How To Diagnose Old Car Electrical Problems

As with any electrical repair, you need patience and knowledge to diagnose old car problems. A good plan of attack is to break the electrical system into smaller categories. In no particular order, it would look like this:

diagnosing electrical problems

Components Of Automotive Electrical System

  • Low tension side (battery, ignition switch, contact breaker, primary winding of the coil)
  • High tension side (distributor cap and rotor, spark plug wires, spark plugs)
  • Charging system (alternator/generator, voltage regulator)
  • Starting system (starter, starter relay, starter solenoid)
  • Electrical components (blower motor, wiper motor)
  • Fuses and fuse box
  • Wires, switches and relays
  • Light bulbs (interior and exterior)

Before tackling any electrical problems, start with a fully charged battery. Have a factory service manual for your make and model car and study the wiring diagram.

Magnifying glass with large viewing area for reading small prints

If the diagram is too small to read clearly, pick up a magnifying lens with a large viewing area and transcribe the information you need onto a larger piece of paper.

At least one company offers large, laminated posters of electrical wiring for individual vehicles, but these are limited by the vehicle's popularity.

Wires and Harnesses

As cars get old, electrical wires get dirty and brittle and lose conductivity. The wiring harness and switches may be 50 years old or older, and never designed to last that long. Distributors and coils cannot work efficiently if they do not get the full voltage they require.

old car fuse box

Many old car electrical problems are as simple as loose or corroded connections, a blown fuse, or a melted fusible link. If several electrical components aren't working, that may indicate a fuse or ground connection.

Repairing A Broken Wire

Once a broken electrical wire has been discovered, the two pieces of wire need to be spliced and reconnected. This can be done by using either crimp connectors, or soldering the two wires together. Crimp connectors are much easier to use, but soldering is more permanent.

Electrical Connectors

Although your classic car's wire terminals were originally soldered on, solderless terminals may be used if properly connected. Electrical connectors come in various types and sizes, and the procedure for installing them is straightforward. After cutting the old terminal off, strip the wire insulation about 3/8", then crimp the terminal barrel on with a wire crimper. When done, give them a slight tug with two fingers to make sure they're secure.

Heat Shrink Connectors

With heat shrink style terminals, after crimping them on, apply heat with a heat gun or lighter while rotating the connector. The heat shrink tubing will reduce in size and adhere to the wire.

heat shrink spade end electrical connector kit

Shop: Heat Shrink Spade Connector Kit

Electrical Test Light

A test light, also called a circuit tester or continuity tester, is invaluable for finding electrical shorts.

Be sure to use an automotive test lamp which works with 12 volt systems. One end goes to a positive and the other end goes to a ground.

how to diagnose old car electrical problems

Shop: 12 Volt Test Light

When diagnosing an automotive electrical problem, always start by checking the test light on the car's battery. Depending on what you testing for, you may have to turn the ignition switch on.

Most test lights have a sharp pointed end. Use this to pierce the plastic insulation on a wire. This allows you to test the circuit without disconnecting anything.

Digital Multimeter

A multimeter is a test tool instrument designed to measure electrical values such as voltage (volts), current (amps), and resistance (ohms).

There are two main types of multimeters, analog and digital. Digital is by far more common, they have an easy-to-read numeric display, and one can be had for less than $20. Although you can run dozens of tests with a multimeter, I use mine for about two.

electrical multimeter

Shop: Digital Multimeter

To check if a wire is good or bad, connect one test probe to one end of the wire, and the other test probe to the other end of the wire. With the meter on ohm setting, it should read .02 or less. Anything higher than that, replace the wire and/or the terminals.

Battery Cables and Terminals

Check the battery cables; make sure the cables fit firmly and securely on the battery posts. There should be no wiggle in them. Also make sure that the cables going to the terminals are in good shape. If not, replace them.

Ground Straps

Another area of concern is the condition of the ground straps. All old cars need a good electrical ground from engine to frame and engine to firewall. Often, these straps are old and grimy and need to be cleaned or replaced. If you start looking through your car, you may notice several ground straps from the factory, and some may have disappeared over the years.

Aside from the cable from the negative post on the battery to the engine block, there may also be a strap from the engine to the firewall, from the dash to the body, from the body to the frame or the battery negative post, and a strap from the frame to the engine block. Make sure your grounds have clean connections and are in good shape. If you're not sure, replace them or add another one.

Checking Low Tension Side

To perform a simple check, switch the headlights on, then watch them while a helper cranks the starter. They should dim slightly, and the starter should turn at it's usual speed. If the headlights go dim while the starter turns slowly, either the battery is low or it's connections are bad. Check positive and negative battery cables to be clean and tight. Also check ground straps from motor to frame.

Parasitic Draw

The easiest way of finding a parasitic draw is to use a multimeter. With the vehicle off, disconnect the negative battery terminal. Check that the car doors are shut and other accessories turned off. Set your multimeter to 10 amps DC, touch the positive lead to your disconnected negative battery cable, and touch the ground lead to your negative battery post. If your meter shows anything above 50 milliamps, you have a parasitic draw.

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Read: Battery Jump Pack Review