Fuel Pump burned up

[RD911T]

I'm starting a new thread on my fuel pump problem that I identified earlier today. The title is more appropriate and this is about where I go from here.

I got a new fuel pump and decided I needed to trace the old wire and find a place to fuse it. I followed it up into the engine compartment and found it burned up at a connector here:

[IMG][/IMG]

So I unwrapped the tape to follow it. You can see it here goes back into the main harness that to goes the front of the car, I assume. It's the red wire on top. You can maybe tell that the insulation is damaged by the heat generated:

[IMG][/IMG]

So, as I mentioned in my earlier thread, I'm not real swift at electrical. What are my options here? Do I need to replace that wire all the way through the harness? Is there a way to test to see if it's ok? Why wouldn't they have fused the power to the fuel pump? That's what I was planning to which is why I started following the wire to find a good place to do that. It doesn't look as bad down below, the worst is around the connector in the engine bay. I appreciate any help/advice that you can offer.

Tom

[Porboynz]

Ouch! That red green fuel pump wire goes pretty much straight to fuse #7 on the top fuse panel up front, but on the battery feed side, not the fused side as you discovered. #7 is bridged to #8 and there is a red and black feed wire coming from the ignition switch. I would check the condition of this red black wire at the ignition switch and then as far into the loom as you want to go, same with the red green wire to the fuse panel. Then you need to make a call on replacement. If they look basically intact you might run with them OK. The big problem here is what other wires has the heat damaged? They may have melted through the insulation of adjacent wires in the loom. If you continue to use them and they short through to other wire(s) in the loom interesting things can happen. Replacement will give peace of mind if you are at all uncertain, you can easily run a new wire inside the tunnel but outside the main loom from the engine bay to the fuse panel. Same from the fuse panel to the ignition switch. Then you can insulate off the old wires and likely not have any issue. You might even consider adding an inline fuse, Porsche must have thought that fuel pumps are more likely to go open circuit than short circuit.

[timmy2]

Trace all of the wires in the same loom back to the source as mentioned above and replace them.
Remove the wire loom jacket as needed to inspect all wires and replace them if there are ANY signs of damage. Install a fuse and wrap the wire loom back up.

Please don't scrimp on inspection and repairs. these cars don't burn nicely...and your life is on the line.

[RD911T]

My fuel pump was installed on a bracket under the drivers side torsion tube. I was thinking of moving it to the front near the steering rack as I've read it may have originally been. Where does the other end of the loom come out at the front? Is it pretty straightforward to fish a wire to that location from the fuse panel?

The red wire had heat damage, but none of the other wires around it seemed to have any damage, but I'll keep looking.

As far as running a new wire if I decide to keep the pump in back, what are some tricks/techniques for doing that? Open to suggestions.

Tom

[wwest]

For some reason the FP fuse and holder are subject to overheating and subsequent failure. Fuse will sometimes blow due to local fuseholder heating, NOT FP related. So it is highly likely that at some point in the past that fuse/holder conbination had to be properly bypassed, I used an inline fuse & holder.

Someone may have done it improperly, thinking the fuse "failed" due to not enough current rating. With the proper, factory, fuse rating that should NOT happen. The FP is of the electromagnetic "pulse" design wherein the momentary current flow "pulse" exceeds the fuse rating substantially. So if the FP stalls in pulse sequence the fuse will/should blow instantly.

[RD911T]

I bought an inline fuse holder to install. What size fuse is appropriate ?

The power to the fuel pump was scorched at the pump for a few inches, the melted significantly at the connector in engine bay. It has some signs of melting where it joins the harness. What causes these hot spots? Is it likely to happens all along that wire all the way to the fuse block and ignition? I checked position 7 on the upper block and it looks fine, in and out. Position 8 looks good too.

[RD911T]

I stripped the wire loom jacket back a couple feet and all the wires including the red/green look pristine. Do I need to go further?

[Porboynz]

I will jump in with a theory on the cause of the hot spot you discovered near the FP connector, open to ridicule because its just a theory. The FP feed wire is the same guage copper wire all the way from the fuse block to the connector, so no reason for any particular section to heat any more than any other section when subjected to a short circuit current (probably 50-60amps plus based on 12 volts and 0.2 ohms resistance end to end) However at the point where the wires meet the connector they are no longer wrapped in the loom and my theory is that the loom section acts like a heat sink to take away the heat more effectively than the point near the connector. Plus the actual connector to wire join is probably a slightly higher resistance causing that section to heat more. As the connector join heats, that small section of wire goes into a runaway heating cycle just like an overloaded fuse. Well, its a theory.

The fuel pump does draw a significant surge current at switch on, so I would select a fuse on the basis of 2 times whats needed. You can measure the operating current using a multimeter, most have a 10amp hi-current range using a seperate lead connection. I would guess about 8 amps running so a 16 or 20 amp fuse, just a guess. The point is that the fuse has to handle repeated thermal shocks from that high initial current at switch on, not just the running current.

Why did Porsche run an unfused battery wire to the FP? Its the same for the ignition so maybe the factory figured anything that can stop a Porsche engine in a race is to be eliminated?

Last of all, only you can make a call on re-using the FP wiring, it may be fine all the way or there may be an overheated section deep inside the loom somewhere thats about to make contact with another wire in the loom, or not. I think I would check it for low resistance to the chassis earth with each end disconnected, if its good (Hi resistance) then try it out with a suitable in line fuse. Based on what you have found so far its maybe just the section by the connector you need to replace, you got to be lucky sometimes.

[Walt Fricke]

Moving the pump to the front is a good deal pretty much all the way around. It is much cooler up there. Calls for some fuel line replumbing. Pressures for your carbs shouldn't be a problem.

Whether you move the pump or not, why not get a 30 amp relay (NAPA will have, inter alia) and use that to feed the high current to the pump. You can go directly from the battery, or from one of the large battery wires which feed the fuse panel, to the relay, and then from the relay to the pump. You can ground the pump separately to any convenient ground. You can control the relay with the wire from the ignition switch which currently controls the pump. I think that, starting maybe in '72??, a relay is used in this way? Your wiring diagram for your '71 ought to tell you that. Or post a picture of your fuse panel - I don't know just when the very early panel got changed to a later style (though someone here does).

If you leave the pump in the back, you can get the power from the battery connection at the starter, and use your repaired wire to actuate the relay. Now that wire won't be carrying any significant amount of current.

I like the Kiwi theory here - some corrosion/oxidizing on the exposed wires leading to a local resistance. Sounds as good as any.

The fuel pump fuse on a 3.2 is 25A. Ought to be right for your pump also.

[RD911T]

Pelican is a great community!