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

[Roger Morrall]

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

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.