Volvo 740 Blowing Fuse #1 (fuel pump fuse)

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

 

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.

 

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

 

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.

 

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.

 

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.

 

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.

 

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.

 

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

 

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.

 

I am not comfortable with running on the brink of empty.

 

Fair comment. When I do it, I am always aware of the distance to the
nearest fuel station, and the opening hours too.

 

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

 

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

 

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.

 

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.

 

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

 

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.

 

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"