Automotive Electrical / Electronic System Tools

Electrical system service and diagnostic tools are discussed in the following paragraphs. This discussion does not cover all of the tools you may need; rather, these tools are the most commonly used by the ser vice industry. Many automotive systems are electrically controlled and operated; therefore, these tools are also used in those systems. Details of when and how to use these tools are presented in Section 3 of this guide, as well as in the sections that discuss the various other automotive systems.

Computer Memory Saver

Memory savers are an external power source used to maintain the memory circuits in electronic accessories and the engine, transmission, and body computers when the vehicle's battery is disconnected. The saver is plugged into the vehicle's cigar lighter outlet. It can be powered by a 9- or 12-volt battery.

Circuit Tester

Circuit testers are used to check for voltage in an electrical circuit. A circuit tester, commonly called a testlight, looks like a stubby ice pick.

Its handle is transparent and contains a light bulb. A probe extends from one end of the handle and a ground clip and wire from the other end. When the ground clip is attached to a good ground and the probe touched to a live connector, the bulb in the handle will light up. If the bulb does not light, voltage is not available at the connector.

!WARNING! Never use a 12 V testlight to diagnose components and wires in computer systems.

The current draw of these testlights may damage the computer system components.

High-impedance testlights are available for diagnosing computer systems.

A self-powered testlight is called a continuity tester. It’s used on open circuits. It looks like a regular testlight but has a small internal battery. When the ground clip is attached to one end of the wire or circuit and the probe touched to the other end, the lamp will light if there is continuity in the circuit. If an open circuit exists, the light won’t illuminate.

!WARNING! Don’t use any type of testlight or circuit tester to diagnose automotive air bag systems. Use only the vehicle manufacturer's recommended equipment on these systems.

Voltmeter

A voltmeter has two leads: a red positive lead and a black negative lead. The red lead should be connected to the positive side of the circuit or component. The black should be connected to ground or to the negative side of the component. Voltmeters should be connected across the circuit being tested.

A voltmeter measures the voltage available at any point in an electrical system. A voltmeter can also be used to test voltage drop across an electrical circuit, component, switch, or connector. A voltmeter can also be used to check for proper circuit grounding.

Ohmmeter

An ohmmeter measures resistance to current flow in a circuit. In contrast to the voltmeter, which operates by the voltage available in the circuit, an ohmmeter is battery powered. The circuit being tested must have no power applied. If the power is on in the circuit, the ohmmeter will be damaged.

The two leads of the ohmmeter are placed across or in parallel with the circuit or component being tested. The red lead is placed on the positive side of the circuit and the black lead is placed on the negative side of the circuit. The meter sends current through the component and determines the amount of resistance based on the voltage dropped across the load.

The scale of an ohmmeter reads from 0 to infinity (_). A 0 reading means there is no resistance in the circuit and may indicate a short in a component that should show a specific resistance. An infinity reading indicates a number higher than the meter can measure, which usually indicates an open circuit.

++++An ammeter with an inductive pickup is called a current probe

++++The volt/amp tester checks batteries and the starting and charging systems.

++++Typical multifunctional, low-impedance multimeter.

Ammeter

An ammeter measures current flow in a circuit. Cur rent is measured in amperes. Unlike the voltmeter and ohmmeter, the ammeter must be placed into the circuit or in series with the circuit being tested. Normally, this requires disconnecting a wire or connector from a component and connecting the ammeter between the wire or connector and the component.

The red lead of the ammeter should always be connected to the side of the connector closest to the positive side of the battery and the black lead should be connected to the other side.

It’s much easier to test current using an ammeter with an inductive pickup. The pickup clamps around the wire or cable being tested. The ammeter determines amperage based on the magnetic field created by the current flowing through the wire. This type of pickup eliminates the need to separate the circuit to insert the meter.

Volt/Ampere Tester

A volt/ampere tester (VAT is used to test batteries, starting systems, and charging systems. The tester contains a voltmeter, ammeter, and carbon pile. The carbon pile is a variable resistor.

When the tester is attached to the battery and turned on, the carbon pile draws current out of the battery.

The ammeter will read the amount of current draw.

The maximum current draw from the battery, with acceptable voltage, is compared to the rating of the battery to see if the battery is okay. A VAT also measures the current draw of the starter and current out put from the charging system.

Multimeters

A multimeter is a must for diagnosing the individual components of an electrical system. Multimeters have different names, depending on what they mea sure and how they function. A volt-ohm-milliamp meter is referred to as a VOM or DVOM, if it’s digital.

A digital multimeter (DMM) can measure many more things than volts, ohms, and low current.

Most multimeters measure direct current (dc) and alternating current (ac) amperes, volts, and ohms. More advanced multimeters may also measure diode continuity, frequency, temperature, engine speed, and dwell, and/or duty cycle.

Multimeters are available with either digital or analog displays. DMMs provide great accuracy by measuring volts, ohms, or amperes in tenths, hundredths, or thousandths of a unit. Several test ranges are usually provided for each of these functions. Some meters have multiple test ranges that must be manually selected; others are autoranging.

Analog meters use a sweeping needle against a scale to display readings and are not as precise as dig ital meters. Analog meters have low input impedance and should not be used on sensitive electronic circuits or components. Digital meters have high impedance and can be used on electronic circuits as well as electrical circuits.

++++ Two types of lab scopes.

++++ The screen of a graphing multimeter.

Reproduced under license from Snap-on Incorporated. All of the marks are marks of their owners.

Lab Scopes

An oscilloscope or lab scope is a visual voltmeter. A lab scope converts electrical signals to a visual image representing voltage changes over a specific period of time. This information is displayed in the form of a continuous voltage line called a wave form or trace. With a scope, precise measurement is possible. A scope displays any change in voltage as it occurs.

An upward movement of the voltage trace on an oscilloscope indicates an increase in voltage, and a downward movement of this trace represents a decrease in voltage. As the voltage trace moves across an oscilloscope screen, it represents a specific length of time.

The size and clarity of the displayed waveform is dependent on the voltage scale and the time reference selected. Most scopes are equipped with controls that allow voltage and time interval selection. It’s important when choosing the scales to remember that a scope displays voltage over time.

Dual-trace scopes can display two different wave form patterns at the same time. This type scope is especially important for diagnosing intermittent problems.

Most new lab scopes can display more than two waveforms and are hand-held units. Some scopes have an electronic library of known good signals, which allow technicians to compare what they see with what they should be seeing. Some also include wiring diagrams and additional diagnostic and testing information.

Graphing Multimeter

One of the latest trends in diagnostic tools is a graphing digital multimeter. These meters display readings over time, similar to a lab scope. The graph displays the minimum and maximum readings on a graph, as well as displaying the current reading. By observing the graph, a technician can detect any undesirable changes during the transition from a low reading to a high reading, or vice versa. These glitches are some of the more difficult problems to identify without a graphing meter or a lab scope.

Battery Hydrometer

On unsealed batteries, the specific gravity of the electrolyte can be measured to give a fairly good indication of the battery's state of charge. A hydrometer is used to perform this test. A battery hydrometer consists of a glass tube or barrel, rubber bulb, rubber tube, and a glass float or hydrometer with a scale built into its upper stem. The glass tube encases the float and forms a reservoir for the test electrolyte. Squeezing the bulb pulls electrolyte into the reservoir.

When filled with test electrolyte, the hydrometer float bobs in the electrolyte. The depth to which the glass float sinks in test electrolyte indicates its relative weight compared to water. The reading is taken off the scale by sighting along the level of the electrolyte.

++++ A hydrometer is used to measure the specific gravity of a battery's electrolyte.

! WARNING! Be careful not to allow the battery's electrolyte to drip on you or the vehicle. The electrolyte is a very strong acid and can burn your skin and damage the vehicle's paint.

Wire and Terminal Repair Tools

Many automotive electrical problems can be traced to faulty wiring. Loose or corroded terminals; frayed, broken, or oil-soaked wires; and faulty insulation are the most common causes.

Wires and connectors are often repaired or replaced. Sometimes an entire length of wire is replaced; other times only a section is. In either case, the wire must have the correct terminal or connector to work properly in the circuit. Wire cutters, stripping tools, terminal crimpers, and connector picks are the most commonly used tools for wire repair. Also, soldering equipment is used to provide the best electrical connection for a wire to another wire and for a wire to a connector.

Headlight Aimers

Headlights must be kept in adjustment to obtain maximum illumination. Sealed beams that are properly adjusted cover the correct range and afford the driver the proper nighttime view. Headlights can be adjusted using headlamp-adjusting tools or by shining the lights on a chart. Headlight-aiming tools give the best results with the least amount of work. Many late-model vehicles have levels built into the head lamp assemblies that are used to correctly adjust the headlights.

Most headlight aimers use mirrors with split images, like split-image finders on some cameras, and spirit levels to determine exact adjustment.

When using any headlight-aiming equipment, follow the instructions provided by the equipment manufacturer.

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