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Fiddling with Rambler's since 1995

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Electrical Systems Primer

There are two systems on cars that leave most people at the mercy of someone else. The first is paint and body work (Rambler's have alot of galvanized steel body panels, failure to prep them correctly causes paint failure), and electrical systems repair. Both of these areas are considered "black magic" by those not accustomed to working with them.

The purpose of this page is to introduce you to the basics of electrical work. I'm not going to try to teach you how to rewire your car (which, in many ways, is easier than repairing what you already have), but what I hope you'll get from here is a good understanding of how your electrical system functions so you can find and fix the problems you have.

A few basics you need to understand...

One of the most confusing parts of electrical work for new comers are some of the terms that get used. The four  you are most likely to see are circuit, current, voltage, and resistance. A Circuit is the path that electricty takes as it goes from the positive side of the battery to the negative side. Current is the amount of electricity that flows through an electric ciruit over a given amount of time and in a certain direction. Current is measured in Ampres (you'll usually see this indicated as a number of amps). Voltage is a measurement of how much power the current supplies. Most cars run on a 12 volt system. Normal voltage is 10.5 to 14.5 volts. Resistance is the tendancy for a part of the circuit to reduce the amount of voltage passing through a circuit. Resistance builds heat. Resistance (for the most part) is bad except in the case of light bulbs.

Electrical current, just like water and any other non-solid matter, always flows from an area of high pressure (the positive side of the battery) to an area of low pressure (the negative side). The speed at which it flows (which is the current), determines how long a battery will last before it is drained. Batteries have several ratings which are of concern to you as you work on electrical systems.

The most important (and usually ignored) rating is its Ampere pre hour rating. A higher rating here means that you have more reserve power than a battery with a lower rating. If you are doing a custom wiring job, or you are adding extra accessories to your car (like amplifiers for the radio), you will need to figure out the amount of current used and make sure the Ampere/hour rating of the battery (and generator/alternator) is close to this. Please note that adding up the amp numbers on your fuse block is not the correct way to get this number. You need to know the current requirements of each item you will be using (headlights, fan, radio, etc..) excluding the starter. To make matters worse, the Ampere/hour rating is not always advertised but is expressed in minutes of reserve power (which wouldn't be bad if they listed the load this was tested under).

The next most important rating is the battery's Cold Cranking Amp ( or CCA) rating. The starter motor requires more current than any other item on a car (usually several hundred amperes). The colder the engine and oil temperature, the more current the starter motor needs to do its job. If you have a battery with a border line Cranking Amp rating (which is how much current it can supply at room temperature and is always higher than the Cold Cranking Amp rating), you will have problems starting the car in cold weather. Keep in mind that as a battery ages, its Cranking Amp (and Cold Cranking Amp) rating decreases. This is why many batteries die in the winter.

The final rating we are concerned about is voltage. Most cars use a 12 volt electrical system and require a 12 volt battery. If you put the wrong voltage battery in a car, you can cause all sorts of problems. To small of a volt rating and you can cause the battery to either leak acid or explode from overcharging. If the battery is to large (voltage wise), you can damage the rest of the electrical system very quickly. Since most cars use 12 volts (higher and lower voltages would be special order), you shouldn't have problems resulting from the battery putting out to much voltage.

A simple circuit...

Lets take a look at one of the simpler circuits on a car (keep in mind that almost every circuit operates on the same principals, the devices in the circuit are the only differences): the dome light (the type that comes on when you open the door). For now, act like the light can't be turned on with the headlight switch. The components in the system are: the battery (A), a fuse (B), the dome light fixture, a light bulb (C), the door jamb switch (D), the wire going between all the components, and the car body (E). Where the wiring contacts the car body (points E), is called a ground.

In order for the dome light to come on, every component in this circuit must be working. A bad light bulb or a bad (blown) fuse are the most common reasons a simple circuit like this fails.

circuit.gif (1263 bytes)
figure 1 -  Dome light circuit

What if I've tried a new bulb and fuse and the light still doesn't work?

When you get into a situation like this, the usual problem is that one of the switches in the circuit failed or the last item in the circuit has a bad ground. If these check out okay (I'll get to test procedures shortly), you will need to check the light fixture and the wiring from the fuse panel to the light and from the light to the door jamb switch. This switch, like most others that mount into a metal part of the car, do not have a separate ground (negative) wire. The contact between the switch and the body of the car is how they ground. To go any farther in our diagnosis, you will need some tools specific to electrical work.

The tools of the trade..

The electrical system, just like many other areas on cars that are not serviced regularly, require a few specialized tools. The minimum you need to do correct repairs are:

  • Factory level wiring diagram (this is the most important "tool" you have for troubleshooting)
  • Wire stripers
  • Wire cutters (I have both manual and automatic adjusting types)
  • A crimp tool
  • A test light
  • An electronic multimeter
  • A soldering iron and/or gun

If you go the cheap route, you can probably get all of these tools for less than $60. Just keep in mind that spending a little bit more initially can save you money in the long run. At any rate, you're still not done shopping. Now that you've gotten the tools, you need some


This is a hard area to give specifics on. Generally, you should keep the following around:

  • Electrical tape (UL listed)
  • Heat shrink tubing and/or shrink tape (UL listed)
  • Quality crimp connectors (MotorMite, Ideal, and Ilsco are some of the better ones.).  These should all be UL Listed.
  • Rosin core solder. DO NOT use acid core! It will destroy your connections. Acid core is for plumbing, rosin core is for electrical work.
  • Wire ties. 4", 6", and 8" lengths should suffice for most jobs. Longer ones are available if you need them. You can get these in all sorts of colors if you want. I prefer black since it hides well. The better ones are UL listed.
  • Automotive grade wire. Standard wire will not hold up to the harsh environment in a car (especially under the hood). What happens with standard wire is that all of the heat and goo under the hood dry out the insulation and it cracks and falls off.  Make sure the wire is UL listed. Small spools of 12, 14, and 18 gauge wire will cover most circuits.
  • Harness tape (early cars) or split tubing (later cars). This may seem like a strictly cosmetic item, but it serves a few important functions. First, it supports the wiring. If you rely on the connectors alone, the wire will come out of them or break from all of the flexing. Second, it protects the wiring from anything sharp or abrasive. Long story short, use it.
  • Fine emery cloth. No, this isn't UL listed, but it comes in real handy for cleaning contacts.

At this point, you may be wondering, just what in the heck is all this 'UL listed' business? A UL listing is basically an independent labs certification that the item the mark is on meets or exceeds (in most cases) the specifications for which it was designed for. 'UL' is short for 'Underwriters Laboratory' and is the most prevalent certification lab in the US. The mark on metal components will be 'UL' enclosed in a circle. Every connector, wire tie, or whatever that is certified will have this mark on it somewhere. Note: in countries other than the US, your connectors may not have a UL listing.

Now that you have all of your tools and UL listed supplies, you may be wondering

How in the heck do I use this crap?

Wire cutters: These are obviously used to cut wire. Don't be tempted to cut huge wires (like battery cables) in one cut. If the wire (or bundle of wires) is thicker than a No. 2 pencil, make a few smaller cuts instead. What happens when you cut large wires is that you deform the cutting jaws. This causes problems when you need to cut smaller wires.

Wire strippers: These are used to remove the insulation from a wire. In general, the sharper they are, the easier it will be to strip the wire. I've seen guys remove large amounts of skin with good ones. Be careful where you leave them. The way you use them is to put the wire in the correct slot, close down on the handles, and rotate them around the wire to make sure the insulation is cut cleanly. Once the insulation is cut, pull the wire out of the jaws of the strippers. The insulation will fly off the end of the wire once you are near the end of the wire. If you need to remove more than about 1/2", strip the wire off in chunks.

Crimp tool: This tool is used to crush (or crimp) the metal sleeve around the end of the wire. How you use the crimp tool depends on the design of its jaws or dies. If the tool has two semicircles for its jaws (A in the picture), place the connector on the end of the wire, find the split in the connector and make sure the split points to the middle of one of the jaws. When everything is lined up, squeeze the handles together until the connector is on tight. It the tool has one jaw (or die - B in the picture) that points up and the other side has a corresponding groove, follow the above procedure except make sure the split in the connector faces to the top of the jaw with the groove. elec1.gif (938 bytes)

Test light: This device looks like a screw driver with a wire coming out of it. The way to use this is to connect the wire to a good ground and you place the sharp end into a connector in the circuit you are testing. If it lights up, you have power.

Electronic multimeter: There are lots of different types of these. Consult the users manual.

Soldering iron and/or gun: This is used to melt the rosin core solder and join two wires. Unless you plan on repairing delicate electronic circuits, a 100 watt gun will be the easiest to use. The way you solder is to twist the ends of the wires you are joining together (make sure the ends are clean), touch the iron or gun to the wires, and after it has had 15 or 20 seconds (this time may be longer depending on the wire size), touch the solder to the wires you are joining. If the joint was hot enough, the solder will be sucked into it. If this doesn't happen, let the joint heat up more.

Back to the business at hand...

Okay, you've got your tools, you understand how they work, but you're light still doesn't work. The first thing to check is the ground connection. Once you get past the replacement of fuses and bulbs, this fixes 85% of your problems.  Remove your door switch and clean the area of the door jamb where the switch makes contact and reinstall the switch. If this doesn't fix it, try the next simplest thing: the switch itself. To check this, simply unplug it and touch the wire to bare metal. Okay, lets assume you're car has decided not to cooperate with us so far. Before you are tempted to do something drastic (like shoot your car), get out your wiring diagram (in our case, figure 1) and study the circuit in question. In our case, we either have a bad wire or the light fixture is bad. Let's check the fixture first.

To check the fixture, disconnect the wiring and get your multimeter. Set the meter to its continuity function and put a lead on each connector. If you have continuity, your meter will show you according to its manual. If there is no continuity, try wiggling the bulb (removing the bulb and checking it is a good idea also). If this doesn't fix it, check the contacts. If they are dirty, clean them and try again. If this doesn't fix it, replace the fixture.

So your fixture was okay, huh? Now were into the nether regions of electrical work (this is where about 5% of your problems will be but it'll seem like more because this area of the system is a pain to work on). Since you've eliminated every other device in the system, we are left with a wiring problem. Get out your test light and connect it to a good ground. Connect the other end to the wire coming from the fuse. If the light comes on, you're problem is with the ground side of the circuit. If it doesn't check the other wire just to make sure. If it still fails to light, you need to repair the positive wire.

Before you go replacing any wiring, there are two things you need to check. The first is to make sure the ground connection the switch has is good. Set your multimeter to check continuity and then connect one side to the negative terminal of the battery and the other side to the switch body. If there is no continuity, you have a ground problem. The other thing to check is the voltage where the positive side connects to the fuse. Its rare, but the fuse holder or block (depending on the circuit) could have a problem. If both of these check out, go ahead and replace the indicated wires.

Adding some more realism...

As you've probably noticed, our first circuit was really simple. Figure 2 shows what the circuit in your car actually looks like. Notice that two switches have been added. Switch F is another door switch and switch G is the headlight switch. What this means is that there are two more connections that could be bad and you could also run into the situation where opening a door doesn't cause the light to come on (or it stays on all of the time).

Notice how the second switch is wired in series with the first? This means that there could be a problem with the wiring (or a connecter) going to D that affects F. The splice that goes to G could also be a problem. If it is corroded, it could keep the circuit from grounding through any of the switches.

circuit2.gif (1617 bytes)

figure 2 - real world dome light circuit

This is where a wiring diagram (which is absolutely critical to have for most circuits) will come in handy. Before you can trouble shoot your car's electrical system, you need to know exactly what is in the circuit and what impact it can have on the operation of the circuit. Every additional device adds at least three more things you need to check. It can add a bad ground, bad connections, or bad wiring. As circuits become more complex, it is necessary to check the device ground first and then start from the power source (almost always the fuse panel) and work towards the defective item. If there is a connection, you will need to get access to it and check that the connection is not the problem. This is why electrical shops charge what seems to be a fortune to fix seemingly simple problems.

Wiring tips...

  • Always support the wire every 12 inches and right before where it connects to each device, if possible. If you don't support the wiring, you will have problems with crimp connectors falling off and internal breaks in the wire.
  • Avoid using crimp-on wire taps (these are sometimes called vampire taps). They cause corrosion problems in the wire where you use them and if the insulation falls off (which is a big problem with some of them), you can have a problem with the connection shorting out.
  • After you make any sort of connection, gently tug on the connection to make sure it is secure. If you are soldering a splice, tug on the wires before you solder.
  • When you are extending a wire by using a butt-splice, push the strands of the wire into each other and then twist them together. This results in a better splice than simply twisting the wires together.
  • When you solder, heat the connection enough to melt the solder. Don't use the iron/gun to melt the solder.
  • If you need to tap into an existing wire, strip about 3/4" of the insulation from the area  you want to splice into. Strip 1/2" from the wire you are adding. Separate the existing wire and push the new wire into it. Twist the new and old wires together and solder. Wrap with shrink tape or electrical tape.
  • If you don't have access to a heat gun to shrink heat shrink tubing, you can use a butane or refillable cigarette lighter to shrink it. Move the flame back and forth slowly under the tubing being careful not let the flame touch the tubing.
  • After you solder a connection, give it a few minutes to cool off before you slide the heat shrink tubing over the connection.

Product Sources:

Perfect Performance Products, Inc.
9505 Santa Paula Driv
Fort Worth, Texas  76116
(817) 244-6898

Motormite(division of R&B, Inc.)
3400 East Walnut Street
Colmar, Pennsylvania  18915
(215) 997-1800

Radio Shack
100 Throckmorton Street Suite 1800
Fort Worth, Texas  76102
(817) 415-3200

Last Updated 01/05/04 08:56:33 PM