AxeHealey GRM+ Memberand Dork
2/16/21 12:48 p.m.

When I removed the dash of the E21, I discovered a complete rats nest of spliced and hacked wiring. Since none of it goes anywhere anymore (heater/ac, interior lights, etc.) I'm sort of tempted to remove it rather than cap it off and secure it. 

What's the best practice? Just be super careful, trace things back to the fuse box and remove? I've got a wiring diagram but it's not the best I've dealt with. I just don't want to go to start the car in the spring and have to trace an unknown issue. 

I have an irrational fear that, for some reason, power for the fuel pump is somehow linked to the heater fan and the thing won't run. It's not, but you know, conceptually.


Any tips/tricks would be appreciated. 

Error404 Reader
2/16/21 1:02 p.m.

Based on my experience with my project car I would suggest positively identifying anything you intend to cut and cap. If you can do that, go for it. It also never hurts to clean up any ugly splices. Just go slow and work one thing at a time. When in doubt, label. 

wae UberDork
2/16/21 1:11 p.m.

If you can start with a wiring diagram, that will tell you what you can and can't get rid of.  Once the harness has been hacked up, though, all bets are off.   I went and found the wiring harness for the Neon and then made a spreadsheet that looked like:

"primary color","secondary color","circuit number","function","diagram page"

And then a second sheet that looked like:

"function","primary color","secondary color","circuit number","diagram page"

Each of those got printed out and put into my build binder for the car.  Now I can quickly identify any wire - if I want to know what function this wire serves, I look at the first sheet.  If I need to know which wire I need to find for the whatever on the car, I look at the second sheet.  Super fast, super easy.

Once you know that, you can trace the unneeded wires back to their connector and then un-pin them and remove them entirely.  That way you don't need to worry about a bit of electrical tape coming loose.  And if you ever need to put it back together, use your wiring diagram and you can simply re-pin it.  Alternately, if you need to run other things and you have "extra" wires in a harness, you can depin the other side of the connecter, put your wire in its place, and then pick it up on the other side of the harness.  That can be easier than trying to push a new wire or five through the firewall, but be sure to note the wiring change in your spreadsheet!

You can get a set of de-pinning tools for around ten bucks on Amazon.  They take a little bit of fiddling with to learn, but they're worth having around.

FieroReinke New Reader
2/16/21 3:49 p.m.

I have some experience doing this when I built the wiring harness for my locost 7 from the stock miata harness.  I was removing everything from a fully loaded NA miata to just what it took to run the engine, ie windows, stereo, lights, etc.  I pulled the harness out of the donor miata and laid it out on a 4x8 sheet of plywood and completely stripped the harness of all wrapping and loom.    I use screws into the plywood and zip ties to loosely hold the harness to shape and then started labeling everything based on the wiring diagram.   If I couldn't identify something, I labeled it with a ? until I could.  Eventually I could label everything.  As I labeled and traced certain wires, they would lead to the unknown items helping to clarify what they were.  I then went through the factory wiring diagram to confirm everything I needed was accurately identified against the diagram.  Only after I was 100% confident I had everything labelled correctly did I start cutting.   I only cut one wire at at a time and completely removed it from beginning to end before I moved onto the next one.  The scariest part was the splices for power.   They were clear in the wiring diagram where they spliced.  I wasnt as concerned with ground splices.   When I did it, I started at the device and worked back to the fuse block. 

It wont be as easy if the harness is in the car, but if it is a project / race car, it might be worth pulling the harness to repair if it is as bad as you describe.  

2/16/21 5:18 p.m.

Easiest to do with the entire harness out of the car.

Did it when I used the Miata harness in the Volvo and had to decouple the engine management and a few other unneeded circuits from the harness. The factory wiring manual was very useful. The actual work was more an exercise in patience tracing one circuit at a time..


AxeHealey GRM+ Memberand Dork
2/16/21 7:57 p.m.

Thanks everyone, as always, for the good tips. 

The hackery seems limited to what I've found under the dash so it's not a lot but it annoys me. We could probably debate this statement but I don't think pulling the harness is worth my time on this car. I'll either be able to trace all the wires in place or I'll just tidy it up and leave in a slightly better state than it currently is. 

I'll take some time and give myself a cheat sheet of wires/colors and see how much I can ID and tag and go from there. 

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