Main Relay (Computer/Fuel System): Testing and Inspection

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

Refer to Engine Controls Power And Ground Schematics.

CIRCUIT DESCRIPTION
The main relay is controlled by the Powertrain Control Module (PCM). The PCM provides a ground for the relays coil anytime the ignition switch is On. The main relay supplies ignition positive voltage to many engine control systems and components. The following components receive power from the main relay:
^ The fuel pump relay
^ The fuel injectors
^ The Mass Air Flow (MAF) sensor
^ The EVAP control system solenoids
^ The exhaust gas recirculation (EGR) valve
^ The Idle Air Control (IAC) valve
^ The Powertrain Control Module (PCM)

DIAGNOSTIC AIDS
Check for any of the following conditions:
^ Check the resistance of the main relay. The resistance across terminal 1 and terminal 2 is 79-95 ohms at 20°C (68°F). The resistance across terminal 3 and terminal 4 is infinite.
^ 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.
^ Check for a faulty electrical connection to the PCM.

An intermittent malfunction may be caused by a fault in the main relay electrical circuit. Inspect the wiring harness and components for any of the following conditions:
^ Any backed-out terminals
^ Any improper mating of terminals
^ Any broken electrical connector locks
^ Any improperly formed or damaged terminals
^ Any faulty terminal-to-wire connections
^ Any physical damage to the wiring harness
^ A broken wire inside the insulation
^ Any corrosion of electrical connections, the splices, or the terminals

Main Relay Circuit Diagnosis:

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

TEST DESCRIPTION
The numbers below refer to the step numbers in the diagnostic table.
1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
6. This step will verify whether the PCM is not providing a ground path for the main relay circuit. When terminal 2 of the main relay is grounded, the main relay turns on.
9. This step checks for ignition positive voltage at the PCM. The PCM will not operate the main relay if there is no power to terminal 20.
11. This step checks for an open in the PCM electrical grounds. The PCM will not operate the main relay if there are no electrical grounds.