408 : How to route a new wire from battery compartment to boot?
 

I want to run a single new wire to take power to the electric fuel pump in the boot (trunk).. .The existing power to the pump is tapped into the power for the fuel sender which seems asking a lot and I'd like to get it off there.. .The original wiring harness disappears into the bodywork above the righthand rear wheel arch.. So I doubt I can add a wire to its route.. . All I can think of is to lay it under the carpet along the door edge area.. Is there a better solution, please?. It will be carrying 8amps.

David
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David Womby
1964 408-7023 Florida/Nova Scotia

Hi David,
I am actually in the process of doing the same thing. I intend to have a fuel pump isolation switch on the dash board, not as an anti theft device, although it may help a little. My real reason for the cut off switch is because a few years ago I had the unfortunate experience in one of my other cars, a V12 Jaguar, where the accelerator jammed at full throttle on acceleration. It’s surprising how quick these cars reach 90 mph in such a short time, fortunately all ended well.
Anyway, back to the point; I have taken a new live wire via a newly installed fused distribution board (and relay) in the battery compartment, from there my wire goes behind the dashboard to the switch position, before dropping down to the prop shaft tunnel below the carpet, the cable runs to and through the cross member supporting the rear seat base and then below the rear seat and into the compartment behind the steel plate in front of the petrol tank (the steel plate is directly behind the back of the rear seat), once there it’s a straight forward job to push the cable into the boot area in the vicinity of the fuel pump.
I suppose there are easier routes but as I have my back seat out, I have chosen this way.
Additionally, I have also added a few additional wires as a spare should I ever need it for something else.
Hope this helps.
Regards
Brian

Thanks, Brian. Firstly, having owned two XJ12s myself, I can imagine how quickly a Jaguar V12 can hit 90mph. A very scary thought!

I have my rear seats, and the steel bulkhead that is in front of the tank, out at the moment so your routing sounds very doable. Thanks.

As an aside, I couldn't work out what was powered by two wire harnesses that disappear through the metal under the rear seat. Finally, when I got the car on stands and went under, it became obvious - wiring for the (long since removed) Selectaride shocks!

David

I take it that this means that it is tapped in at the tank, in which case it seems to me somewhat surprising that either the pump or the fuel gauge has ever really functioned. The sender is a variable resistance which acts to vary the current through the fuel gauge so I’d expect the voltage at this point to be varying quite considerably, somewhere between 12 volts at "full" (actually if the sender fails completely, so there is no resistance at all in the circuit, the gauge will show over full) and close to zero volts at "empty". So the pump will be seeing a very variable voltage, so I’m surprised it worked consistently, and conversely the fuel gauge will be seeing the pump resistance in parallel with the sender which I think means that the gauge will also have been reading optimistically high.

On the 410, which came with an electrical pump from new, the feed to the pump was taken from the white input side of the fuse that fed the green switched circuits, so wasn’t protected by any fuse. I’d suggest that an inline fuse might be sensible and, as underneath the carpet isn’t actually all that benign an environment, I’d also recommend use of a cable with two layers of insulation

By the way the top of the fuel tank is a bit higher than the carburettor float chambers which means that fuel will flow (through an SU pump, I don’t know about others) into the float chambers if the tank is reasonably full whether or not the pump is switched on. So any switch (and this includes the ignition switch) in the pump circuit will not necessarily cut off fuel to the carburettor, it all depends on the amount of fuel in the tank and whether your facing up or down hill etc.

Roger, thank you. The reply is much appreciated.

My fuel gauge has never worked. I am hoping it might after I get the pump wiring off it. The pump has always been loud. It was actually installed upside down! I am hoping righting it, mounting it on rubber isolators and giving it a good 12v supply will help that.

I am running a new 12v line from a relay I have installed in the battery compartment. It has its own fuse there too. The relay will be turned on by the 12V from the circuits that are live when the ignition switch is 'on'. There is an inertia cutoff switch in the boot by the pump and I will leave that in the new 12v supply circuit. I am in the process of running the new 12v wire under the carpet and have ordered a sleeve for it to add a bit more insulation to it.

David