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Need some help with battery cabling (front mount)

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    Need some help with battery cabling (front mount)

    First of all...I didn't do this.


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    The car was like this when we bought it, and sorting it out properly was pretty low on our list of priorities. After recently replacing the battery I decided it was time to get this back to some semblance of factory spec. I've done a bit of research and have not had much luck figuring out what I need. Most of the information out there is for trunk-mounted batteries...or for CONVERTING to a trunk mount. I don't really have any interest in doing this, and would prefer to get it back to the way it looked when it came off the assembly line.

    The previous owner did unspeakable things to this car...most of which I have corrected over the years. The cable running from the fuse box to the + terminal is VERY short, so that needs to get sorted. and the tray where the cables run along the firewall...that's missing as you can see.

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    So...what do I need in terms of parts to get this looking better and more reliable? We've run into a few problems over the years where after going over a large bump the cable running to the fuse box gets pulled out. I'd also like to find a way to secure the battery to the shelf its sitting on. I've already fabricated a new negative cable using some #4 welding cable.

    I really appreciate any feedback or assistance with this!
    1988 Alpineweiß 325iC
    2003 Audi A4 Avant 1.8T

    You don't need any help. You can sort this out!


      I'm sure I can do some (or all) of the physical work. I just need help identifying (and possibly sourcing) the parts I need.

      1988 Alpineweiß 325iC
      2003 Audi A4 Avant 1.8T


        Well, it is going to be a bit of work for sure. How much experience do you have with wire harness construction and wiring in general? Or rather, how familiar are you with best practices as those apply to this?

        First, if you don't already have a copy of this, here you go:

        (All E30 diagrams here)

        The relevant diagrams to this repair are in section 670-2 mainly. I am not super familiar with the M20 engine harness and connections, but I just completely rebuilt an M42 harness and the entire front chassis electrical harness for my car, so I am at least up to speed on some stuff.

        It looks like there are 7 wires you need to deal with.

        1) 2.5 mm2 red wire to diagnostic connector
        2) 0.5mm red2 red wire to Motronic for its stand-by supply
        3) 4 mm2 red wire to the main relay, which supplies power to the Motronic and most things in the engine controls + fuel injection
        4) 20 mm2 black wire for the starter motor
        5) 4 mm2 red wire to the ABS unit and OVP relay
        6) 1.5 mm2 red wire to the fuel pump relay
        7) 10 mm2 red wire to the fuse box, via C100 (as far as I can tell from the photos and diagrams...very different than the M42 cars)

        Connections 1, 3, 4, 6 & 7 look like they should be items that can potentially be replaced outright with full-length used ones without being an insane amount of work. You will need some de-pinning tools, and I know of some economical kits that I can recommend which work with the AMP/TE terminals used in our cars. Basically, assuming my assumptions are correct about these wires' routing and termination, it just becomes an issue of removing them from the existing sheaths and snaking the replacements through. I am not sure if there is harness tape in there anywhere, and if so then it has to be removed.

        Note that #5 is not in the list. In my car's case, it is integrated into the front chassis harness, and accessing/replacing it requires completely un-taping everything between the fuse box and ABS unit. Now that I have re-taped the whole front chassis harness and replaced the crapped out connectors, it is not too bad of a job, although having the radiator out is a major help. Anyway, maybe it is different in your case.

        Also note that #2 is not there either. Again, I am not sure how much tape there is on the factory M20 harness between the engine compartment and ECU in the glove box, and if it is anything like the M42's then you will need to un/rewrap it just before the big ECU connector to replace that wire, although removing the terminal from the Motronic connector is fairly easy with the right tool.

        OK so with that said, some additional pics of the harness and where the wires route into and out of would be helpful here (and I am just sort of curious about the M20 setup since I don't have one). Is most or all of it in black PVC sheathing, or taped, or both? Tape is actually easier to deal with IMO, although you really want gloves, rags and a bunch of 99% rubbing alcohol to clean all of the awful old adhesive goop off before putting new tape on. I'd recommend building a list of each wire's endpoints and hot they seem to route so that you can form a strategy. Proper planning is everything with wire harnessing.

        If you do end up needing to re-tape a harness, the stuff you 100% want to use is Tesa 51036. This is the latest and greatest in high performance, high temperature German engine wire harness tape, and since it uses an acrylic adhesive it won't become disgusting like the old rubber-based stuff. There is a LOT of knock-off stuff on Amazon and eBay, which will not work well, so I'd get it here:

        The other option is to cut and splice new ends on to get stuff back to the original length. That is mostly fine for the smaller gauge stuff, but the starter & fuse box wires would require some more specialized crimpers to do it properly.

        Now after all that typing, I would guess that it will be cheapest and easiest to just find an entire used engine + starter harness to swap in so that you can toss the hacked wires and sheaths entirely. The ABS power wire will still take some work probably, and the best fix would be to route an unmolested replacement in, but it is small enough that it could be repaired with the right tool and splice. M20 harnesses come up all the time on here, with convertibles being a little less common, but just look around for part-outs in the classifieds.