Alan

Another possibility as a contributor to Carl's problem is the dual feeders from the jump post to the CE panel (3 feeders on older cars). I consider this bad practice in the primary wiring. Each conductor connects individually in parallel to the CE panel, if either (any) has a poor connection or becomes disconnected completely - all the current flows though the other remaining wire(s). Smaller conductors are easier to route (more flexible) and of course cheaper. However - these are fixed in position, they do not flex with engine movement and are well protected physically and don't need to be moved for any routine maintenance.

This was a bad configuration choice by Porsche IMO.

Alan

FredR

I reckon Porsche did what they did simply to reduce the bending radius given the cables had to do an immediate 180 bend after leaving the hot post- a rather **** poor piece of design in and of itself. Ultimately whether they ran it as one cable or two would have made no difference to the end result given insulation break down was destined to happen with the resulting arc burning through the steel bulkhead- in simple terms that car became a welding machine, the heat released set fire to plastics, tyres and anything else that would burn.

Strangely enough those two cables superficially appear to hold up well with the sheath usually retaining its red colour unlike the feed to the hot post that goes "cable cancer brown" as I often refer to it. When I replaced my engine harness I quite deliberately routed the power cables separately in two layers of heat shrink and a sleeve material so I could open them up and take a look in due course.

To better understand what happened in the wiring of that car I built a simulation model in excel that demonstrated that to drop 12 volts to earth in that system something in the region of 1000 amps could flow instantaneously. Given the battery can deliver around 800 CCA and the car was indoors it would have been quite interesting to see how that thing glowed and caught fire - damned scary! 1000amps at 12volts is 12kW of energy let loose!

Given the alternator churns out 120 amps, the fans run a total of 30 amps, the assumption is that under normal operational circumstances some 90 amps max be flowing continuously through the cable from the alternator to the central electrics and theoretically some additional transient loads from things like seat motors or whatever. This made me wonder whether it might be possible to install some form of protection like a fast acting fuse rated at say 120 amps [?] at the point where the cable leaves the starter motor on its way to the alternator and then bifurcate the line to feed the alternator and the ABS post. I assume [but do not know for a fact] that it would be too hot down there to mount some kind of MCB.

Bottom line had a 120 amp fuse been in that line at the point I mentioned maybe that car would not have torched the way it did. Doubtless some protection specialist somewhere should be able to confirm such.

dr bob

We are straying from the fuel pump voltage drop, and might end up in a separate thread.

The failure you describe with those two parallel wire sections is exactly what happened in the Carl incident and fire. Two smaller wires with no short-circuit/fault protection. A fault at the CE panel or a client circuit demanded more current than those primary wire sections could carry. They heated up to the point where the insulation melted, and made an even shorter path to ground on the fenderwall. One of the two heats faster, the other catches up quickly though as they race to failure. Protection for those wires needs to be at the jump-post ends, in the form of fuses or fusible links, or maybe self-resetting breakers. Whatever the method, two in parallel, one for each conductor. I like fusible links because they are self-contained with no friction-slip connections to corrode. But they would be a little difficult to replace on the side of the road somewhere, assuming you made it to the side of the road without being hit if you are driving when you have a problem. The self-resetting breakers option is good, but means the undersized wires will need more-undersized breaker ratings to do much good.

Carl's suggestion for a mega-breaker or fuse at the battery is a no-go, as it might only protect the positive cable to the starter. The rest of the primary wiring would be considered sacrificial with the same failure.


Too much fun!


Fun stuff!

Alan

Of course there is no reason at all why that 180 bend should be there, even for smaller cable/wire it is less than ideal - stressing the insulation, and making it a more likely friction point over time.

Back to the topic at hand

Alan

FredR

Bob,

Fun stuff indeed!

One of the questions I asked at the time was whether there were any signs of scorching in the central electrics and the answer was none. The car was shutdown at the time so theoretically the cables were live but no current flowing through them. For current flow to damage the cables there would have had to be a major fault down stream. My take is that there was a latent defect in the insulation of one of the two cables and for reasons unknown the thing just got out of hand. Difficult thing to visualise but if the insulation was damaged by degradation the only logical explanation I could think of was that there was some electrical leakage adjacent to the bulkhead and at some point it just let go and whoosh- up it went- the hole in the steel bulkhead where it arced tells its won story. Whether there was one bigger cable or two smaller ones would make no difference to this scenario or so I would think..

jpitman2

My pump is only a few 1000mi old, and is correct for CIS, and a recent correct relay is in place. I have measured the pump at 7A at the fuse panel.
jp 83 Euro S AT 57k