1977 911 fuel pump replacement

[dm36415]

I've pulled the old non-working fuel pump from my car (has never worked since I acquired it). The black wire was connected to the + terminal, the red wire was connected to the - terminal. Logic dictates this is backwards.

Any guidance?

Di I reconnect as before, or follow my logic?

[ruf-porsche]

Are you sure it red and not brown? PORSCHE uses brown as ground.

Easy way to find out is to take a volt meter and test the two wires.

[Formerly Steve Wilkinson]

My 1977 wiring diagram shows a black and brown wire going to the fuel pump, and it doesn't show any difference between + and -. No red wires anywhere in the area...

Stephan

[dm36415]

It is brown, thought it was red, but dirty

[Jim Sims]

The factory shop manual 1977 911 wiring diagram shows black as the hot (+) wire between the fuel pump relay and the fuel pump. A brown wire is shown between the fuel pump and ground. The DC electric fuel pumps can be damaged if connected with the polarity backward. This would be a good time to loosen the fuel tank drain bung and drain the tank of old gas and check the bung screen for rust and other debris. Also replace the rubber fuel lines between the tank, pump and center tunnel lines. Cheers, Jim

[Formerly Steve Wilkinson]

That's wierd. I too have the multi-volume factory manual with the color wiring diagrams (at least for the '77s) and it doesn't show any + or - on the pump--just the motor symbol and the black and brown wires.

Stephan

[Jim Sims]

Follow the black wire back through the fuel pump relay (J16) and the ignition switch and eventually (after several jogs through the fuse block busbars and wiring) the circuit ends at the positive post of the battery. The black fuel pump wire is not explicitly marked with a positive symbol (+) (nor is the battery) but both polarities can be deduced with a little schematic reading. Cheers, Jim

[Formerly Steve Wilkinson]

And you're sure the polarity matters? I should think that either the terminals and posts would be of different sizes or the schematic would be clearer, if it did matter. Electricity is magic to me, so I'm trying to learn.

Stephan

[Jim Sims]

The Bosch OEM pumps tended to have asymmetric terminals and a matching wiring harness plug that ensured the correct polarity. The replacement fuel pumps come with instructions stating reversed polarity will damage the pump and void the warranty. I haven't looked into the exact reason but I suspect the roller cell pump elements do not "like" being mechanically turned in reverse.

[randywebb]

- Black IS the hot (+) wire in some circuits for German cars - I forget which, but there is an old thread that discusses the wiring conventions

- This diagram etc. may help with the general idea behind the "newer" Bosch roller fuel pumps. http://www.delavanagpumps.com/roundup/works.html

They are a high quality item. Richar Parr (PMO) told me not to buy any new fule pump for my carb conversion, but to keep the Bosh FI pump.

Stephan - Your local library will have some books to start you off at whatever level you want. Analogies to water flow are common & you just need to understand resistors, capacitors & inductors & you're there. The rest is electronics, not electrics.

[ruf-porsche]

Jim is correct, there is a + and a - terminal on the fuel pump. Just replace the fuel pump on my 82 sc and the pump was marked with a + and - and the posts terminals were a different size.

If you didn't pay any attention to the wiring hookup, maybe you should go back in and look at it again before you ruin the pump.

[unclebilly]

I've run both the Bosch and a universal fit Holley pump on my car. Both worked fine without any problems. The Bosch pumps seem to get loud from time to time - I've had 2 new ones.

[dm36415]

Big thanks to all y'all for the advice. Installed a new relay and Bosch fuel pump (brown is negative, black is positive). The posts are a different size.

Fired up for the first time since I bought it.

Now for the oil change, tune up, interior work, paint job, clutch adjustment, and the list goes on and on...........

I'll be back for more help later. Hopefully I'll learn enough to help someone else one day!