How to Test Your Honda Main Relay

Main relay diagram for testing reference.
Refer to this chart detailing the terminals of a Honda main relay. about.com library

From time to time, vehicles can experience a wide range of electrical issues—even those that are reliable, like Hondas. One of those problems can involve the main relay. Fortunately, you might not have to rush off to a mechanic to check the relay in your Honda. You can do it yourself by following some simple steps.

What Is an Auto Relay?

Just about every vehicle on the road today includes automotive relays. These components are basically switches that open and close a circuit by electromechanical or electronic means. The relay works as a conductor in between a low amperage circuit, allowing it to turn a higher amperage circuit on or off.

These parts are vital to ensure proper electrical function—not to mention safety—in a vehicle. For example, if you plugged your headlights directly into a headlight switch, you may surpass the amperage rating and cause an electrical fire. Relays can also switch multiple things on simultaneously while using one output, allowing, for instance, you to turn on your car's radio at the same time the antenna extends. The main relay in a vehicle controls the fuel pump and supplies power to the injectors. Knowing how to test a Honda main relay can help you better pinpoint the cause when an electrical issue arises.

How to Test the Main Relay in a Honda

There's a pretty simple way to tell if your main relay is affected. Simply turn on the vehicle and see if it continues to run. If so, the main relay is okay. Should the engine shut off, the main relay could be impacted.

If you suspect you have a bad main relay in your Honda, you should perform the following test to be sure. It can save you time, money, and aggravation, as most parts suppliers don't take returns on electrical components. This is why it's important to avoid buying a part you don't need.

If your Honda is suffering from hot start problems that affect cranking, this diagnostic test might be helpful, too. Consider printing it out to help you while you're testing the relay. Here's how to do it:

  1. Consult your owner's manual or vehicle repair manual and remove the main relay.
  2. Attach the battery positive terminal to the No. 4 terminal and the battery negative terminal to the No. 8 terminal of the main relay. Then check for continuity, using a circuit tester or multimeter, between the No. 5 terminal and No. 7 terminal of the main relay. If there is continuity, go on to Step 3. If there is no continuity, replace the relay and retest.
  3. Attach the battery positive terminal to the No. 5 terminal and the battery negative terminal to the No. 2 terminal of the main relay. Then check that there is continuity between the No. 1 terminal and No. 3 terminal of the main relay. If there is continuity, go on to Step 4. If there is no continuity, replace the relay and retest.
  4. Attach the battery positive terminal to the No. 3 terminal and battery negative terminal to the No. 8 terminal of the main relay. Then check that there is continuity between the No. 5 terminal and No. 7 terminal of the main relay. If there is continuity, the relay is just fine. If the fuel pump still does not work, keep testing down the wiring harness toward the fuel pump. If there is no continuity, replace the relay and retest.