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The EFI (Electronic Fuel Injection) Main Relay is the heart of your Honda
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It operates the fuel pump, AND supplies power to the injectors. Without this thing in proper working order, your car is going nowhere. As stated elsewhere, this device is prone to failure due to cracked solder, which results in intermittent/nonexistent current flow to the fuel pump or the injectors.

If you listen very carefully, or reach around behind the dash and put your fingers on the Main Relay, you'll hear/feel THREE CLICKS as you start the car. Check the green text on the images to see when those clicks should appear. Most Main Relay problems that cause starting issues result in one or more of those clicks being missing, making the clicks a handy diagnostic tool.

The Main Relay will click three times during the starting process. When problems arise, one of those clicks (usually the third) is missing. If you consistently hear/feel all three clicks, the Relay is fine. DO NOT REPLACE IT.
Battery current passes through the main relay every second the car is running, so it gets fairly warm. The constant heat/cool cycle that occurs as the car is used then shut off flexes the solder and eventually it cracks.

The fuel pump's default state is OFF. There is only one specific condition that will result in the pump running, so pretty much anything that goes wrong inside the Main Relay will turn the pump off, and your car won't run.

Understanding the operation of this device will help you understand why cracked solder causes problems, and will help you troubleshoot those problems.

This page shows the operation of a correctly functioning Main Relay. Damaged relay problems are shown on the next page.
IMPORTANT NOTES:

The graphics below show the Main Relay for Multi-Port FI (one injector per cylinder). Dual-Point FI (two injectors in the throttle body) Main Relays differ in two respects:
1) The injectors get their power from the line that runs between Terminal 7 and the fuel pump, and
2) Terminal 3 ONLY goes to the ECU, and has no connection to the injectors.

As of about 2005, Honda separated the Main Relay into two assemblies that plug into the under-dash fuse box. These graphics do not cover that design, which should prove to be quite a lot more durable than the arrangement covered by these pages.

The pin assignment numbers listed are common to many Civics and Integras, but are NOT true for ALL Honda models! Honda's shop manual diagrams are all the same, but the actual pin assignments differ from model to model and from year to year. Before using the diagrams below, you MUST either be very savvy with electrical troubleshooting, or have the wiring diagrams for your car.

Without further ado, we start with Step 1...

Main Relay Step 1

Even with the key out, Terminal 1 receives straight battery voltage (yellow line) at all times. That voltage can't go anywhere because the first relay (the one on the left) is open, and that relay can't close until the ignition (red line) is turned on.
Main Relay Step 2
When you first turn the key on, the ECU (the computer) wants to charge-up the fuel rail pressure. It does this by running the fuel pump for two seconds. It's assuming that the rail already has fuel and just needs a bit of a pressure boost to get it ready for a startup. You'll find that your Honda will start faster if you allow it to sit for five or ten seconds after the Check Engine light goes off, before operating the starter.

As you can see, all sorts of things are happening here.
1) Terminal 2 is always grounded. The ECU monitors current flow here, but cannot shut that ground off. This means that the first relay is always closed  as long as the ignition is ON.
2) The injectors now receive power from Terminal 3. Since the first relay is already closed, the injectors get power at all times that the ignition is ON.
3) The ECU uses Terminal 8 as its activation line for the fuel pump. The ECU controls the second relay, and thus fuel pump operation, by either applying ground or removing it at Terminal 8.

In this picture, the ECU is grounding Terminal 8, which allows the second relay to be pulled closed. Current can now flow from the feed tapped from near Terminal 5 to Terminal 7, and then to the fuel pump.

I'm showing two clicks in Step 2. I kinda fibbed a bit when I said earlier that there were THREE clicks in total. There are actually FOUR, but these two happen so close together than you really have to listen to tell that it's actually two clicks.
Main Relay Step 3

One thing Honda does not want is a flooded engine, or one spraying gas all over the place after a collision. The ECU studies the signals it receives from the Crank Angle Sensor (inside the distributor on cars with distributors). It knows from this device whether the engine is turning or not. Note that the engine does not have to be running, just turning, so the fuel pump will operate even if you push-start the car.

Since the ignition has just been turned on, and our engine is not turning at this point, the ECU cuts ground to Terminal 8 after two seconds, the second relay pops open and the fuel pump stops.
Main Relay Step 4

The key has been turned to START. The ECU has re-supplied ground to Terminal 8 because it sees the engine turning, all lines are now carrying current again, and the fuel pump runs again.

Notice something here: We have a redundant power feed to Terminal 8, and therefore to the second relay. Not only is there battery feed through a voltage-dropping resistor from Terminal 1, but the starter terminal on the ignition switch is also feeding power all by itself through Terminal 4, but with no resistor.
Relays take more current to energize than to hold closed. The starting position provides full current to initially energize the relay, then the voltage through the resistor holds the relay closed while using less current and generating less heat. It would appear that Honda is trying to ensure reliable fuel pump operation during cranking by feeding boosted voltage to the second relay.
Main Relay Step 5

This is the final configuration, the one that will persist until you shut the car off again.  When you release the starter, all that happens is the START circuit loses power as the ignition switch rotates away from it. Otherwise it's the same power distribution you saw in Step 2.
Now, what happens if it goes bad and solder cracks? This is what.

Last update: March 27, 2008