Volvo 740 Blowing Fuse #1 (fuel pump fuse)

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 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

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.

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.

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.

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

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

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

1. 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.

2. 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.

1. 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"