Wiring advice (930 fuel pumps)

cowtown · 08-11-2003, 01:18 PM

Hi, I've been busy working on my '75 911S (w/ 930 engine) fuel system, and have come to the dreaded electrical wiring phase of the project.

Background: I've plumbed Aeroquip -6AN stainless braided hose from the tank to the engine, and have all the plumbing hooked up. I've installed an SC pump in the front using a factory bracket and cushions, and a new Turbo pump in the rear. Now, I'm adding relays and wiring to get the pumps to run. This car didn't previously have any fuel pump relay at all.

So, my questions are:

--I'm thinking of using 10-ga stranded wire to carry the current to the pumps (on the switched side of the new relays). Is this appropriate/too big/too small/wrong?

--Is there a specific brand or type of wire that I should look for for this application? OEM correctness is not a big deal on this car, but I want a high quality wire that will last 25 years. How should I protect the wire in the tunnel...shrinkwrap/tape/plastic sheath?

--Can I/Should I fuse the power supply to the pump? If so, what amperage fuse is appropriate?

--Is it a bad idea to ground fuel pumps to the chassis? If not, I could just run the positive lead to the pump, but I don't see the factory doing this. I'm assuming I will need to run + and - wires, but would like to know why this is necessary.

--any other tips appreciated!

I can't wait to drive this car. It's been up on jacks for about 2 months while I added a new Sachs clutch/pp and flywheel, installed an RSR front bumper, giant Setrab cooler, and the fuel system. It's getting close! I'll post pictures soon.

Thanks,
Colin

dean · 08-11-2003, 03:01 PM

Hi Colin,

Find out how many amps protect the pump on say an SC, and fuse the pumps the same. Also cross referance the wire size for that many amps. Any auto store would have fairly good wire for you to use. I would fuse the power supply for the pumps. I would ground the pumps to the chassis. Make sure you clean the paint off so you get a good ground. Then you can put some white lithium grease on it. I would protect the wire with plastic sheath and try to keep it away from the throttle and shifter.
Good luck

304065 · 08-11-2003, 03:40 PM

SC used 25 amp fuse in the fuel pump circuit along with a "red" diode relay. See thread under my name and "relay" for good instructions from Warren and others.

What's your pump pull? If you look in the little white book for an SC it should tell you. A "T" Pump pulls 2.5 amps, and the Walbro in my fuel cell draws 7 amps at 2 bar. With an SC pump putting out like 4 bar, the amperage load should be higher, let's figure 15 amps, okay?

Check out this article on how to size the wire:

http://www.aeroelectric.com/articles/wiresize.pdf

If this were a race car, I would use lightweight aviation tefzel wire of the absolute minimum gauge required to drive the pump. You may want to err on the size of bigger.

RoninLB · 08-11-2003, 04:15 PM

Quote:

Originally posted by Blown

I would ground the pumps to the chassis.

ditto

cowtown · 08-11-2003, 05:32 PM

Thanks for the replies. John, from the article you sent me to, it looks like 10 ga wire is more than enough for what I'm doing. I'm not going to worry about weight down to the level of wire sizes for this car.

The stock Turbo uses a 25A fuse (I never thought to look in my spec books!). I don't have the SC spec book, though.

Anyone know why Porsche used a ground wire on the pumps? Was it a reliability feature (to prevent a corroded ground from sidelining the car)?

Thanks guys.
Colin

dean · 08-11-2003, 05:43 PM

Quote:

Originally posted by cowtown

Anyone know why Porsche used a ground wire on the pumps? Was it a reliability feature (to prevent a corroded ground from sidelining the car)?

Thanks guys.
Colin

That is how they complete the circuit.

cowtown · 08-11-2003, 07:50 PM

Quote:

Originally posted by Blown
That is how they complete the circuit.

I guess I wasn't clear with that last question...why does Porsche complete the circuit with a complete wire run back to the panel, instead of grounding the pump to the chassis right near the pump mounting location? There must be a good reason that they add that extra wire and its weight and cost...?

Thanks,
Colin

dean · 08-11-2003, 07:57 PM

I didn't know they did that and I don't know why they do

RoninLB · 08-11-2003, 08:14 PM

Quote:

Originally posted by cowtown
...why does Porsche complete the circuit with a complete wire run back to the panel, instead of grounding the pump to the chassis right near the pump mounting location? There must be a good reason that they add that extra wire and its weight and cost...?

electric grounds are more complicated than they seem, IMO.. there are various areas of the tub that are "proper" ground areas. you can find some by looking around.. there are "ground loops", etc. problems. there is one on the bulkhead on the passenger side, etc.. these areas are more critical on our cars because of the engine starter being a long way from the battery.. a common situation is where the tub to tranny ground strap has a poor connection and the clutch cable becomes the ground by default, thus the cable gets "eaten".. another example is when a elec. unit is battery grounded. if the starter is looking for more ground then the battery grounded unit becomes the default ground.. then computer ignitions and fuel systems add to the fun.. computers are very ground sensitive. an interesting example is the lunar module computers crashing one week before lift-off.

I know that Warren and Jim Sims could give a clearer answer.