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How to Test Your Main Relay

The main relay is an important part of the vehicle because of what it controls. It operates the fuel pump and supplies power to the injectors. To check the main relay you will need to know how to use a multi-meter or a test light. They are available at hardware stores and are fairly inexpensive. Purchase a multi-meter that reads Ohms or a less test light.

Step 1: Start the Car

Place your hand on the main relay as you attempt to start the car. You should hear or feel three clicks. Look at the ignition. There are different points of starting the vehicle. They are labeled as IG1, ST, and IG2. The first click should turn the key to IG1 but will not start it.  The second click would turn to ST, and wait for the check engine light to go off. The third and final click turns to IG2 and should happen when the car actually starts.

Step 2: Remove the main relay

For the next couple of steps refer to the layout of the main relay as follows:

  • Number 1 terminal goes to the car battery.
  • Number 2 terminal goes to the ground.
  • Number 3 terminal goes to the ECU. (Engine Control Unit)
  • Number 4 terminal goes to the ST on the ignition.
  • Number 5 terminal goes to IG1 on the ignition.
  • Number 6 terminal goes to the ST on the ignition.
  • Number 7 terminal goes to the fuel pump.
  • Number 8 terminal goes to the ECU. (Engine Control Unit)

Step 3: Test the Continuity between No.5 and No.7 on the Main Relay

Connect the car battery negative terminal to the Number 8 terminal and positive terminal to the Number 4 terminal on the main relay. Then check continuity between the Number 5 and Number 7 on the main relay. If continuity is present, go on to step 4. If no continuity is present reinstall the relay and test again.

Step 4: Test the Continuity between No.1 and No.3 on the Main Relay

Connect the car battery negative terminal to the Number 2 terminal and the positive terminal to the Number 5 terminal on the main relay. Then check for continuity between the Number 1 and Number 3 on the main relay. If continuity is present go to step 5. If no continuity present reinstall the relay and test again.

Step 5: Check the Status of Relay

Attach the car battery negative terminal to the Number 8 terminal and the positive terminal to the Number 3 terminal on the main relay. Then check for continuity between the Number 5 and Number 7 on the main relay. If continuity is present the relay is OK.

Step 6: Check the Wiring Harness

If the fuel pump is still not working, continue to test down the wiring harness towards the fuel pump. If no continuity is present reinstall the relay and test again.

What you are checking for is a break in the circuit or in the continuity. There can be a number of reasons for the problem such as a break in the line or a loose connection. A continuity test will tell you if the problem is between point A and point B. Without the main relay functioning properly many of the car electronics won't work.

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