ase 6 - electrical electronic systemsfaculty.ccbcmd.edu/.../books/a6studentworkbooks131/... ·...

ASE 6 - Electrical ElectronicSystems

Module 8Circuit Faults

AcknowledgementsGeneral Motors, the IAGMASEP Association Board of Directors, and RaytheonProfessional Services, GM's training partner for GM's Service Technical College wish tothank all of the people who contributed to the GM ASEP/BSEP curriculum developmentproject 2002-3. This project would not have been possible without the tireless efforts ofmany people. We acknowledge:

• The IAGMASEP Association members for agreeing to tackle this large project tocreate the curriculum for the GM ASEP/BSEP schools.

• The IAGMASEP Curriculum team for leading the members to a single vision andimplementation.

• Direct contributors within Raytheon Professional Services for their support oftranslating a good idea into reality. Specifically, we thank:

– Chris Mason and Vince Williams, for their leadership, guidance, and support.– Media and Graphics department under Mary McClain and in particular, Cheryl

Squicciarini, Diana Pajewski, Lesley McCowey, Jeremy Pawelek, & NancyDeSantis.

– For his help on the Electrical curriculum volume, Subject Matter Expert, KenBeish, Jr., for his wealth of knowledge.

Finally, we wish to recognize the individual instructors and staffs of the GM ASEP/BSEPColleges for their contribution for reformatting existing General Motors training material,adding critical technical content and the sharing of their expertise in the GM product.Separate committees worked on each of the eight curriculum areas. For the work on thisvolume, we thank the members of the Electrical committee:

– Jack Davis, Community College of Baltimore County - Catonsville– Jim Halderman, Sinclair Community College– Megan Kuehm, Community College of Allegheny County– Frank Longbottom, Camden County College– Jeff Rehkopf, Florida Community College at Jacksonville– Randy Peters, Des Moines Area Community College– David Rodriguez, College of Southern Idaho– Ed Schauffler, Longview Community College– Vince Williams, Raytheon

ContentsModule 8 – Circuit FaultsObjective .......................................................................................................... 4

Short to Ground .............................................................................................................. 4Short to Power ................................................................................................................ 6Open ............................................................................................................................... 7Open in a Series Circuit .................................................................................................. 8Exercise 8-1 .................................................................................................................... 9Exercise 8-2 ...................................................................................................................11Exercise 8-3 .................................................................................................................. 13

© 2002 General Motors CorporationAll Rights Reserved

ASE 6 - ElectricalElectronic Systems

Module 8 - CircuitFaults

8-4

Student WorkbookObjectiveAt the end of this section, the technician will be able to describe thedifferent types of circuit faults and their effects on automotive electricalcircuits.

Circuit MalfunctionsThere are several ways that a circuit can malfunction. Following Is a list ofcircuit malfunctions:

Short to groundShort to powerOpenToo much resistanceToo little resistance

Short to GroundA short to ground occurs when current flow is grounded before it wasintended to be. This usually happens when wire insulation breaks and thewire touches a ground. The effect of a short to ground depends on thedesign of the circuit and on its location in relationship to the circuit controland load.

Figure 8-1, Short to Ground Before the Load




8-5

Student Workbook

In this illustration, the short is located between the switch and the load.The resistance is lower than it should be because the current is notpassing through the loads. The fuse blows only after the switch is closed.Lower resistance means the current flow is higher than normal. The fuseor other circuit protector will open. An automatically resetting circuitbreaker will repeatedly open and close. If there is no circuit protector at all,the wire can get hot enough to burn.




8-6

Student Workbook

Figure 8-2, Short to Ground Before the Switch

This illustration shows an example where a short to ground has occurredafter the protection device, but before the load and the control. As a result,the current flow is not being controlled and because there is very littleresistance, the protection device will open the circuitThis is sometimes referred to as a “dead short to ground. If the protectiondevice is a fuse, it will blow open. Replacing the fuse without repairing theshort will cause the replacement fuse to also blow open upon installation.If a short to ground occurs close to the intended ground connection, youprobably will not notice any effects.

Short to PowerA short to power occurs when one circuit is shorted to the voltage ofanother circuit. Such a short can also occur between two separatebranches of the same circuit. The cause is usually broken or damagedwire insulation.The symptoms of a short to voltage depend on the location of the short inboth circuits. One or both circuits may operate strangely. For example, inthe illustration below (View A), the short Is before the switches on bothcircuits. This means both switches control both loads.




8-7

Student Workbook

Figure 8-3, Short to Voltage

A different problem occurs if the short is after the load on one branch andbefore the load on the other (View B). The load In the second branch willoperate normally. The load in the first branch will not come on at all, andthe current flow will be so high that the fuse will blow (direct short toground). If there is no circuit protector, the wire could get so hot that itactually catches on fire.

OpenAn open is a break in the current flow of what should be a closed circuit.Current cannot flow because there is no completed path.Opens can occur anywhere in the circuit and are usually the result ofloose, dirty or corroded connections; a broken wire or a failed component.Keep in mind that a blown fuse, fusible link or tripped circuit breaker canalso cause an open as a result of an-other circuit fault - a short to ground.A poor electrical connection is the most common cause of an open.




8-8

Student WorkbookOpen in a Series CircuitThere is only on path for current flow in a series circuit if that path is open,there is no current flow and the circuit loads cannot work.If there is an open in a series circuit, the voltage drop across the load inthat circuit will be zero volts. At some point in that circuit you are able tomeasure applied voltage. If you measure the voltage across the openends of the circuit (on either side of the break), you will measure theapplied voltage. If you measure available voltage, from the power sourceto the component, you can find where the open exists.There is no continuity (infinite resistance) between the source and ground.You measure zero current flow at any point on the circuit.

Figure 8-4, Open in a Series Circuit




8-9

Student WorkbookExercise 8-1Read each question carefully and anew by filling in the blanks.Turn to the schematic for HVAC: Blower Controls, Manual C6O In theservice manual specified by the Instructor, and review the followingconditions.

Condition AYou suspect a problem in the blower resistor assembly and want to checkthe resistance across its various terminals.

1. Which DVOM Input terminal do you use? _________________

2. To which position do you turn the rotary switch? _________________

3. If you measure infinite resistance across terminals C and B of theblower resistor assembly, is it possible for the blower motor to workcorrectly in positions LOW and M1? Answer Yes or No______

Condition BMeasure voltage at Terminal 30 of the blower relay and ground, whilemoving the blower switch through the various positions. The modeselector is turned to MAX and the engine is running.

4. Which DVOM Input terminal do you use? ______________

5. To which position do you turn the rotary switch? ___________




8-10

Student WorkbookOpen in a Parallel CircuitThe effect of an open on a parallel circuit depends on where in the circuitthe open Is located, and on the design of the circuit.

Figure 8-5, Open in a Parallel Circuit

If an open occurs in the main line, none of the loads on the circuit canwork. In effect, all of the branches are open. In this example, an open inthe ground path will have the same effect as an open in the mainline.If an open occurs on a branch below the main line, only the load on thatbranch is affected. All of the other branches still form closed circuits andstill operate.




8-11

Student WorkbookExercise 8-2Use the exterior-lights schematics in the service manual assigned by yourInstructor to answer these questions. Assume that the:Ignition is ONTurn signal switch is in LEFTHeadlight switch is OFF

1. Which loads will operate if the circuit is operating property?

_________________________________________________________

2. Which loads will operate if there is an open circuit between the turnflasher and the turn/hazard-headlight switch assembly In Circuit 16?

_________________________________________________________

3. Which loads will operate if there is an open circuit between the turn/hazard-headlight switch assembly and the splice at P101?

_________________________________________________________

Which loads will not operate?

_________________________________________________________

4. Which loads will operate if there is an open In Circuit 18 between theturn/hazard-headlight switch assembly and the first connector?

_________________________________________________________

Which loads will not operate?

_________________________________________________________




8-12

Student WorkbookToo Much ResistanceIntermittent concerns or components that operate abnormally are oftencaused by high resistance connections.A build up of dirt or corrosion can increase the resistance of n electricalconnection. These higher resistance results in lower current and maycause intermittent circuit behavior. For example, a bulb may flicker or lightdimly.Voltage drop tests are often used to locate high resistance faults onoperating circuits.




8-13

Student WorkbookExercise 8-3Read each question carefully and answer by filling in the blanks.

1. Name the three basic circuit faults: ___________, _____________and_______

2. A short to ground usually happens when ______.

a. one circuit Is shorted to the voltage of another circuitb. there is a break in the circuitc. wire Insulation breaks and the wire touches a groundd. a connector becomes corroded

3. When a circuit shorts to grounda. circuit resistance goes up and causes the fuse to blowb. current flow goes downc. current flow goes up because of less circuit resistanced. current flow goes up because of more resistance In the circuit

4. The type of circuit fault that causes current flow to stop is a (an)

_________________________________________________________

5. An open in the main line of a parallel circuit will not affect the individualloads or branches.a. trueb. false




8-14

Student Workbook6. An open in a series circuit will cause the current flow to stop.a. trueb. false

7. A component is working abnormally. For example, a headlamp is dim.This is usually caused by what type of circuit fault?a. openb. low resistancec. short to voltaged. high resistance

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