How to test the Main Relay

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Ways you can test the EFI Main Relay

The problem with testing the Main Relay is that it often malfunctions when hot. When it cools off, it frequently resumes normal functioning, which drives everybody batty because they can't reproduce the problem when their favorite mechanic is looking at the car.

Generally, the symptoms are usually enough to condemn the Main Relay: Clicks are missing, the fuel pump does not run when the Check Engine light is on, the car starts then stalls as soon as the key is released, that sort of thing. If you still want to test it, here are five you can perform.

All the tests require that the Relay be REMOVED from the vehicle. Note that the first three require you to feed (and ground) 12V battery power to the Relay while checking for continuity or the presence of voltage. The other two require that you have NO power going to it. In these two you simply test for continuity.

Ignore the appearance of the switches inside the Relay pictures. They appear to be closed, which they may or may not actually be in a given case, but it's not important here to show them accurately. The important thing is where power goes, and what terminals you check.

Also, where simple continuity is involved, it does not matter what orientation the probes are in. Positive can go on either side. I put different colors just to make it clear that the negative side was not going to ground.

Test #1

With battery power being fed to Terminals 1 and 5, check for voltage with your multimeter by connecting the + side to Terminal 7. Then remove the + probe from 7 and move it to Terminal 3.

There should be battery voltage available at both locations.

This test confirms that both relays are closing.

Test #2

With battery power being fed ONLY to Terminal 5, and with Terminal 2 grounded, connect the probes of your multimeter between Terminals 3 and 5.

There should be continuity between both locations.

Test #3

With battery power being fed ONLY to Terminal 3, and with Terminal 8 grounded, connect the probes of your multimeter between Terminals 5 and 7.

There should be continuity between both locations.

Test #4 (NO battery power!)

Connect the probes of your multimeter between Terminals 2 and 5.

There should be continuity between both locations.

Test #5 (NO battery power!)

Connect the probes of your multimeter between Terminals 3 and 8.

There should be continuity between both locations.