I am not comfortable with running on the brink of empty.
"Roger Hunt" <nospam (AT) nospam (DOT) demon.co.uk> wrote

John Robertson <johnnr (AT) optusnet (DOT) com.au> wrote
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

On Oct 2, 9:15*pm, James Sweet <jamesrsw... (AT) gmail (DOT) com> wrote:
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

On Oct 10, 2:57*pm, Darien <cajuemich... (AT) gmail (DOT) com> wrote:
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.

On Oct 10, 7:37*pm, James Sweet <jamesrsw... (AT) gmail (DOT) com> wrote:
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.

Darien <cajuemichica (AT) gmail (DOT) com> wrote:

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"