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  1. Honda Acura Electrical and Mechanical Repairs -

    (mainrelay) Main Relay Repair Ignition System Ignition Switch Overheating VSS Downloads About Contact Search




    Soldering Relay

    Honda and Acura: Main Relay Problems and Other Intermittent Starting Problems & Solutions.


    Name: Main relay
    Type: Electrical/Mechanical
    There are many reasons why your Honda or Acura won't start. They can range from a simple corroded battery terminal or a bad distributor rotor to the most expensive of repairs. But, since the main relay is the most reported problem two pages are dedicated to the main relay.
    Main relay symptoms are:
    Car won't start when hot outside (fuel injected PGM-FI models only.)
    Car does not start after parked for just a minute.

    Car starts but quickly dies.
    Windows have been rolled up on a hot day when problem occurred.
    The heater was on before turning the car off.
    Tapping your finger or keys on the main relay will start the car.
    No (humming) sound from the fuel pump the moment the ignition switches ON.
    Check engine light comes on (rare cases) or stays on longer than usual and/or no sparks (electronic timing only.)
    10-15 of waiting and the car pushed to a shade, will usually start.
    Ignition switch symptoms are:
    The dash will go dark when it dies (no warning lights, no shift indicator, no electrical).
    Car stalls then can be started, then stalls after driving or idling for a few seconds.
    Car runs fine when the key slightly held at start position.
    See Ignition Swith page for details.
    Igniter/ignitor symptoms are:
    The tachometer quits abruptly or jerks around before stalling.
    The car quits abruptly or jerks around before stalling.
    Engine cools down and starts, stalls under normal operating temperature.
    No spark at the coil.
    See Ignition System for details.
    Coil symptoms are:
    No spark at the coil.
    Cranking shows no signs of sputtering.
    The distributor was disturbed before it died.
    The coil looks extremely dry, flaking or cracking.
    See Ignition System for details.
    Engine cranks slightly faster than usual.
    Open the throttle halfway usually starts.
    Once started, a plume of smoke is seen from the tailpipe.
    Press the gas halfway down to release fuel vapor and allow more air to fuel ratio usually starts.
    Shutoff the fuel supply (unplug EFI and fuel pump fuse) for 15 seconds while cranking. Then by putting the fuse back in usually starts.
    Some late model cars will shut off the injectors after 15 sec of starting at full throttle.
    Engine cranks but won't start when it rains.
    Signs of moisture on the ignition wire.
    Shutoff the car and come back a couple hours later will start.
    Read more regarding this problem.
    Bad starter solenoid, dirty console, clutch switch:
    Will only click at the starter solenoid but won't start.
    Click here to troubleshoot starter related problems.
    Corroded battery terminals, bad ground straps symptoms are:
    Sometimes the dash will go dark and stalling while driving (no warning lights, no shift indicator, no electrical).
    Car won't start (no relay clicks, warning lights, no electrical)
    Usually a single snap is heard when attempting to start.
    TW (coolant temperature) sensor:
    Won't start when cold outside or on a cold engine.
    Disconnecting the TW sensor usually starts.
    Low compression:
    Won't start when stored for a month or so.
    Pouring a cap of motor oil into the cylinder usually starts.
    Compression reads low. (normal: 165-180 psi)

    If your symptoms best describe the main relay then your best bet is to either repair or replace the main relay. Coils and igniters cannot be repaired. An ignition switch can be repaired in some cases, but most often not. To replace the main relay see instructions below.

    I can't find my main relay, where is it at?
    Turn the ignition to ON. You should hear two clicks (a thousand of a second apart) under the left or right side of the dash board. One click is for the ECU/Injectors the other is for the fuel pump. At this moment the check engine light should come on.
    Listen carefully at the moment the clicks engage (usually somewhere underneath the dashboard.) Reach in and touch the main relay.
    2 seconds later you should feel a click if the main relay is at least working or wait until it works. At this moment the check engine light should go out.
    Turn the ignition to start and you should feel a click, turn engine off you should feel another click.
    Most main relays should have seven terminals. Please check if the location is already listed below.

    How do I check if my main relay is working or if my fuel pump is pumping?
    Close your window and turn the ignition to ON. You should hear two clicks (a thousand of a second apart.) One click is for the ECU the other is for the fuel pump. At this moment the check engine light should come on.
    Listen for the hum of the fuel pump (at the moment the clicks engages) at the rear seat where the fuel tank is located. (You can also listen for the hum at the fuel filler nozzle with the cap removed.)
    2 seconds later you should hear one last click. At this moment the check engine light and the fuel pump should go out.
    If the fuel pump doesn't run, the clicks don't click or the check engine light doesn't go out then the main relay is most likely dead.
    Turn the ignition off and try again.

    How do I check for fuel pressure without a pressure gauge?
    Remove a spark plug.
    Put the spark plug onto the spark plug wire.
    Lay the metal part of the spark plug onto a suitable ground (e.g. a bolt, steel tabs.)
    Crank the motor. You should smell or see a mist of fuel coming from the cylinder using a flash light if the fuel injectors are operating.
    Caution: It is dangerous to check for fuel pressure by releasing the fuel filter bleed bolt since it's unpredictable as to how much fuel pressure is retained and which direction the fuel will spray. Do not smoke or work on an overly heated engine.

    How do I check if my fuel injectors are working?
    With a test light, connect one end of the test light to the positive battery terminal.
    Probe each terminal on the back of the injector connector (try one terminal at a time.) Crank the motor and you should see the test light blink.
    The method to check for fuel pressure above should tell you if your injectors are working.
    How do I check if my fuel pressure regulator (FPR) is working?
    Place a rag over the fuel return line at the bottom of the fuel pressure regulator to prevent damaging the hose.
    With a vise grip, grip the hose (including the rag) to prevent fuel from returning to the fuel tank.
    If the fuel pressure regulator is dead then the fuel pressure (40-45psi) should be restored and your engine should start.
    How do I get out the main relay?
    Steps: I drew four ways below so you can get it out. First you have to reach high up in there and do all this with one hand and a flashlight.

    A. Unbolt the bolt using a 10mm socket (picture A) or
    A. Unclip the clip from the steel tab and slide it apart (picture A) or
    B. Separate the steel tab from the plastic case (picture B) with a flat screw driver jam and twist between the plastic case and the tab or
    C. Unclip two clips I pointed with two arrows (picture C, or below) and then yank it out (D.) This is the preferred choice. See detailed relay photo.

    Accord main relay replacement procedure.
    90, 91, 92, 93 and-94 Accord relay location:

    A main relay should look like this

    or this.

    The main relay is located behind the dash on the driver side to the left of the steering column. It's hidden above the silver cruise control box. After removing the cruise control box, you should see the main relay with a gray and rosy colored base.
    Remove the access panel under the steering column and look upwards.
    Remove the cruise control box using a 10 mm socket in order to see the main relay.
    Remove the relay, including the steel tab.
    Squeeze in the connector's locking tabUnplug the connector from the relay
    Installation is the reverse of the procedure, although it isn't require to bolt it back at its original spot. Move the relay to a low, suitable spot where it is cool and away from heat vents.

    86, 87, 88 and 89 Accord and some Acura Integra relay location/repair procedure:

    A main relay should look like this

    or this.

    The main relay is located just right above the hood release, inside the dash panel bolted to the left kick panel with a bolt that will fit a 10 mm socket. It has seven wires from the connector. Older Acura Legends is located behind the fuse panel near the firewall.
    Remove the coin box (for a better view.)
    With a 10 mm socket or wrench., reach deep inside and at the top of the relay there you will feel a bolt.
    Unbolt it. (If you cannot unbolt then you may dislodge the relay from the the steel tab using a large flat head screw driver.)
    If you damage the tab, you may be able to flatten the tab back to shape.
    Bring the relay down and unplug the connector by squeezing the locking tab then separate the connector from the relay.
    See installation at the bottom of this page.

    The picture below shows the location for the 99 Accord.
    99 Accord

    96, 97 and 98 Civic Main Relay Removal Procedure
    The relay is located to the right of the glove box along the wall of the right kick panel. This is where it is reported to be. If you find it elsewhere, please report it to the author of this website.

    Remove the glove box by removing the bottom panel under the glove box (several screws.)
    Look upward and remove the screws holding the glove box's hinges.
    Lower the glove box, if not possible then squeeze any tabs necessary (do not bend till it breaks.)
    With a 10 mm socket or wrench, reach deep inside and at the top of the relay there you will feel a bolt.
    Unbolt it. (If you cannot unbolt then you may dislodge the relay from the the steel tab.)
    Bring the relay down and unplug the connector by squeezing the lock flap then separate the connector from the relay.
    Installation is the reverse of the procedure.

    85, 86, 87, 88, 89, 90 and 91 Civic Main Relay Removal Procedure
    The 85-91 Civics is located right above the fuse box. It can be felt while leaning down from driver's seat. The color of the relay is usually black with a brown base. The wires coming from it are usually seven. It can even be seen without access panels removed. It's to the right and above the hood latch release. It's pretty high up. Removing it may require removing several parts surrounding it.

    A main relay should look like this

    or this.

    Remove the coin box (for a better view)
    With a 10 mm socket or wrench, reach deep inside and at the top of the relay there you will feel a bolt.
    Unbolt it. (If you cannot unbolt then you may dislodge the relay from the the steel tab.)
    Bring the relay down and unplug the connector by squeezing the lock flap then separate the connector from the relay.

    92, 93, 94 and 95 Civic Main Relay Removal Procedure

    A main relay should look like this

    or this.

    The main relay is to the left of the fuse box, wedged up against the left above the hood latch in a tight spot.
    Remove the coin box (for a better view.)
    With a 10 mm socket or wrench., reach deep inside and at the top of the relay there you will feel a bolt.
    Unbolt it. (If you cannot unbolt then you may dislodge the relay from the the steel tab using a large flat head screw driver.)
    If you damage the tab, you may be able to flatten the tab back to shape.
    Bring the relay down and unplug the connector by squeezing the locking tab then separate the connector from the relay.
    See installation at the bottom of this page.

    Acura Main Relay
    EFI Main Relay for:
    Honda Civic/CRX/del Sol 1984-1995
    1986-89 INTEGRA
    1990-93 INTEGRA #39400-SH3-003 RELAY (RZ-0063)
    1986-90 LEGEND #39400-SH3-003 RELAY (RZ-0063) (RZ-0046)
    1991-93 LEGEND (rz-0105)
    VIGOR (behind the left front kick panel)
    is located behind left side of the dash and above and behind the fusebox and above the hood release. The removal procedure is the about the same as the Accord's procedure.

    Legend main relay
    Installation is the reverse of the procedure.
    It isn't required to install onto its original location. Move it to a cooler practical location. Eg., away form heat ducts, vibrations or from higher locations. And future troubleshooting would be far easier, considering that the next technician will be able to locate it.

    Fuel delivery Technical details
    Physically moving Fuel: -
    The fuel begins its journey from the fuel tank and into the fuel pump then to the fuel filter and into to the gates of the fuel injectors. It should not be restricted. The fuel cap should not be restricted. The fuel pump's check valve should not be broken. The fuel pressure regulator should hold and maintain pressure. But in the case of a bad main relay, the fuel pump will not work because the fuel pump relies heavily on the mainrelay to send power to the fuel pump. This is a safety feature designed into most cars. During such times when the car is in a collusion, stalling or (on some vehicles) overturned the main relay shuts off the fuel pump when the ECU (main computer) tells it to. The car starts then quickly dies out. This is due to the remaining fuel pressure in the fuel line. Many people complained that the problem is creating anxieties because they are afraid of being stranded or ending up with an expensive repair bill.
    When heat from the cabin, heat due to the fuel pump, heat from the relay coil windings expand and contract the poor quality/applications solder the joints inside the main relay develops a dry crack. Thru time arc contaminants will grow in the crack(s) and the main relay will not work effectively. That is why Honda Acura owners need to remove the old solder and replace it with a higher quality solder. This should not cost much requiring that you use the proper tools and materials on the next page.
    Electronically controlling fuel: ECU.
    The ECU detects the crank signal from the distributor. The ECU uses this signal (during idle) to close the main relay which powers the fuel pump. If the engine shuts off so will the fuel pump. Carburetor model uses the coil tachometer or starter signal to keep the fuel pump running. On a cold motor the ECU sends one long 7ms burst of current per cycle to the fuel injectors to obtain a rich mixture.
    On some vehicle going full throttle during starting for 15 seconds or more will shutoff the fuel injectors for several seconds. After this time the injectors will automatically turn back on once flooded gas has a chance to clear. Check your users manual if yours have this feature.
    Electronically controlling fuel: Main relay.
    Technically, the relay is made up of two relays, one for the fuel injection system and one for the fuel pump. To prove this you can hear two clicks almost in sync when you turn on the key. The relay sits on a PC board inside a plastic case, The PCB board contains resistors and some diodes.
    Usually the fuel injection system relay may be the second to fail. If the fuel injection relay fails, a "check engine light" will turn on longer than usual after the ignition is first turned to ON. And diagnostic trouble code (DTC) 16 may be sought, which should point to a fuel injector open circuit problem - which is most likely a main relay problem.
    The relay will close, pressurizing the fuel rail for 1 second and 1/3 of a second when the ignition is switch ON. After this time the fuel relay will open - this can be heard as a second "click." The second click means that the relay is working but doesn't necessarily mean that the relay has powered the fuel pump. Switching ignition to START will close the relay, pressurizing the fuel rail until the ignition key is released from START. The ECU will (in addition to other battery power source) obtain battery power from the main relay. If no battery is found a "check engine light" will come on and stay on and the car won't start and won't produce any sparks on cars with electronic timing.
    There are cases where no sparks have been linked to a bad main relay. The ECU will send timing signals to the igniter before the coil can produce sparks. Since the ECU relies on the main relay for power, a bad relay usually produce no spark on these models (88 Civic and above, 90 Accord and above.)
    The main relay supplies battery voltage to the injector resistors when the ignition is switched on and, on some cars, the heated oxygen sensor.
    On run mode, the main relay will supply power to the fuel pump. The information needed to keep the pump running originates from inside the distributor via crank sensor (as stated before.)
    Do not run the starter for longer than 20 seconds at a time. Doing this will burn out the starter motor or starter motor brushes. Crank for less than 20 seconds if you are attempting to obtain a DTC code. Continuously running the starter will not help but a slight banging with a fist on the dash board might help to start the car.
    Schematic of main relay is shown connected to other components.
    Diagram of its internal circuits.
    Main Relay
    Relay manufacture
    Mitsuba, made in Japan.
    Numbers of relays inside

    Rated Amps

    Installed on Carburetor models

    Expected life span
    8-10 years
    Price ranges