mj1pate

I've seen some references to the susceptibility of shorted ABS power cables causing unchecked full current shorts and subsequent fires. Has anyone identified a proper fusing mod to prevent this?

Thanks

Mike

Alan

Yes - fuse it at the battery (and if you like you can take out the fuse at the CE panel).

Porsche runs unfused small(ish) gauge connection from the battery to ABS, Cooling fans (2) EZK & LH. For the fans & ABS there is a fuse there - for the EZK & LH there is no fuse anywhere...

If you get a short on any of these wires - the battery is quite capable of melting them and setting fire to anything near them - even with the ignition off...

This is quite horrible design practice and the responsible engineer should have his brains removed and flushed down the toilet (he might not notice the difference though...).

Alan

mj1pate

Alan; Is there a recommended "overarching" battery fuse size?

Little confused here, Alan. You mention that there are "unfused connections from the battery to ABS, Cooling fans", but "For the fans & ABS there is a fuse there". I know it will help when I go over the wiring diagram. Can you elaborate, though?

Thanks;

Mike

mj1pate

and perhaps an "overarching" fuse in line with the battery cable insn't the answer. Not sure. Do you, Alan (or anyone) recommend fuses (size ?) more specifically inline with the power cable to the ABS, the fan, the EZK & LH? Is there an existing mod for this?

Alan

Mike

When I saiid there is a fuse 'there' - I meant there is already a fuse for those (Fans/ABS) at the Central Electric panel - but the seperate feeder wires to the CE panel for those are not fused - so any short up to that point is not protected and the wire will just melt/burn - of course its routed with a bunch of others... so not only is there a risk of fire but even if you escape that your rear wiring loom is probably all melted together... very nasty...

The solution is not a fuse in the battery main battery line - you just can't practically fuse that - you'd need about a 500A fuse for the starter and then you could still melt these wires with that much current.

Note that a short on the main battery cable is nasty but safer overall - the big wire itself will not melt - it may get hot but it will kill the battery well before it melts... however whatever caused the short may start burning up at the point of contact due to spectacularly high current...

Back to the feeder wires:

Since these route directly from the battery terminal (you will see them bolted on seperately) - you can just fuse them there. For the fans and ABS - just match the fuses that exist (30A for the fans and 15A for the ABS) - then you can remove the fuses from the CE panel (or leave them) - I have removed mine with a soldered & shrink wraped connection on the back of the CE panel instead. For the fans particularly you need very good fuseholders - big connections to handle the 25A+ drain on each. This also saves your CE panel from the crispy issues many have had around the fan fuses that often overheat. For the ECU's you'd need 30A for low noise rather than operating current.

I have done all these and while its fiddly to do it actually isn't really difficult. However - you have to build this really well for high current and safety - include wire retention devices so the wire can't come off the fuseholder - use big connections etc... or else you may just be creating new risks - its probably not the first electrical project to undertake.

But when I realized how the car was wired - I was quite horrified at the implications. There is another thing Porsche did wrong too - the main CE panel connections from the alternator & jump post are not fused and they really should be (needs a really big fuse though! - in the 150A-200A range). It also really needs a single big wire to CE form the jump post where it has 2 small(ish) ones... this is not good design practice for high current - you really need a fuse value that could protect either wire alone and those wires aren't good for 150A+ each - so you cant really do it right...

Alan

worf928

IIRC the stories of the ABS power causing fires are in reference to the far end of the ABS's power wire where it is routed behind the power steering pump reservoir and behind the ABS pump bracket. In these stories the insulation of the wire, due to age or damage, is compromised and shorts against the ABS pump or bracket.

One simple way to guard against this specific failure mode is to help out the insulation of the power lead with another layer. A 6" to 12" piece of heat shrink will do the trick.

mj1pate

Thanks, Alan....This suggests that before I go probing around for possible insulation compromises, that I disconnect the battery first. My background is EE, but have been in computer networking for quite some time, since. I'll investigate reliability in high current DC connections, before undertaking the mod.

mj1pate

Yes, this sounds like the first step, prior to making a more significant mod.....

Alan

Yes by all means reduce risk where you can and patching or replacing wires with damaged or just deteriorating insulation is smart.

But the whole concept of the wiring plan was flawwed - not one of Porsche's finer moments. I think they wanted to keep all the fuses to the CE panel - which is in some ways a good idea... but they didn't do that right and then over time made it worse still with extra direct feed wires...

Alan

mj1pate

Right you are, Alan...This makes me think seriously about maintaining calibrated strippers/crimpers, and researching reliable industry connection techniques for high current connections. It would seem easy to make things worse, by less than careful preparation.