Volvo 740 Blowing Fuse #1 (fuel pump fuse)

Discussion in 'Volvo 740' started by Darien, Sep 19, 2009.

  1. Darien

    Darien Guest

    I am having a problem with my otherwise reliable 1988 Volvo 740. For
    several years now when I turned the car in very cold weather the idle
    would falter a little. If I touched the gas pedal it would be just
    fine. Then I started noting several months ago that when I was driving
    in the highway (~70mph) occasionally when passing or going up hills,
    the RPM would completely drop and the car was coasting. If I let go of
    the accelerator completely and step on it again, the car would
    continue right away just fine.

    Well, at long last one day I was driving in the highway and after
    about 1 hour. The car lost power and coasted to a stop. I couldn’t
    restart the car. And after being towed home, I noticed fuse #1 was
    blown. So I replaced it, but the fuse blew again when I tried to move
    the car. Spoiling my victory dance, I might add :)

    I am in a new town, and my new mechanic didn’t want to look at the
    fuel pump (small town). I took it to another guy who noticed that fuse
    #1 will only be blown if the distributor was connected (he loosen the
    wire harness from underneath the distributor and showed me). So he
    replaced the distributor with a new one ($400 ouch!).

    Unfortunately after driving the car home, the next day when going up a
    hill near my house, fuse #1 blew again, and again, and again. So a
    neighbor helped me move my car home, where it is parked now.

    Following some advice on the internet about checking the fuel relay I
    short-circuited to terminals, but fuse #1 simply blew right away. So
    today I got under the car and unplugged the fuel pump, thinking that
    the fuel pump was draining too much current. BUT when I cranked up the
    engine with the fuel pump wires disconnected (from under the car). The
    fuse just blew up again, and again.

    I remembered reading on the internet that fuse#1 was also the fuse for
    the air sensor (I am not sure of this). So I went and unplugged the
    wire harness for the air sensor (because it is so easy right next to
    the air filter). And no difference, fuse #1 blew up when cranking the
    car.

    What else might be fuse #1 protecting? Right at this moment I am
    thinking that the wires from the fuel pump to the fuse box have a
    short circuit somewhere, but I can’t tell. I cut my hand trying to
    remove the plastic of the 2 wires that connect the fuel pump, but they
    look ok (I didn’t remove the whole plastic). I am removing the front
    seat to see if I can track the wires from the fuel pump to the fuse
    box.
    Any other ideas or suggestions? Anybody knows what else could fuse #1
    protecting, or what could be making the short circuit? Thank you very
    much for your help, I’d appreciate any insight or suggestions. Thank
    you.

    Darien
     
    Darien, Sep 19, 2009
    #1
  2. Darien

    James Sweet Guest



    With the distributor disconnected the ignition system won't report
    engine rotation to the ECU and the fuel pump won't be turned on. I
    cannot imagine what the mechanic was thinking by replacing the
    distributor, there is absolutely no way that a bad distributor can blow
    the fuel pump fuse, there is just no connection, so first things first,
    find a mechanic who actually has a clue, this guy is incompetent. Look
    for someone who specializes in European cars.

    Diagnosing this problem is really simple. Reach under the car and unplug
    one of the wires from the fuel pump. Put in a new fuse and crank the
    engine, if the fuse doesn't blow, the pump is probably bad. If it does
    blow, unplug the fuel pump relay and try again. If that stops the fuse
    from blowing, then there's probably a short in the wiring from the relay
    to the pump, it comes up through a rubber bung above the pump and runs
    along the edge under the carpet up to the relay in the dash.
     
    James Sweet, Sep 19, 2009
    #2
  3. Darien

    Darien Guest

    With the distributor disconnected the ignition system won't report
    James thank you very much for your help. I live in a 15k little town
    in central Missouri, I couldn't find any mechanic that knows about
    European cars, unfortunately.

    I did unplug both wires in the pump from under the car and the fuse
    keeps blowing with BOTH wires disconnected. Do you know if there is
    anything else I should be looking for? Is fuse #1 protecting anything
    else besides the fuel pump?

    As you suggest, I will try replacing the wires going from the fuse/
    relay to the pump, but it is a such a cramped space. I don't know how
    to work there effectively. I wonder if there is some particular trick
    I could use. Else I'll do what I can. Thank you very much for your
    help.

    Darien
     
    Darien, Sep 22, 2009
    #3
  4. Darien

    James Sweet Guest


    Ah, yeah the West coast over here is swarming with Volvos, it's rare to
    find a single residential street without at least a couple of them in
    driveways, so specialists are a lot easier to find here. These cars are
    very easy to work on in general, however they are different, and someone
    who is not familiar with the difference can have a lot of trouble.

    What year is this car again? There was a redesign of the 740 at some
    point, '90 I think, I'm less familiar with those but on the earlier
    700's the fuel pump relay is in the fuse box which resides behind the
    ash tray under the radio. In those, if you pop off the little cover
    around the cigar lighter there's a screw, take that out, and you can
    pull out that whole pocket assembly under the radio. Once you've done
    that, there's a pair of clips you press and the whole fuse/relay panel
    lifts up and pulls out on an umbilical cord of sorts which gives you a
    lot more room to work with.

    Either way, when you find the relay, unplug it and see what happens. If
    the fuse still blows, then something else is wrong, but I suspect the
    culprit is something simple. I would carefully inspect the wires that
    come from the pump where they go up through the rubber bung in the floor
    pan and pull up the edge of the carpet to make sure they have not been
    damaged there. Also, don't forget the pre-pump which is located in the
    fuel tank and runs on the same circuit. You might have a short in the
    wiring back in that area too, on the sedans you can get to the top of
    the tank easily though a cover panel under the carpet in the trunk, I
    don't recall where it is in a wagon though.

    One other helpful trick that will save you some fuses. If you have an
    old headlight bulb laying around, use some clip leads and wire the high
    beam filament in place of the fuse then place the bulb somewhere where
    it will not melt the carpet. If the bulb glows brightly there's still a
    short, if it glows dimmer or not at all, the short has been cleared.
     
    James Sweet, Sep 23, 2009
    #4
  5. The 740 has a little primer pump in the tank which delivers fuel to the
    main pump.If the in tank pump has gone the main pump works overtime and the
    fuse melts in the holder or blows or both in my wifes 740 1988.The tank pump
    needs to be replaced .This is most common when the tank is low on gas (very
    common with women drivers to run the tank till its almost empty) .Mind you
    if you keep the fuel at a sensible level you avoid getting condensation and
    other issues in your fuel.
     
    Jon Robertson, Sep 30, 2009
    #5
  6. Darien

    James Sweet Guest


    I just had to deal with that this week on my brother's 760. The fuse
    didn't blow, but the car did die. I found that the pre-pump had failed
    and the bellows connecting it was split. Lots of fun laying in the trunk
    removing the hoses and then maneuvering the pickup assembly out of the
    bung hole and through the little access panel.
     
    James Sweet, Oct 1, 2009
    #6
  7. Darien

    Roger Hunt Guest

    I usually run my (petrol) V40 until the tank is almost empty, and I am
    not a woman.
    Last night for instance, I travelled for miles after the fuel warning
    light came on, then put 58 litres in (60l tank).
    I don't need to "keep the fuel at a sensible level". It's always at a
    sensible level as far as I'm concerned.
    I fill it to the brim, run it to almost empty then fill it to the brim
    again, and think that doing it this way keeps the whole fuel system as
    flushed as possible.
     
    Roger Hunt, Oct 1, 2009
    #7
  8. Darien

    James Sweet Guest


    That's what I do as well, the pumps are fine as long as you keep fuel
    flowing through them. They don't need to be submerged in fuel to stay
    cool, the rotor which is where the heat is produced is submerged in fuel
    whenever fuel is flowing through the pump.

    They normally last in the ballpark of 200k, and by the time they're worn
    out, the rubber bellows has usually split too.
     
    James Sweet, Oct 2, 2009
    #8
  9. Darien

    Darien Guest

    Thanks everybody for the suggestions, I have had no time to look into
    the wires more. But I'll try this weekend. My 740 Volvo is a 1988
    Sedan. I will look at those wires. When I did remove the pump relay
    the fuse did not blow. But it did blow at once when I replaced the
    relay with a wire (to shortcircuit the relay). I'll try checking
    continuity of the wires between the fuse box and the pump. I just need
    to try to get good access to the cable which seemed hard last time I
    tried. I will report back as soon as I get a chance. Again, thanks for
    your suggestions.

    Darien
     
    Darien, Oct 2, 2009
    #9
  10. Darien

    James Sweet Guest


    As I recall, you have to remove the side trim strip under the door and
    then you can pull up the carpet enough to get to where the wire goes
    through the floor, or maybe I'm thinking of a 240, I don't recall, but
    the 700 series pump is much further forward.

    You can also unplug the pre-pump in the trunk. There's a three pin
    connector behind the plastic cover over the fuel filler pipe under the
    side of the trunk carpet, it goes to the pre-pump and the fuel level
    sender. I'm guessing one of the wires has chafed somewhere and shorted
    to the body of the car.
     
    James Sweet, Oct 3, 2009
    #10
  11. I am not comfortable with running on the brink of empty.
     
    John Robertson, Oct 5, 2009
    #11
  12. Darien

    Roger Hunt Guest

    Fair comment. When I do it, I am always aware of the distance to the
    nearest fuel station, and the opening hours too.
     
    Roger Hunt, Oct 5, 2009
    #12
  13. Darien

    Darien Guest

    Ok, I finally got to try several of the suggestions here...
    Unfortunately I still can't find the problem exactly. I hope I am
    getting close though.

    1. I removed the two wires to the main pump

    2. I removed the three wire connector to the tank fuel pump

    3. I crank the car and it stills blow fuse #1.

    4. I decided to check continuity with a voltmeter between the wires of
    the main fuel pump, and they are making a contact (beep and 0 ohms in
    the ohmeter). So I said to myself "good, now I know the wires are in
    short circuit". Got under the car and remove all the insulation
    including going through the hole and the cable looks fine. I removed
    the drivers front seat to have access at the rest of the wire (I
    thought), but the cable just passes by and enters the center console
    through another hole. I have not been able to access the wires in the
    center console it is kind of crowded there.

    5. However, I decided to check continuity between the three wires in
    the trunk. The wires going to the tank fuel pump are fine. But two of
    the wires in the connector going to the cabin are making contact too
    (beep). Now I am wondering if the short circuit is in the wires
    running from the main fuel pump to the fuse box, or the wires running
    from the fuel tank pump to the fuse box. The main fuel pump cable I am
    actively tracking right now, the other I don't know how it travels
    between the trunk and the fuse box.

    6. Then I read again somewhere that for the Volvo 1988 740 fuse #1
    also protects the fuel injection system. And this is becoming too big
    to track that many cables/circuits. Are you GUYS completely sure that
    FUSE #1 ONLY protects both FUEL PUMPS?? Or do you think, I might just
    have been looking all the time at the wrong problem, and maybe my
    problem is that somehow the fuel injection system is busted or has
    some cable in short circuit. The fact that the car failed at high
    speeds and after letting off the gas several times fixed the problem
    might be pointing to problems with something in the fuel injectors or
    something, do you think?

    I don't know what to think, right now. I don't know what is making the
    problem with fuse #1. It just is clear that after disconnecting both
    fuel pumps I wouldn't expect the fuse to keep blowing. Of course a
    shortcircuited cable could account for that. But where do you think
    the shortcircuit could be. Betwee the fusebox and the pumps, or maybe
    some other place???

    Any help and hopefully some to the point guidance that would help me
    with the diagnostics would be greatly appreciated. Thanks.

    Darien
     
    Darien, Oct 10, 2009
    #13
  14. Darien

    Darien Guest

    Ok, update two. I finally track the main fuel pump cable all the way
    from under the car to the fusebox where it could be easily
    disconnected. The cable is in perfect shape (surprisingly perfect for
    a 20 year old cable). Anyway, that is surely not the problem. Right
    now I am thinking that since both pumps are NOT the problem (fuse
    blows with both disconnected), and the cable underneath is not the
    problem, and the back cable also looks good as far as I can see it.
    Maybe the problem is not related to the pumps... Has anybody had to
    deal with a fuel injector in shortcircuit or something? What other
    electrical potential culprits are there in the fuel injection system?
    Please help, thanks.

    Darien
     
    Darien, Oct 10, 2009
    #14
  15. Darien

    James Sweet Guest



    The wiring runs up into the big bundle that goes into the fuse/relay
    panel and connects to the relay.


    That's not surprising, the pumps are wired in parallel. If you measure a
    short at one, you will measure a short at the other. Does your meter
    have a low ohms mode? You may be able to use the resistance of the
    wiring to determine which end the short is closer to by measuring the
    resistance at one pump connector then the other with the pumps unplugged.


    It doesn't matter what else is on the fuse. You've determined that
    unplugging the fuel pump relay prevents the fuse from blowing, and
    you've measured a direct short between the wires to the pump. I'd say
    that narrows it down pretty well. My guess is that a bolt or screw has
    been tightened down somewhere and punctured the insulation. If you
    unplug the pumps and the relay, you should see an open circuit between
    the two pump wires. The 3rd wire in the rear is for the fuel level sensor.
     
    James Sweet, Oct 11, 2009
    #15
  16. Darien

    James Sweet Guest


    The cable looks perfect, but you've measured a short. You've eliminated
    the pumps as the cause, and nothing else is connected to that cable. If
    you unplug the relay and both pumps, and still measure a short, then
    it's obvious that the problem is in the cable. You may not *see* the
    problem, but that doesn't mean it isn't there. There may hypothetically
    be miles of perfectly good cable, but one tiny short is all it takes
    for the whole mess to be out of commission.

    Fuel injector short is extremely rare, and would likely blow the
    transistor in the ECU long before the fuse. If what you've reported so
    far is accurate, the problem is in the fuel pump wiring, there's simply
    nothing else it can be.
     
    James Sweet, Oct 11, 2009
    #16
  17. Darien

    Darien Guest

    Thank you James, the ohmeter both times read 0 ohms, so I don't think
    I can tell where is the short that way (cables are pretty thick = low
    resistance).

    I have to get there again, I just have a hard time thinking about what
    to do. I don't know how to replace that cable. It goes into a big
    bundle and disappears in the front of the car (I think)... You said
    that the cable powers nothing but the fuel pumps. Maybe I can somehow
    get the power from some other place and bring the power to the
    relay... I wonder. right now I think the shortcircuit is in the front
    of the car somewhere not in the wires going back although it could be
    in the fuel tank fuel pump wire (from dash to the trunk). I just can't
    bear having to throw my Volvo because of a single short circuit in
    some tiny cable somewhere... Thanks for your help and guidance.

    Darien
     
    Darien, Oct 21, 2009
    #17
  18. Darien

    James Sweet Guest


    Well worst case you could run new wires from the relay socket in the
    fuse panel back to the pumps under the carpet, it wouldn't be very hard,
    but if it were me I'd follow the bundle until I found the short. It's
    fairly simple to pull the seat and lift the carpet to expose the whole
    thing. I would look around the relay socket for shorts or chafes, or the
    bolts that hold the seats down. You might also try passing a bit more
    current through the wires, for example jumper a headlight bulb across
    the fuse terminals and bypass the relay, then measure voltage at the
    bottom of the relay socket and then at the pump wires under the car.
    With the 5 Amps or so flowing through the wires it should register at
    least a few tenths of a volt if the short is any distance away. You can
    use that technique to feed the cable from any end, car battery charger
    in series with a headlamp bulb will work too.
     
    James Sweet, Oct 21, 2009
    #18
  19. Darien

    Randy G. Guest

    On one of our first day trips in our used motorhome a fusible link
    blew at the solenoid and the thing died in the road. We tried jumping
    the burnt through section and it was a dead short. What to do? If you
    have ever worked on a motorhome you would know that the wiring it a
    freaking nightmare!

    The next day I bought a wire tracer. One portion is a battery operated
    transmitter that you wire in series with the circuit or wire in
    question. The other part is a wireless receive with a short antenna
    that you follow along he wire and it leads you to the open or short.

    I found the short in less than one minute once the device was hooked
    up. The device, which came in its own carrying case, is now a
    permanent part of the RV's kit! I don't leave home without it! it cost
    less to buy the tool than to have the motorhome toed the 1/4 mile back
    to my house.

    __ __
    Randy & \ \/ /alerie's
    \__/olvos
    '90 245 Estate - '93 965 Estate
    "Shelby" & "Kate"
     
    Randy G., Oct 22, 2009
    #19
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