It's Electric! Part 2

Practical knowledge for the muscle car owner

It's Electric! Part 2
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Feature Article from Hemmings Muscle Machines

July, 2005 - Ray T. Bohacz

With the basics of electrical theory established in Part 1, it's time to roll up our sleeves and tackle some electrical gremlins. But not before we cover some test instrument fundamentals.

There are a three different meters and a few application-specific tools that will be required to work on your car's electrical system. They are a volt/ohm/amp meter, a test light, jumper leads and a soldering iron. Other than these items, your investment will be in knowledge instead of equipment.

Meters with needles are called analog meters. There's nothing wrong with an analog meter, but digital meters are coming down in price, and are much easier to read. The digital readout is usually accurate to four places. That means you can read exactly 1,615 ohms or 12.36 volts or 0.541 amps. Better automotive-style digital meters have an internal resistance of up to 10 million ohms. That's overkill, for an early-model muscle car, but you'll still find the digital meter highly accurate and easy to use.

The most practical meter for an enthusiast to buy is what is known as a multimeter. This combines a voltmeter, ammeter and ohmmeter in one unit. The one disadvantage of a multimeter is that the measured current draw is usually limited to 10 to 20 amps. This means that it is good for almost every electrical circuit in an older car except the starter.

When buying a meter, one suggestion is to buy a one with fewer "bells and whistles" and, instead, put the money toward a meter with accuracy and quality. A good-quality multimeter for the enthusiast will be less than $200 and can be used on any electrical circuit, not just automobiles.

An ohmmeter measures resistance and can only be used on an unpowered circuit. In addition, an ohmmeter has no polarity, so it does not make any difference which lead is connected to either end of the circuit. The only time this does not apply is when checking a diode. An ammeter measures the load that is being consumed or used by the circuit and is polarity sensitive. A traditional ammeter needs to be connected in series, with the meter installed as part of the current path. (Some expensive meters use a clamp that clips around the wire to become part of the circuit.) A voltmeter is connected in parallel. Like the ammeter, it requires proper polarity when being connected to the test point. For example, if you wanted to check the voltage of your car's battery, just attach the meter leads to the proper terminal without disrupting the connections at the battery.

In all cases, read your manual to familiarize yourself with the scales, settings and connections of the instrument before attempting any diagnostic procedure. There's nothing worse than trying to diagnose an electrical problem while figuring out the test instrument.

Don't forget to clean, tighten and check all connections and every aspect of the circuit before you go chasing gremlins all over the car. When in doubt, always go to the battery positive and ground connections first.

A good diagnostician thinks and analyzes first. Does the activation of another component have an impact on the circuit in question? Are the battery fully charged and the alternator working? Is the circuit wired as you think it is? As with many things in life a little more pondering and a little less action usually pay the greatest dividends.

One last note: Whenever using any style meter, it is best to start on the highest scale and then work downward to a range that would provide the greatest amount of resolution for the most accurate reading. This holds true for both analog and digital equipment.

Electrical theory holds true regardless of the application. For example, a dim headlamp diagnostic procedure would be the same as a slow-cranking circuit or power window motor. So follow along as HMM provides some tips for the novice auto electric mechanic.

Resistance check of component or circuit
Tool: Multimeter set on ohms scale
Possible uses: To check a light bulb, fuse or any other circuit for continuity.

Simply attach the terminals of your ohmmeter to opposite ends of the circuit. Some meters have an audible continuity tester that will beep or buzz when a circuit is complete. This frees you from looking at the scale and is a quick way of checking circuit integrity.

Checking for a draw with the ignition off
Tool: Test light or multimeter set on amperes
Possible use: Finding out why your battery goes dead when vehicle is stored.

After charging the battery, a draw can be found with a simple test light. Remove the battery ground cable and install a test light in series to the ground path. If the light illuminates, there is a draw. Now go around and start removing fuses from the fuse box until the light goes off. When it shuts off, you've found the circuit that is staying powered with the ignition off.

Note: On newer cars, it is better to use an ammeter connected in the same manner to actually identify the amount of the draw. A modern electronic radio with station memory or a clock will illuminate the test light since they are drawing current. But these circuits consume so little energy it would be hard for them to kill a battery even after months of vehicle storage.

Checking alternator output
Tool: Multimeter set on volts
Possible use: Confirming charging circuit output

With the engine off, connect the voltmeter leads to the battery positive and negative terminals. The reading should be a minimum of 12.6 volts and can be as much as 2 volts higher due to the accumulation of what is called a surface charge. Now start the engine and check the battery voltage. The reading should be higher than with the engine off. Next, rev the engine and have a helper turn on electrical loads such as the headlamps. At 2,000 rpm, the battery voltage should be about the same reading as it is at idle with no electrical load.

Procedure: Check battery cranking capacity
Tool: Multimeter set on volts
Possible use: Determining whether the battery has enough energy to crank the engine

With the voltmeter attached to the battery, have a helper crank the engine over. You may want to unplug the coil wire so that the engine can crank without starting. With a fully charged battery the voltage should stay above 9.6 during crank. If the voltage drops below that, it is possible that the battery is weak or the starter is drawing an excessive amount of current. If the voltage stays high and the engine turns slowly, there is a problem with the starter circuit such as a poor ground, but the battery is fine.

Looking for an unintentional ground
Tool: Multimeter set on ohms
Possible use: Checking for a circuit that constantly blows a fuse.

Disconnect the battery. Connect one end of the ohmmeter to a good ground and the other lead to the suspect circuit. A reading of zero indicates an unintentional ground. Then locate the offending wire and start disconnecting components until the area where it is grounding can be identified with the same test procedure. For example, if the wire is leading back to the rear tailamps and there is a connector in the trunk, open the circuit there. If the ohmmeter reads infinity at the fuse box, then you know the unintentional ground is located rear of the connector.

Finding a short
Tool: Multimeter set on ohms
Possible use: One circuit is activated when another is turned on or a circuit is completely nonfunctional.

With the battery disconnected, check in between all of the circuits in the fuse box. If there is continuity (see first procedure above) between two fuses, those are the circuits that are shorted together. Then, using a wiring diagram, identify the wires and disconnect the suspect circuits at every possible connection. Using the ohmmeter leads, check for the short in each segmented circuit until it is located.

Checking a diode
Tool: Multimeter set on ohms
Possible use: Confirmation of inoperable component that employs a diode.

Diodes are used in alternators and in many early air conditioner systems. The diode was usually placed across the compressor clutch circuit to avoid a voltage spike back into the charging circuit when the clutch shuts off. If the diode is open, the circuit will not work.

Gain access to the diode. A good diode will pass current only one way. This will mean that the ohmmeter should read little or no resistance with the leads connected one way, and infinity with them connected in reverse. Anything else and the diode is defective and needs to be replaced.

Voltage drop test
Tool: Multimeter set on volts
Possible use: The best method to check a ground or feed circuit for integrity

A voltage drop test can be used on either the positive or negative circuits and is the most accurate method for diagnostics. The term drop describes the loss in the circuit and can explain poor electrical performance of any component. This test is an especially important way to check a ground path. It should be used instead of a resistance test because it will prove the circuit's ability to carry a high-current electrical load. Since an ohmmeter is powered by a small internal battery, a ground may look good under those circumstances, but when asked to carry a high rate of electrons, performs poorly.

When doing a voltage drop test on a ground circuit, you are measuring the amount of electrons stuck in a "traffic jam" at the poor ground path. To perform a voltage drop test: Connect the voltmeter red lead to the power (positive) side of the component, circuit or connection to be tested. Connect the voltmeter black lead (negative) to the ground side. Set the meter scale to exceed the expected voltage, and turn the circuit on (current must be flowing through the circuit for resistance to be found) and read the voltage. Scale the meter down to achieve the most accurate reading. This reading represents the voltage lost as current is pushed through the resistance. Dirty, corroded or loose connections or switches can be diagnosed in this manner.

To check for a good ground on a power window motor, the meter positive lead would be attached to the battery negative and the meter negative lead to the ground of the power window motor. In most instances, a jumper extension would be required. A good ground will have a voltage reading of less than 0.2 volt (except for high amperage circuits such as the starter, which can be as high as 0.4 volts). Anything higher and there is a traffic jam of electrons, meaning the ground path is poor.


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