMS2000 to determine electrical load on the engine. When electrical loads are switched on the generator will require more electrical energy, which will in turn create a greater load on the engine. The output is a PWM modulated signal.

If the PWM signal fails the engine may exhibit poor idle speed stability as the electrical loads are increased.

Main Relay Operation
Upon receiving the wake up signal from the ignition switch, KL15 supplying power to fuse 34, the EMS2000 provides a switched ground signal via Pin 97 to the Main Relay. This energizes the Main Relay, providing operating power to the EMS2000. Output of the Main Relay is monitored so that if the relay is energized and the power is not received by the EMS2000 a fault code is set.

Function
The main relay is used to ensure the power supply to the fuel injector valves, to the camshaft sensor and to the crankshaft sensor as well as to other system components. The relay has an additional protective function, whereby it is not activated in the event of incorrect battery polarity. The relay is activated when the engine control module connects to ground.

Installation location
The main relay is located in the fuse box in the engine compartment.

Failure
If the relay remains open continuously, the engine control module does not become active. The engine does not start. If the relay remains closed without interruption, the battery is discharged.