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Wiring 101

Discussion in 'The Hokey Ass Message Board' started by Crazy Steve, Nov 5, 2013.

  1. Several members have asked for a 'wiring basics' thread to remove the 'magic' that seems to attach to it, so here goes... I'll do this in several parts as there's a LOT of material to cover. I'll try to dispel some myths and misconceptions, and just general misinformation that's out there.

    First, my bona fides; I'm a retired electrician with over 30 years in the industry at multiple levels. I've designed systems and taught electrical theory, so while I won't claim encyclopedic knowledge, I know more than enough to talk about automotive wiring. Now, the first objection I'll hear is 'but that kind of wiring isn't the same as car wiring!'. Yes, that's correct, but electricity doesn't know where it's working, a car, house, etc, the basic theory is the same no matter what. AC or DC, no difference....

    The OEM wiring/harness that came in your vehicle is engineered. The factory has full data on all connected loads, uses many purpose-built components, and knows to a large degree just how much many of these loads will be used at any given time. They do make compromises in the design many times, mostly for cost reasons, and occasionally get it wrong. But by and large, a factory-designed wiring system is an extremely reliable part of a car and rarely needs repair during the 'normal' life of the vehicle. But over time, wire insulation deteriorates and switches fail, the system gets altered, and all the other calamities you can run into, to where a replacement can be easier than repair. Add in major modifications to the vehicle, and a new system can become nearly mandatory. If you're starting with a vehicle older than about the early 60s, the wiring used in the older vehicles will be failing in multiple ways by now unless the vehicle has been very well preserved.

    So, you need new wiring. Now, there's multiple vendors offering 'replacement' harnesses, with many claiming how easy theirs are to install. But at best, these are only semi-engineered; they've used 'typical' values for circuit sizes that may or may not be correct for your particular install, and worse yet, some ignore what engineering data is out there when sizing circuits and related parts. Sometimes this is just ignorance, sometimes it's for cost savings, allowing them to limit inventory and reduce their costs to build the harness.

    What I'm going to do is attempt to spell out (in simple language) what you need to think about when doing wiring. While a pre-built harness may be just what you need, I'll try to give you the knowledge to make an informed decision and/or be able to ask the right questions to your vendor (and know if he knows what he's talking about! LOL). If you're wiring a very 'basic' vehicle (lights, horn, minimum creature comforts) this can be pretty simple. But if you're upgrading with power accessories, AC, big stereo, etc, it can get complicated.

    I'm going to be conservative, so that if you follow my recommendations you should end up with a wiring system equal to or better than OEM for years of reliable, safe service.
    Last edited: Nov 6, 2013
  2. brady1929
    Joined: Sep 30, 2006
    Posts: 8,596


    Cool, thanks.
  3. ehdave
    Joined: Feb 28, 2009
    Posts: 119


    Thanks for this Crazy Steve, this will come in handy for both of my builds
  4. Karmaize
    Joined: Oct 5, 2013
    Posts: 48

    from Mass

    This should be a good thread. Thanks in advance for taking the time to write it up.

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  5. 1964countrysedan
    Joined: Apr 14, 2011
    Posts: 1,131

    from Texas

    This hints as a potentially great thread.
  6. Subscribed. Thanks!

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  7. 3wLarry
    Joined: Mar 11, 2005
    Posts: 12,804

    Member Emeritus
    from Owasso, Ok

    note to I really want someone named Crazy Steve wiring my car? :eek:
  8. rfraze
    Joined: May 23, 2012
    Posts: 2,003


    You are going to wire it. He is just crazy enough to try to help you do it. Beats Sparky. Let's roll.
    clem and gotta56forme like this.
  9. Subscribed. Looking forward to this. Thanks Crazy Steve.
  10. arturo7
    Joined: Jan 5, 2013
    Posts: 30



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  11. Bells Dodge
    Joined: Jun 9, 2009
    Posts: 47

    Bells Dodge

    Subscribed Crazy Steve. Thanks.
  12. I am in as well ! Wiring is not my strong suit , that's for sure !
  13. todztoyz
    Joined: Jun 21, 2008
    Posts: 176


    Just pulled my wiring outta the box over the weekend to start laying it out. Perfect timing!
  14. Clik
    Joined: Jul 1, 2009
    Posts: 1,904


    Question: Other than cost is there any drawback to using oversize wire?
  15. Weasel
    Joined: Dec 30, 2007
    Posts: 6,693


    Getting ready to wire my '40 - perfect timing - thanks Crazy Steve....
  16. Ok, as the first 'how-to' installment, let's talk about the 'logic' of the harness. I'm not going to even try to get into specific circuit design as there can be just too many variations due to needs and what parts your using, but I will give some general guidelines later.

    There should always be a certain logic about how the harness is constructed. If you think about how a factory harness works, some items will be powered with the key off, some will work when the key is turned to 'ignition', some will work when the key is on 'accessories', some will work when the key is turned to 'start'. I'll note here that unless you're building an absolute 'bare bones' vehicle (ignition/starting system, head/tail/brake lights, horn and little else), use an ignition switch that has an 'accessory' position. I'll get into all the reasons why later.

    So, what should go where?

    You want basic safety items to be powered at all times; headlights, taillights, brakes lights, horn, emergency flashers if so equipped. Any sort of courtesy lights (interior, trunk, underhood) should be included if present. If you have remote entry and/or solenoid operated doors/locks, those need to be powered ahead of the switch. Cigarette lighters/power points should be powered at all times (lighters in particular; they're a large load), as well as clocks and electronic radios that need power for their memory (virtually all radios of this type will have a separate wire for this). Alarm systems if you have one.

    For the 'on/ignition' position of your switch, this should have only the ignition system hooked to it. Nothing else should be connected here, and the main reason is to reduce load on the switch contacts. If you're doing a 'bare bones' car, you can hook the gauge power here. Remember, if these contacts fail, the car quits...

    For the 'start' position, this is where the starter solenoid wire goes. The only other thing that may connect here would be a secondary connection for any warning/indicator lights if present (excluding oil or charge lights). This is an easy way to provide a test of the lamps so you know they work.

    The 'accessory' position should get everything else; heater, AC, wipers, turn signals, power goodies (windows, seats, etc), radio/stereo, etc. One nice thing about having the accessory position is you can check these items without having the ignition on for an extended time and risk damaging it.

    By and large you want to follow this guide, but there can be exceptions. Some like power seats powered ahead of the key so they can be adjusted with the key off, and some AC units may need at least part of the system powered off the ignition position; check with your vendor and/or the information you have on the unit (there's no reason for the compressor clutch to be on if the motor isn't running).

    Now, because you have four 'kinds' of circuits as listed above, this will effect how your fuse panel will be configured. The first thing is your ignition and starting circuits don't need to be fused. The 'basic' ignition systems most of us are using won't present a short-circuit issue if they fail, the same thing goes for the start circuit; the only real danger here is if the power supply wire itself goes to ground which can be addressed by careful routing. Installing a fuse to 'protect' the ignition/starting circuits really only introduces an additional failure point that's not needed; few if any of these vehicles had these circuits fused from the factory. If you're running EFI, then that may not be true, but then that isn't very HAMB friendly either... LOL. But if you're tackling that, then you should know most of this already...LOLOL...

    So what you need is two fuse panels, or one that's 'split' internally. All the unswitched circuits go to one, all the switched 'accessory' circuits go to the other. Ideally, each panel will have one power feed wire that will feed one side of each fuse off a common 'buss', with the other side going to the individual circuits. This will reduce your wire count considerably. Depending on the size of the panel and the loads involved, this may not be possible. This is something to ask about when selecting a fuse panel vendor or buying one on your own.

    So the very first thing you need to do when deciding to wire your car is to figure out just what items you'll have, and which type of circuit it is; unswitched or switched. If you have some 'wish list' items that you're thinking of adding later, be sure and include them as it's far easier to set the harness up out of the gate for them than it is to add them later.

    Next installment I'll go into wire sizing and ampacity... and I'll start upsetting applecarts..... LOL!
    Last edited: Mar 20, 2014
    indianbullet likes this.
  17. The37Kid
    Joined: Apr 30, 2004
    Posts: 28,053


    Thanks! Too tired to read it all tonight. I just hope you can post it all before it gets torn apart by others that come from the other school of electricity. Bob
    LOU WELLS likes this.
  18. jkski
    Joined: Jan 27, 2009
    Posts: 137


    great topic!,i'am all in.
  19. The37Kid
    Joined: Apr 30, 2004
    Posts: 28,053


    As the most clueless can I ask a question? Why are you starting with the ignition switch? This is like hanging wallpaper and the sill plates haven't been attached to the foundation. Bob
  20. "but electricity doesn't know where it's working, a car, house, etc, the basic theory is the same no matter what. AC or DC, no difference...."

    Amen brother! Trons is Trons. Provide the understanding and remove the fear so many can benefit.

    High School, Navy, and College have all taught me the same thing. Last 2 cars I wired from scratch. Teach the masses!
  21. Yea I'll watch this. Always looking for helpful hints
  22. I will be watching this one .Been there & done it but keeping it fresh is always good
  23. Model T1
    Joined: May 11, 2012
    Posts: 3,309

    Model T1

    I've wired many cars from scratch and used lower cost kits. My son has been wiring cars both ways since he was under 10. He's 24 now and knows more and easier ways than I and many others.
    Yet we can always learn and relearn. Thanks for walking us through this magical thing, electricity!
  24. Good thread.. subscribed!
  25. sedan33
    Joined: Mar 27, 2009
    Posts: 100


    Never to old to learn,,,,subscribed!
  26. Great thread, keep em coming !
  27. Larry Pearsall
    Joined: Apr 7, 2007
    Posts: 1,462

    Larry Pearsall

    I too have wired many cars from scratch, making my own harness..But I will read this for any clues that will help.. I see "subscribed" by by readers of this.. How do you start that ?
  28. brooksinc1976
    Joined: Dec 4, 2009
    Posts: 259

    from P-Town

    Well looks like I'm hooked on another one. Subscribed. Read through the tech info and had flash backs to flight training electrical day. Keep it comming.

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  29. woodbox
    Joined: Jul 11, 2005
    Posts: 1,144


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