Technical Rewiring assistance

[JCCS]

All,
As I have gone through my very very bad 1980 Fiat 124 Spider 2000 I have found that the FI wiring harness is bad and some of the main wires are bad. I have made the painful decision to rewire the whole car. I found a used FI wiring harness for this model and if it is good I will use that and rewire the rest of the car.
I can say that I am no expert when it comes this project so I am looking for advice and counsel on the best way to go about picking the best wiring. If there are kits that would be a good choice, let me know. If not, I would love advice on how to approach this.
As you can see from this picture, it is now a blank slate. Please Help!!

 

[s130]

Have never rewired a car from scratch.

What I do know is that older Fiats used large cross sectional area wires (overkill/over rated) with thick insulation (which went harder with time).

Suggest you go back to the drawing board and work out what guague/current rating of wires you really need.

e.g. a 21 Watt brake light only requires 2 Amps. So a 4 Amp rated cable would be fine.

Also modern cables have better rated current capacity. So a good 6 Amp cable will be thin/lean/mean and easily wired and routed compared to the old Fiat "chunky" wires.

When dealing with bundles of cables then try to arrange the higher current demand cables on the outside of the loom, or separate them off into separate looms.

Lastly it is unlikely that you can get all the multicolored and striped cables. So you need to i-dent the cable at both ends and preferably at stages in-between start and end of run. Cable ring tabs are cheap, easy to fit/slide on and could save you a big headache when possibly diagnosing an issue in the future.

 

[AndyRKett]

How to pick the right wire sizes

American wire gauge - Wikipedia

en.wikipedia.org

Also with this sort of job I would start with the earths to everything first then start putting in the other wires after the earths, do one system at a time and you shouldn't get lost. Make sure everything is properly fused and make sure you have good joins between the wires and the connectors. you can solder connections in your car as you are doing it from scratch which will be way more reliable and robust than OEM.

one thing you can do is nail the old loom to a large board to work out all the connections and routing of the wires, once its nailed up, cut off all the old loom binding tape. Also don't use insulation tape to bind the new loom, use the proper tape.

 

[124BC1]

(OP - just in case you haven't already checked it out)

Here's a U.S. website that stocks a variety of 124 Spider wiring components, plus some replacement looms.

Fiat 124 S-C Wiring

Shop from a wide selection of Fiat 124 S-C Wiring at Midwest Bayless, the leader in Fiat and Lancia auto parts.

midwest-bayless.com

 

[vexorg]

There are companies that make pre-made looms, all with right connectors and wire colours.
If you have no experience then it must be an option, even if a bit more costly.

 

[124BC1]

There are companies that make pre-made looms, all with right connectors and wire colours.
If you have no experience then it must be an option, even if a bit more costly.

Do you mean something like :-

https://www.autosparks.co.uk/fiat-124-abarth-rallye-wiring-harness-set

Wrong model, wrong year, very expensive. They don't list any other for the 124 Spider.
I had already searched for replica wiring looms/harnesses in the U.S. but could find none for the OP's model.

My approach has always been to check and repair what is already there (but then again I was a qualified Auto-Electrician back in the day).
Also, modern wiring insulation seems to be very attractive to rodents....

 

[vexorg]

My approach has always been to check and repair what is already there (but then again I was a qualified Auto-Electrician back in the day).
Also, modern wiring insulation seems to be very attractive to rodents....

Depends on skills vs how much you want to pay. The OP gave the impression they didn't know much about electric. And for some odd reason most car people cannot get their head round wiring.

 

[JCCS]

My approach has always been to check and repair what is already there (but then again I was a qualified Auto-Electrician back in the day).
Also, modern wiring insulation seems to be very attractive to rodents....

I appreciate the posts. As I indicated I am not an expert and I would like to do as you suggest. Before pulling this trigger and buying all new, I am evaluating the current wiring. I am tracing every circuit and checking continuity. Thus far I am not finding any breaks in continuity. I did find a repair on the main brown wire near the starter that had a failing repair by a PO.
With that said, in using original are there any updates that I should consider as 'musts' as I evaluate this? One I am planning is updating the headlight wiring with new relays and fuses.

 

[MarkAlfa68]

I would keep the old harness and just repair it as necessary- most of it probably works. If you need help with
a specific issue then we can show you how to go about troubleshooting it
These electrical systems are pretty simple, not much to go wrong

 

[JCCS]

Well. I posted that initial decision that I was going to redo it all. After I pulled it, I tested all the continuity and read and reread and reread again all the diagrams, so I understood it all. I felt a lot more confident with the original wiring. I did get that replacement FI harness and testing the heck out of that to ensure it was good. I actually put it all back in on Saturday but haven't tested it yet. I think in the next week or two I should be able to have it all together.
If it is successful I will post again.