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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.
Main Relay will click three times during the starting process. When
problems arise, one of those clicks (usually the third) is missing. If
hear/feel all three clicks, the Relay is fine. DO NOT REPLACE IT.
- Turn ignition to ON (but not to START):
- Check Engine light goes off: Click 2
- You now turn the key to START: Click 3
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.
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
Understanding the operation of this device will help you understand why
cracked solder causes problems, and will help you troubleshoot those
This page shows the operation of a correctly functioning Main Relay.
Damaged relay problems are shown on the next
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
from the line that runs between Terminal 7 and the fuel pump, and
2) Terminal 3 ONLY goes to
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
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
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...
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.
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.
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
2) The injectors
receive power from Terminal 3. Since the first relay is already closed,
the injectors get power at all times that the ignition
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,
these two happen so close together than you really have to listen to
tell that it's actually two clicks.
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.
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
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.
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
Now, what happens if
it goes bad and solder cracks? This is
update: March 27, 2008