Of all the jobs classic cars need, the one that scares people most is wiring issues. Whether it’s chasing down wiring troubles or re-wiring your car, it’s as if we’re afraid voodoo is involved. And maybe it is—electricity is, after all, something you can’t see. With a little understanding, electrical issues and wiring in your classic car doesn’t have to be a mystery. For those just getting started, let’s go over some of the tell-tale signs that you may have a wiring issue as well as some key things you need to learn before delving into working through it.

Common Problems Caused By Wiring

  • Ignition/engine performance problems
  • Mysterious issues with Accessories (lights, blinkers, radio, etc) that may only happen “sometimes”
  • Parts powered by electricity aren’t working properly—like a starter or an electrical fuel pump

Why do old cars have so many electrical issues? A number of factors contribute. First, you’ve got wiring that is decades old—which means aging as well as corrosion. Coming in at the ends of the wires, corrosion then travels through the wire itself causing extra resistance, meaning electricity can’t travel through it as easily. The endpoints also get effected by corrosion, which can effect the ground they’re creating. Grounds are necessary parts of complete circuits. Without a complete circuit, electricity can’t travel properly—bringing us to our next part…

Basic Electrical Concepts to Wrap Your Head Around Before You Start Wrenching

Closed Circuit

In order to keep electricity flowing in car’s electrical systems, you need to have a closed circuit for it to travel through. This means you have a load or draw of electricity all the way down through the ground of the circuit. An opening in any part of the system will disrupt the flow of electricity and therefore cause an issue in your car.

Ohms Law

Voltage x Amperage / Resistance = 1

  • Voltage: pressure or force
  • Amperage: volume
  • Resistance: restriction

Formulas aren’t exactly what most people want to see when they go to work on a car, but in Ohms law case it’s just the principal behind the formula that you should understand. Electricity is the pressure or force in this equation, it’s a flow of electrons. When voltage goes up, amperage will also go up. If resistance goes up, then voltage and amperage will go down. I’m no electrician or physicist—and am honestly just beginning to grasp these concepts myself—so I won’t attempt to describe this in detail. But a good primer on electrical concepts can be found here.

Before allowing ourselves to get too confused, let’s get down to business for figuring out a car’s wiring issue…

4 Questions To Ask Yourself to Figure Out Any Electrical Problem

1. How do I get power?

Is it getting the power from a battery, switch, or what?

2. How do I get ground?

Is it the item’s housing, a ground wire, or case that is providing grounding?

3. What do I have to do?

Is there a switch that needs to be flipped, temperature or pressure that it needs to be at? Whatever has to be present in order to make what needs to happen, happen.

4. What does it have in common?

Does it share a ground with anything? Does it share power with anything? Does it have anything in common or shared with anything else?

As you go through a process in doctoring out electrical gremlins in your car, remember to always…


6 Responses

  1. JP Kalishek

    You forgot the greatest fun benefit of overly old or poor wiring, especially with certain Italian, Swedish, and British cars, but plenty of US and Japanese made as well and I had a Japanese model do this once but it wasn’t wiring caused.
    Nothing like a good fire to get ones attention.

    • Kristin Cline

      You’re right JP! Nothing like the obvious… but that certainly needs to be stated! Thanks!
      P.S. British cars and wiring are not the best of friends!

      • JP Kalishek

        Positive”earth” and Lucas Lord of Darkness. Not a great combination.
        I used to drive an old Iveco truck for work. Also not a well wired machine. Some Italian cars looked worse than some hack job rewires by someone who didn’t know how and had no money, but these were factory.
        Fiat: Fix It Again, Tony
        The Ferrari 308 a buddy worked on was not impressive either.

  2. Russ Bellinis

    Check for voltage drops all along the circuit. Get a good wiring diagram for your car. For older cars that are common, I think Motors have wiring manuals. If your old car is a 1952 Ferrari type of which only less than 10 were built, good luck! Most of the common American car post WW2 will have manuals available. When trouble shooting always start with the battery, unless it is a charging problem in which case start with the generator or alternator if it is newer or has had a conversion.

    Your wiring diagram is like a road map. Your trip starts at the battery, goes through a switch of some sort, and then through what ever components it needs to the component it is controlling, and then to ground.

    Take an example, your lights don’t come on when you turn on the light switch. Start at the battery, Do you have good voltage with everything off? 12-13 volts on a 12 volt system. 6-6.5 volts on a 6 volt system. Turn you ligth switch on, check you voltage at the battery again. Did it drop? If your voltage was slightly above 12 volts or 6 volts and it dropped to 12 volts or 6 volts +/- .5, the battery is good. If it dropped 2 volts or more, the battery has a bad cell, and needs replaced.

    The battery is good, but the lights still don’t come on. Do any lights come on? More importantly what lights don’t come on? If everything comes on except one headlight or one tail light, look for a bad bulb. Tail light bulbs are pretty easy to check by taking them out and looking. Sealed beam head lights are more difficult to tell. If it is a headlight on an older car with single headlights, use your dimmer switch to go from high beam to low beam. If the high beam comes on, but the low beam doesn’t you need a new headlight.

    If both headlights don’t come on, you probably have another problem, unless you have been driving for a while with only one head light and then that also quit! If that is the case, you need to improve your vehicle maintenance

    Lest stay with headligths for our example and lets say you have a 1955 Studie like Kristin. You will have a ligth switch mounted in the dash, a fuse for lights in the fuse block, and a dimmer switch on the floor. Check your fuses. Older cars will have the glass “tube” style fuses. When one blows, it is obvious. If you find a fuse blown, good luck. You are going to need to work a little. Fuses don’t typically blow without a cause, usually a short. Did you have a “little” fender bender recently? Look to make sure that your wiring to your headlights did not get damaged. If you find a place on the wire where it chafed on a damaged fender, Use your electrical tape to fix it temporarily to get home, and fix it correctly the next morning. That may involve putting in a new wire and knocking the fender out a little so it does not chafe the wire until you can replace or repair the fender. Replacing blown fuses never fixes a problem, and putting in a larger fuse or a heavy gauge wire instead of the fuse is a great way to start a fire!!

    Fuses are ok, now check voltage at the fuse. Is it full voltage? Good. If it drops significantly either coming in or going out of the fuse, you have a dirty connection right at the fuse block. Clean it.

    Ok your fuse and fuse block were good. You have good voltage going out of the fuse block to your headlights. In our example of the 1955 Studie, you have a dash mounted headlight switch, and a floor mounted dimmer switch. They will be mounted in series with each other. Series means that power goes to one switch, out that switch and to the other switch before it goes to the headlights. Normally it would go first to the dash switch, and then to the dimmer switch before going to the headlights. do you have voltage to the dash switch? If not you have a broken or corroded wire between the fuse block and the dash switch. If you have voltage into the switch, do you have a voltage drop going out? A large voltage drop or no voltage going out means you need a new switch. If you light switch checks out, check the dimmer switch the same way. If you have good voltage out of the light switch, but low voltage to the dimmer switch, you have a problem with the wire running from the light switch to the dimmer switch.

    This comment has gotten kind of long, but I’m trying to show that you need a logical step by step trouble shooting process to find find and fix electrical problems. Just think of your wires as a road for electricity to travel on, and your wiring diagram is the map. I have seen guys just start throwing parts at a problem. That is an expensive way to fix your car! Auto parts stores will not usually take electrical part back once you walk out of the store, so you can end up with your own auto parts store in your garage if you are a parts changer.

    A couple of quick tips before I close this comment. If one or more headlights have a dim yellow glow instead of shining brightly like they should, look for a bad ground. If your dash lights go out at night, you have probably blown your taillight fuse. Auto manufacturers typically put the tail lights and dash lights on the same fuse circuit so that you will know when your tail lights quit working. That is because, if you don’t walk around and check your lights before driving, you won’t know if your tail lights are out.

    • Kristin Cline

      Thanks for the input Russ! “…you need a logical step by step trouble shooting process to find and fix electrical problems.” You are precisely correct!

  3. Russ Bellinis

    I have one other brief comment. Some cars, like Rolls Royce in the 1950’s & 1960’s have the battery in the trunk. Rolls Royce uses two six volt batteries in the trunk tucked into each fender behind the wheels. Volkswagen beetles have the battery under the back seat. You may need some really long jumper wires to do some of the checks on your vehicle if the problem component is a long way from the battery. You can make long jumper wire by buying 20 feet of light cord from a home center and a box of small alligator clips from either the same home center or an auto parts store.


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