Fuel pump short switch

[anisotropy]

Hi, I'm planning on installing a wire from the fuel pump wire to earth with a hidden switch. The idea being as a cheapo security precaution -start the car without switching the switch off and you'll create a short blow the fuel pump fuse and won't be going anywhere (hopefully). A lot of blood and tears have gone into this car, it has no alarm and I know my insurance would give me peanuts for it. So a little question -the fuse is a 15A do I need to use a switch rated to more than 15A (eg. a 30A) to blow the fuse rather than the switch or are the ratings on switches the current they can comfortably deal with rather than the current that'll blow em? Its hard to find higher than 15A switches for my supersecret hiding place. Thanks

[christophbmw]

ive been pondering this idea also, i was planning on running a second fuel pump relay to turn the main pump relay on/off. that way you can use a smaller switch .

[anisotropy]

I hadn't thought of that -though it may be beyond my meager understanding of car electrics. I definitely think blowing the fuse with a short is the best way I bet few thieves carry fuses about. I might just get a few switches and see what happens...

[gearheadE30]

I think that's a great idea except for the fact that I always keep extra fuses in my car... I'll probly just wire a switch in and put it under the driver footwell or under the passenger seat or something.

[Vladi]

Or you could just put a switch on the big green wire going to the ignition contact. That cuts off power to the instrument cluster, starter, fuel pump, almost everything. Hole in one.
I could show you on a schematic if you want and it would be much clearer.

[gearheadE30]

Yeah, thats porbably a better idea, plus the wiring is easier to get to.

[bmwman91]

I got an extra hazard light switch and put it in a blank spot.  From there, I ran 2 wires to the fuel pump power and spliced the switch in.  It was inconspicuous, and all I had to do was switch it off and the car would run for maybe 5 seconds before the fuel rail emptied 100%.

If you do the fuse thing, use the lowest rated fuse.  You want it to blow quick, not heat up & start a fire.  Especially if it is anywhere near the gas tank / pump.

[anisotropy]

mmm but I like the pump idea for a few reasons -the ignition wire is pretty much an immobilizer and a bit more predictable, if someone finds it they can drive off without having to change a fuse (though I suppose there's some you could swap -IF you found the switch so it wouldn't short again) and the fuel pump shorting will seem more like a fault in an old car. The wiring (on uk cars) is just behind the glovebox so pretty accessible as is an earth point. Ordered a few switches rated for 20A and 16A so will have a go and let you know.

I'll have to use good thick wire and test the switches properly but won't be anywhere near fuel.

[christophbmw]

Or you could just put a switch on the big green wire going to the ignition contact. That cuts off power to the instrument cluster, starter, fuel pump, almost everything. Hole in one.
I could show you on a schematic if you want and it would be much clearer.

you would need a big switch, or at least one with a high amp rating.

[AL GReeNeRy]

I got an extra hazard light switch and put it in a blank spot.  From there, I ran 2 wires to the fuel pump power and spliced the switch in.  It was inconspicuous, and all I had to do was switch it off and the car would run for maybe 5 seconds before the fuel rail emptied 100%.

If you do the fuse thing, use the lowest rated fuse.  You want it to blow quick, not heat up & start a fire.  Especially if it is anywhere near the gas tank / pump.

hey bmwman ive actually been thinkin bout doin the exact same thing but i never thought about puttin it in that spot.  sounds like a great idea though, can you explain it a little more? thanks.

[bmwman91]

Sure.

Go under the back seat & pull the larger of the 2 black covers.  You will see a connector running to the top of the pump (from the passenger side direction I believe).  I snipped one of these wires like 4" from the connector.  I butt-spliced on some 16ga wire to each exposed end and ran the length up under the carpet to the center console.  Bear the e-brake I brought it out and just had them run under the console up to the dash.  Using a coat-hanger, I managed to get the wires up to one of the empty switch spots.

Then I just crimped some female spade terminals on and hooked the ends up to the on/off part of the switch's circuit.  That allowed me to turn the pump on & off.  When putting the cover back down, make sure to bend/file the sharp sheet metal edges the wire will be up against.  You do NOT need any sparks in there!

[anisotropy]

You could also do it where the wires come into the car through the firewall behind the fuse box -I believe its the purple and green one - keeps things away from the fuel tank.

[Vladi]

mmm but I like the pump idea for a few reasons -the ignition wire is pretty much an immobilizer and a bit more predictable, if someone finds it they can drive off without having to change a fuse (though I suppose there's some you could swap -IF you found the switch so it wouldn't short again) and the fuel pump shorting will seem more like a fault in an old car. The wiring (on uk cars) is just behind the glovebox so pretty accessible as is an earth point. Ordered a few switches rated for 20A and 16A so will have a go and let you know.

I'll have to use good thick wire and test the switches properly but won't be anywhere near fuel.

No one said you have to put the switch near the steering column, you can run wires and put the switch wherever you want.
That's what the OBC II CODE function does. It taps into the green ignition wire and turns it on/off.