77 Fuel pump relay / harness into a 75. Where to run yellow wire?

[75 911s]

Last summer I moved my fuel pump from the rear of my 75 up to the more conventional location at the front crossmember. I made my own wiring harness with relay and fuse but I wasn't happy with how it looked.



Everything fit under the cover, but it was more temporary in execution.

I also have a 77 911S that I removed the complete fuel harness with red relay from and I thought that would be clean to install in my 75.

My question lies in the best place to attach the yellow ignition wire from the 77 fuel pump harness. The T-connector that contains the yellow and brown typically ties into the harness that runs over to the ignition.





Looking at the wire diagram I can see it goes right to the ignition via a few connectors.



I do have a spot where I removed my seatbelt relay where I could patch into my yellow ignition wire between the starter and the key. I also thought I could find an empty ignition on at the fuse panel and tie it in there with a short jumper from the T -Connector.

I want the fuel pump to turn on with ACC1 as I have carbs and I want to be able to fill the float bowls after sitting.

My 75 didn't come with the fuse panel to the far left. I added that from a spare setup I had.



I think the Red wire shown should actually be on the bottom of the fuse as current flows "down" and the way I have it hooked up now is before the fuse. Right?



As a side note, my 75 did not have a hole in the smugglers box where the harness comes in on later cars, so that had to be drilled out to accommodate the routing of the later style harness.



Has anyone else moved a later harness to an earlier car?

[jimtweet]

I have it connected to the yellow ignition wire in three plugs under cowl. As per how it was connected in 82 I removed the harness from. Correct me if I’m wrong but yellow is only color used for ignition

[75 911s]

Thanks, for that. I'm thinking maybe I'll tie it into the yellow split where the seat belt warning system was inline right behind the speedo. Seems easier to go there than down to the ignition wires under the dash.

[PeteKz]

You may know this, but let's make sure: The SC's used the FP relay to interrupt the power to the FP when the safety switch on the SC air metering unit was not registering airflow (the engine stopped running). The yellow wire to the FP relay provides power to the FP only when the starter is cranked, to allow the FP to run, even though the air meter is not yet lifted enough to open the safety switch. Otherwise, you would have a hard time starting it.

I went through the process of back fittng the SC type FP relay and circuits into my 1973.5 car, so I get what you are going through.

Your 1975 does not have the air meter safety switch (IIRC), so you want to wire your FP relay to run whenever the ignition is ON. The 1977 did have the air meter safety switch (again, IIRC), so it's like the SC's. You do not want to copy those circuits.

However, something I did to take the FP current load off the ignition switch (to make it last longer), was to install a relay to switch power from the hot side of the fuse panel directly to the FP. The relay is switched on by the circuit that used to provide all the current from the ignition switch directly to the FP. Now it only carries a fraction of an amp to activate the power relay. You can use any relay rated for 20 amps or more, including something much smaller that will fit better where you want to put it. Or you could move the relay into the smuggler's box, or somewhere else.

I suggest that new wiring scheme to you.

[75 911s]

Pete,

Your post is extremely helpful, thank you. I didn't know that the relay interrupts it but after your description and looking at the workshop manual, yes it makes perfect sense. I see the air meter switch and it comes in via that T connector on the Red/Brown wire next to the yellow in the FP harness.

So let me ask you this. You mentioned adding another relay to help take circuit load off the FP, but why can't I effectively make this relay perform that function?

I want the stock looking relay and harness but I want it to perform a custom function. How about this?

In the stock relay, the Air meter provides ground when the engine is on to the 85 on the relay correct? Because 86 has a red jumper and that powers the relay coil to close the circuit to let the fuel pump run right?

So in my scenario, If I ground to the red/brown in my T connector then that will make the fuel pump run all the time, with the battery connected.

Obviously I don't want that so I could sever the small power lead from the 87a at the back of the relay that goes over to 86 and instead run a switched power to the relay coil.

So when I turn on the ignition, switched power at some fuse comes on which closes the coil switch in the relay and connects always hot 87a to 30 fuel pump.



Would this work?

[PeteKz]

Duane, yes, your "Modified" diagram is what you should do.

I had to install a relay because my 1973.5 did not have a FP relay--the pump ran whenever the ignition was on.