Volvo 740 Blowing Fuse #1 (fuel pump fuse)

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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.

Reply to
James Sweet

Jon Robertson wrote

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.

Reply to
Roger Hunt

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.

Reply to
James Sweet

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

Reply to
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.

Reply to
James Sweet

Reply to
John Robertson

John Robertson wrote

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

Reply to
Roger Hunt

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.

Reply to
James Sweet

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.

Reply to
James Sweet

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.

Reply to
James Sweet

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"

Reply to
Randy G.

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