2. 22
Chapter
Basic Electrical Tests

3. 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.

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

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

6. Electrical Diagnosis and Repair
Using logical sequence to find problem source

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

8. 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?

9. 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

10. 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

11. 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

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

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

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

15. 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

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

17. 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

18. Using Jumper Wires
(Snap-on Tool Corp.)

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

20. 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

21. Using an Inductive Wire Tracer

22. 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

23. 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

24. 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

25. 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

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

27. Using a Multimeter (Cont.)

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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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)

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. Testing Capacitors (Cont.)

43. 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

44. Testing Other Components

45. 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