No fuse or relay for fuel pump on '75 911S

cowtown · 04-09-2003, 01:43 PM

Could someone who has a the wiring diagram for a '75 (or a '75 911) tell me if I'm reading this right:

There is no fuse or relay on the fuel pump circuit for the '75 model? My diagram show a straight connection from the ignition key to the rear-mounted pump, and my #6 fuse slot is empty. Yet the car runs (or ran before I yanked the engine, anyway).

The '76 diagram clearly shows both a fuse and a relay, but not on the '75.

I'm leaning toward adding at least the fuse for safety reasons. Is this recommended? Any reason not to add one?

Will a fuse pass enough power without blowing to avoid also having to add a relay?

Thanks,
Colin

Jim Sims · 04-09-2003, 05:30 PM

No fuse or relay on the fuel pump circuit in a '75 911; power comes unfused from battery through ignition switch. A fuse is a good idea but needs to be sized for the current but there is also the issue of the wiring to the fuel pump; it must also be the right size (thickness or gage) for the fuel pump current. Are you using the '75 fuel pump? I seem to recall the turbo's required two fuel pumps. If you add a pump to the circuit I would check the wiring gage (you will likely need to go to a bigger wire) and run the fuel pump circuit through a relay to save the ignition switch contacts. I would also put a fuse between the relay and the fuel pumps. Cheers, Jim

cowtown · 04-09-2003, 06:08 PM

Thanks Jim,
I have a new rear pump right now in the stock rear location, and am planning to add a front pump by the tank in the spot where the SCs have it. One of my questions (unrelated to wiring) is whether the supply tube running from the tank to the engine is up to holding fuel pressure instead of suction.

With the wiring, my plan was to run a completely separate pair or wires for the new front pump to the ignition switch, but I like your relay idea a lot better.

I am also somewhat relieved that my '75 never had a pump fuse or relay. Until I studied the wiring diagram, I really believed that the car had some questionable wiring put in by a previous owner. Very surprising that there was no fuse on that circuit, but I guess it worked well enough.

Thanks,
Colin

Jim Sims · 04-09-2003, 06:46 PM

Are the fuel lines in the tunnel metal or plastic? If they're metal I wouldn't worry about them (unless badly corroded or worn thin through vibration against a sharp edge) but if they're 28 year old plastic suction lines I'd replace and ungrade them to metal or pressure rated fuel hose. No easier or better time than now when you can readily access the tunnel. There is a surprising lack of fuses in the 911 electrical system and numerous parallel wires feeding the same component or circuit. Probably signs that the car quickly out grew the original wiring harness design. Cheers, Jim

cowtown · 04-10-2003, 08:14 PM

Jim, the fuel line in the tunnel is metal.

But, at both the tank-to-tunnel and the tunnel-to-rear pump connections, the metal line is joined to standard rubber fuel line with hose clamps.

So...the line's not correct for a front pump, and there are no threaded fittings. I don't like the idea of running 100psi through these connections.

I'm still thinking about all this...any input you have would be more than welcome. One of the things I'm considering is two rear-mounted pumps, but this may negate any benefits to having two pumps in the first place.

Thanks,
Colin

Jim Sims · 09:35 AM

Probably the best thing to do and it's likely relatively easy at this point in your car's reassembly is to just buy an OEM turbo pressure line of the correct vintage for your car. I believe Pelican sells it for $74.00 (Pelican part no. C-356-066-07); it appears to come complete with the correct bango fitting to attach to the front pump. see: http://www.pelicanparts.com/catalog/shopcart/911M/por_911M_FUELCI_main.htm

There are other possibillities; here's a couple:

If there is enough metal tube exposed one could attach a metric Swagelok type tube fitting and then transition to a hose barb fitting and then use a pressure and fuel rated rubber hose (Pelican part no. FH-8-13-HP?) secured with a hose clamp(s). Specialty auto supply stores (speed and racing shops) may be able to supply all the required fittings.

Again, if there is sufficient existing metal tube exposed and one can get to the end of it with a tubing single flare tool then make a single flare on the end, deburr the edge of the flare and force pressure and fuel rated rubber over the bell of the flare and double hose clamp it on the other side.

Cheers, Jim

cowtown · 04-11-2003, 11:05 AM

Jim,
I think I'll play it safe and buy the threaded turbo fuel line when I'm ready to put in the front pump. I'm just not comfortable using clamps on that line in the long term, especially because it will be a little difficult to inspect it at the fittings.
Many thanks,
Colin