Main Relay Circuit Diagnosis

Main Relay Circuit Diagnosis

Circuit Description

The main relay provides ignition positive voltage to many engine control components anytime the ignition is ON. One side of the coil of the main relay is always grounded. The relay is energized when the ignition switch applies voltage to the other side of the coil. The switch side of the main relay receives powered from the main fuse in the underhood fuse block. When the coil is energized, a magnetic field closes the switch side of the relay, supplying power to the following engine components:

* Fuel injectors

* Heated oxygen sensors (HO2S)

* Camshaft position (CMP) sensor

* Evaporative emission (EVAP) purge solenoid

* EVAP vent solenoid

* Engine cooling fan relays

Diagnostic Aids

* Test the condition of the main relay circuits. Measure for 75-95 ohms at 20°C (68°F) continuity across the relay coil, terminal 85 to terminal 86. Measure for infinite resistance across the relay switch, terminal 30 to terminal 87.

* The main relay electrical contacts may be pitted or sticking. Replace the main relay if tapping gently on the relay or wiggling the relay causes a change in the relay's operation.

* The performance of the main relay may be affected by temperature. Check the main relay after sitting outside overnight and after running the engine 30 minutes.

Use the following relay cavity table in order to locate the correct cavities to probe during diagnosis. The table layout corresponds to the cavity layout of the underhood fuse block.