22 Chapter Basic Electrical Tests

Objectives After studying this chapter, you will be able to: Use a systematic approach to find and correct electrical problems. Explain causes of basic circuit problems given their symptoms. Use testing devices to check component operation.

Objectives Use a multimeter to measure circuit voltage, voltage drop, resistance, and amperage. Test basic electric and electronic components.

Basic Circuit Problems Almost all electrical and electronic problems caused by something in circuit increasing or decreasing current, resistance, or voltage

Electrical Diagnosis and Repair Using logical sequence to find problem source

Systematic Approach Verify symptoms Narrow down cause Perform proper tests Make and verify repair

Verify the Symptoms What could be causing the specific symptoms? How many components are affected? Is problem always present, or does it only occur some of the time? Is problem affected by heat or cold? Is problem affected by moisture?

Narrow down the Cause Use symptoms to visualize what is happening inside circuit Use knowledge of specific system and basic rules of electricity If unfamiliar with circuit, use diagram

Perform the Proper Tests If only one component does not work, start tests at that component If more than one component does not work, start tests at circuit’s power source Refer to service manual troubleshooting charts

Make and Verify the Repair After finding problem source, make repair Replace bad component Solder broken wire Repair faulty connector Make sure circuit operates normally If needed, test-drive to make sure shock and vibration do not affect repair

Types of Circuit Problems Four basic types of circuit problems Open circuit High resistance Short circuit Intermittent trouble

Open Circuit Broken wire conductor Blown fuse Burned switch contacts Loose connection Burned or corroded connector terminals Failed electric component

High Circuit Resistance Reduces current but does not stop all current Usually caused by poor electrical connection

Short Circuit Caused by conductor accidentally touching ground or another conductor Hot wire short to ground produces low resistance and extremely high current Usually blows fuse or circuit breaker

Intermittent Problems Occurs only under certain conditions Many intermittent problems caused by poor electrical connections

Using Testing Devices Jumper wire Test light Piece of wire with alligator clips on both ends Bypasses or applies voltage to component or circuit section Test light Quick and easy way of checking circuit for power or continuity

Using Jumper Wires (Snap-on Tool Corp.)

Using a Test Light (Snap-on Tool Corp.)

Inductive Wire Tracer Uses signal generator and inductive pickup to find shorts Signal generator connected to battery Produces weak, fluctuating voltage in wiring When pickup moves over short, audible tone changes

Using an Inductive Wire Tracer

Using a Multimeter Multimeter Also called a VOM Voltmeter, ammeter, and ohmmeter combined in one housing Also called a VOM Volt-ohm-milliammeter Essential tool for automotive technicians

Meter Connections Voltmeter Ohmmeter Measures electrical pressure, or voltage Connected in parallel with circuit being tested Ohmmeter Measures resistance to current in ohms Power must be disconnected Connects to each end of wire or component to check its internal resistance

Meter Connections (Cont.) Ammeter Measures current in amps Must be connected in series with circuit being tested Inductive pickup Meter attachment for analyzing charging systems and hybrid power system operation

Meter Controls Most multimeters have large function knob Rotate select type of electrical reading to be taken Meter control buttons Select other function variables Auto ranging Automatically change to correct scale

Meter Controls (Cont.) Large function knob Meter control Zero meter Minimum/maximum Range Hold Duty cycle Sound alert

Using a Multimeter (Cont.)

Voltage Measurements Help determine condition of circuit or components Voltage drop measurement Determines internal resistance of wire or component

Resistance Measurements Taken when the circuit or component is disconnected Infinite resistance Shown with an ∞ (infinity) symbol Means the circuit path is not complete Zero resistance or no resistance Means perfect electrical path exists between two test points

Amperage Measurements Check condition of various components High current draw Might point to problem producing drag on motor Bad armature bearings or short in winding Low current measurement Might point to problem preventing normal current Poor terminal connections Bad ground

Rules for Using Multimeters Make sure test leads are connected properly Set meter to read high enough value Use digital or digital-analog meter to check computer circuits Place meter in secure location Keep test leads away from hot or moving parts

Rules for Using Multimeters (Cont.) Use specifications in service manual If needed, zero or calibrate ohmmeter before use To find intermittent problems Wiggle wires Use heat gun or freeze spray on electronic components

Testing Connectors Test light Voltmeter Good connector has little or no resistance across terminals Check for power on both sides Check for voltage drop across

Testing Resistors Resistors can break down and change resistance Ohmmeter can test actual resistance Voltmeter can check voltage drop If not within specifications, replace it

Testing Variable Resistors A good variable resistor should exhibit a gradual change in resistance Might be open or shorted May be stuck at a specific resistance value May not vary properly Test with an ohmmeter (Union Electronics)

Testing Switches Good switches block current in one position and pass it in another Bad switches may not turn on and off, or have high resistance when on Test switches with test light, voltmeter, or ohmmeter

Testing a Transducer Switch Internal resistance changes with conditions Temperature, position, pressure Failed transducer switch stays in same position regardless Use ohmmeter to check disconnected transducer switch Use jumper to bypass switch and check circuit operation

Testing Relays Relay problems result from worn, burned, or sticking contact points Ensure voltage is at relay windings Using test light or voltmeter If power is present, check that relay points are closing Check voltage leaving closed relay points If not, replace relay

Testing Fuses Check fuses with test light Light glows when connected to either side of fuse If light glows only on feed side, fuse is blown

Testing Capacitors Always ground capacitor to discharge stored electricity Use ohmmeter or capacitor tester To use capacitor tester, follow manufacturer’s directions To use ohmmeter Disconnect and ground capacitor’s lead Connect ohmmeter leads across capacitor

Testing Capacitors (Cont.) Ohmmeter shows low resistance at first as capacitor takes charge from it As capacitor charges, ohmmeter reading increases to infinite resistance This shows that it is blocking dc current but storing voltage

Testing Capacitors (Cont.)

Testing Diodes Use an ohmmeter Bad diode may have an internal Open PN junction burned and separated Shorted PN junction burned and fused Should have infinite resistance in one direction and low resistance in other If it does not, replace diode

Testing Other Components

Electrical Service Chemicals Electrical solvent Cleans oil and grease from components before soldering Contact cleaner Cleans deposits from contacts, switches, and other components Freeze spray Finds intermittent problems in electronic components