General Electrical/Electronic System Diagnosis

Question 1

How are voltage tests measured?

Answer 1

With the test leads of the DMM in parallel to the component or circuit being tested.
~ In testing applied voltage, the negative or black test lead of the DMM is connected to a battery or chassis ground.
~The positive red lead is then probed near the power source (e.g. a switch or fuse) to determine if the circuit is receiving the proper voltage.
~A handy feature of a DMM is that polarity is not important.
~If the leads are reversed, the display will simply include a minus sign in front of the reading.
~Circuit voltages are tested much the same way.
~Take the positive test probe and go from one circuit’s power source to the next.
~Generally, the applied voltages should all be the same.

Question 2

What is recommended when selecting a DMM to use for testing electronics circuits?

Answer 2

It is recommended that it have a 10 megohm or higher impedance.

• This is necessary to limit the effect the unit might have on accuracy when testing low current flow circuits.
• The lamp in the test light will draw too much current and this can damage the circuit integrity.

Question 3

What should be done if it is necessary to test for voltage in an energized circuit that is in operation?

Answer 3

A convenient way to do this is with a black-probe tool, also know as a “spoon,” at various connectors in a particular circuit.
-This DMM accessory makes it possible to probe into the connector from behind without disconnecting it.

Question 4

Under what conditions should you check current flow in electrical/electronic circuits etc?

Answer 4

Current flow tests are used when a circuit is suspected of having higher than normal current flow, such as a dragging blower motor, or a circuit with a low-resistance short to ground.
-A technician that has a good knowledge of what normal amperage should be can use this knowledge when troubleshooting electrical circuits.

Question 5

In testing current flow with an ammeter or DMM, what is important to remember?

Answer 5

It is important to remember that the test leads are connected in series with the circuit being tested, usually at a point near the power source.
-The circuit must be interrupted at some point to allow the connection of the test leads.

Question 6

When do most DMMs have a 10-20 amp limit?

Answer 6

When measuring amperage directly through the meter.

• Any greater current flow will blow its fuse.
• If you suspect that the circuit carries more than that, then a safe way to test for current flow would be to use a current clamp.
• This device simply clamps over the wire being tested and determines current flow by measuring the strength of the magnetic field surrounding the wire.
• While it is extremely handy to use, it is not as accurate as routing all the current through the DMM, especially in circuits flowing less than 10 amps.

Question 7

What is low current flow usually a result of?

Answer 7

Of excessive resistance in a circuit or low voltage.

- Higher than normal current flow can generally be traced to excessive applied voltage or a shorted component or wire.

Question 8

How would you check continuity in an electrical circuit?

Answer 8

This is one of the most common troubleshooting tests a technician performs.

• Auto-ranging DMMS do not have to be adjusted to the range that is being measured.
• While the purpose of this test is to make sure that a complete current path exists in the circuit being tested, it is important to remember that it is not an accurate indication of circuit performance (e.g., excessive resistance).
• A continuity test is most useful to quickly differentiate one circuit from another, such as trying to locate a specific contact in a multiple pin connector.

Question 9

What are the different ways continuity tests can be made?

Answer 9

• If the circuit is energized, a 12/24 volt test light or voltmeter can be used to check for voltage at various test points.
• Both tools accomplish this by energizing the circuit with low potential current to determine that the circuit can be closed.
• Many MMS have a separate feature on them that will allow continuity tests to be made simply by listening for an audible beep.
• This is handy because multiple tests can be made rather quickly, without having to look constantly at the display for a resistance value.

Question 10

What is essential when testing circuits that include electronic control modules (ECM)?

Answer 10

It is essential to observe the original equipment manufacturer’s (OEM’s) diagnostic procedure to avoid possible damage to the processor.

Question 11

When are resistance checks typically made

Answer 11

When a circuit has unwanted voltage drops or low current flow.

Question 12

What is an ohmmeter?

Answer 12

A device that circulates a small current through a circuit when it is not energized and then measures the voltage drop through it.
- It displays the resistance (or restriction or resistance to current flow) in units know as shms.

Question 13

What is the correlation between resistance value and electron flow?

Answer 13

The lower the resistance value, the less restriction to electron flow there will be a circuit.

Question 14

What does it mean if the meter reads infinity (or high flashing number on most DMMs?

Answer 14

This means that the circuit is open.

Question 15

Except for ___________, when does the lower resistance a circuit has, the better? What is an example?

Answer 15

Except for where resistance is built in to a circuit, such as a blower motor resistor or a load itself.
- For example, when testing a length of wire or a fuse, most will test very near zero ohms.

Question 16

When testing a DMM. what should you always be sure to do?

Answer 16

Always be sure to zero the meter first to compensate for any resistance present in the test leads, especially when testing low-resistance present in the test leads, especially when testing low resistance components.

Question 17

What should you do when making tests with a DMM that is not auto-arranging?

Answer 17

Be sure to select a range that will provide accuracy.
- If you are using an analog meter, set the meter to a range setting that will put the needle in roughly the middle of the scale for the component you are testing.

Question 18

Define a short circuit.

Answer 18

A short circuit is defined as one where the current flow is allowed to ground at a point other than where it was designed, such as a bare wire rubbing against the frame.

Question 19

What causes an open circuit?

Answer 19

Is usually caused by a broken wire or other component not making the necessary connection to complete a circuit, stopping current flow in that portion of the circuit.

Question 20

What is the best way to find shorts?

Answer 20

Finding shorts is best done with an ohmmeter, because in a live circuit, a fuse or breaker will continually blow.

• Do not install a larger fuse; it will risk melting a bundle of wires.
• Divide the circuit into small sections at various connectors (where applicable) while testing for continuity to ground (there should be none) or visually inspect the harness for rub or pinch points.

Question 21

How would you locate opens and why is it more difficult?

Answer 21

Locating opens can be more difficult because sometimes the damage is not visually apparent.

• A good way to test for an open would be to apply voltage at one end and then probe at each succeeding connection downstream until you find no reading.
• Alternatively, an ohmmeter can be used to do the same thing when the circuit is not energized.

Question 22

What is important to remember when testing at connectors?

Answer 22

When testing connectors, especially the smaller ones designed for electronic circuits, do not damage the contacts when probing into them, especially female ones.

• Spreading the tongs on a female contact while testing can create problems.
• Always use an appropriate adapter for testing these types of contacts.

Question 23

While dead batteries are not always caused by a key-off battery drain problem, what is a good idea before you start troubleshooting?

Answer 23

Know the possible causes for this problem and how the system is constructed.

Question 24

What did many systems have before the widespread use of electronics in vehicles?

Answer 24

Many systems had zero current draw with the key off.

Question 25

With so many vehicles having multiple control modules, what will testing for battery drain likely involve?

Answer 25

Testing for battery drain will likely involve disconnecting ECMs or isolating circuits.

Question 26

How would you diagnose key-off battery drain problems in older vehicles?

Answer 26

Disconnect the negative battery cable and connect an ammeter in series with it and the ground ppost.

• Make sure the key is off and all loads, such as dome lights, are turned off. - There should be no current draw.
• If there is still a current draw, an easy way to isolate the problem is to start pulling fuses one at a time until the draw stops.
• Another possible draw is through a defective diode in the alternator.
• Disconnect the positive lead at the alternator to locate this potential problem.

Question 27

How would you diagnose key-off battery drain problems on vehicles with ECMs?

Answer 27

Use a milliamp scale on the DMM and consult the manufacturer’s specifications to ensure that a draw is within parameters: Most will draw well under 50 milliamps with the key off.

• If it measures higher than specified, it might be necessary to disconnect the ECM’s power supply to be sure that there is not another component in the vehicle causing the additional draw.
• Be sure to allow time for the ECM to power down and enter “sleep mode” before taking a final reading.

Question 28

What is a fuse and what happens when it blows?

Answer 28

A fuse is an electrical safety device. Then it blows, it is because of a current overload somewhere in the circuit.

Question 29

What do you do when a fuse blows?

Answer 29

Always repair the problem: Never install a fuse of a higher rating.
Learn to identify the reason for a fuse failure.
- If the metal filament in the center of the fuse melts, it is caused by a current overload.
- On an older glass-style fuses, melted end caps are caused by poor or corroded contacts in the fuse holder itself, not a current overload.

Question 30

What is the difference between a circuit breaker and a fuse?

Answer 30

A circuit breaker performs the same function as a fuse; however, it has a feature that allows it to be reset after tripping, usually automatically.

• Most circuit breakers can be identified as a small rectangular box with two studs attached to it.
• Their maximum current rating is stamped on the housing.

Question 31

What are the two types of circuit breakers used in truck electrical circuits?

Answer 31

SAE #1 circuit breakers cycle when overloaded.
SAE #2 circuit breakers trip when overloaded and do not reset until the circuit is opened or the reset button is depressed.

Question 32

What should you do when testing a circuit that continually blows conventional fuses?

Answer 32

Install a circuit breaker into the circuit temporarily with jumper wires saves both time and money.

Question 33

What are fusible links?

Answer 33

Fusible links are short sections of wire designed to melt and open a circuit in case of overload.
_ They are usually installed near a power source (e.g. battery or starter solenoid) and are normally two to four wire gauge sizes smaller in diameter than the circuit they are protecting.

Question 34

What happens when fusible links melt?

Answer 34

The insulation usually bubbles, but not always, making them difficult to troubleshoot.
- The fuse link has a special high-temperature insulation designed not to separate during an overload.

Question 35

What is the easiest way to test fusible links?

Answer 35

Is to simply give them a good tug at either end.

- if it stretches, it is defective.

Question 36

What do manufactures now using instead of fusible links?

Answer 36

Many manufacturers are now using what are known as maxi-fuses in place of fusible links. which are also usually located near the battery or main power distribution bus.

• They simply look like conventional fuses, although much larger than standard.
• They also are much easier to install and troubleshoot.

Question 37

How should you test fusible links, circuit breakers, fuses, and other circuit protection devices?

Answer 37

Remove the component and test for resistance using an ohmmeter. A good component should show very low.

Question 38

What is a diode?

Answer 38

A diode is simply an electrical “check valve” that allows current to flow in only one direction.
_A symbol of a diode looks like an arrow with a line drawn perpendicular to its point.

Question 39

What is a typical use of a diode?

Answer 39

A typical use of a diode is in an alternator, where it performs the task of converting AC voltage into DC by simply preventing output to the battery during the negative cycle of the sine valve.
- Diodes can also be used to prevent backfeeding of current from alternator excitation circuit to key switch circuits once the engine starts.

Question 40

How would you test diodes?

Answer 40

Use an ohmmeter and check for continuity through both directions. If it is good, current will flow in one direction, but not the other.

Question 41

What are spike suppression devices and what are their purpose?

Answer 41

Spike suppression devices can be in the form of diodes, resistors, or capacitors.

• Their purpose is to absorb or redirect a voltage spike that might come from a collapsing magnetic field, such as when the air conditioner (A?C) compressor clutch coil is switched off.
• By installing such a device in parallel with the coil, the voltage spike is directed back to the clutch coil and prevented from damaging sensitive components, such as ECMs.

Question 42

Define a relay.

Answer 42

A relay is defined as a switching device that uses a small amount of current to control a larger one.

Question 43

What is a solenoid?

Question 44

What is a good example of a solenoid incorporating a relay function?

Answer 44

A solenoid can also incorporate a relay function. A good example of this would be a starter solenoid, which not only moves the starter drive pinion into mesh with the flywheel ring gear, but also makes the high-current connection between the battery and the starter field coils.

Question 45

How many terminals do most mini-relays have?

Answer 45

Most mini-relays have four or five terminals.

• Two small terminals (often specified as #85 and #86) are used to energize the coil that creates the magnetic attraction necessary to cause a connection between the high amperage switch contracts.
• Two other terminals make the high amperage power in (terminal #30) and power out (terminal #87) connection.
• Sometimes a fifth terminal (marked #87a) is used a normally closed (NC) contact, as opposed to #87 being normally open (NO), which closes when the relay energizes.
• The physical size of the power terminals (#30, #87, and #87a) may or may not be larger than the control terminals (#85 and #86), depending on the amperage capacity of the relay.

Question 46

What happens when a relay is signaled to close the high-current contacts?

Answer 46

A small amount of current is fed through terminals #86 and #85, one being battery positive and the other battery ground.

• This signal can come from an ECM, key switch or other low-current switching device.
• The positive side of the high-current contact (#30) is then connected to load side (#87), completing the circuit to the high-current device, such as a horn or multiple light circuits.
• Some relays do not have the same code numbers, but function using the same principles.

Question 47

Some manufacturers use solid-state relays in different areas of the truck. What is the operation of these devices?

Answer 47

The operation of these devices is much like an electronic fuel injector.

• These components can be totally off or totally on, like regular relays/
• What differentiates them is their ability to be turned on to a certain percentage with a pulse-width modulated signal.

Question 48

What is the advantage of the use of solid-state relays?

Answer 48

They can control the current flow to the output device, resulting in a variable speed device.

Question 49

What tool is necessary to troubleshoot solid-state relays?

Answer 49

A scan tool is necessary.
- The scan tool can retrieve trouble codes as well as perform output tests on these circuits during the troubleshooting process.

Question 50

Valley Forge schematic symbols were once considered standard in the United States and Canada, but due to the widespread ownership of out trucking OEMs by European companies, some manufacturers now use European schematic symbols and architecture.

Answer 50

Study the symbols on page 17. Pay special attention to the symbols used for grounds, connections, fuses, diodes, switches, relays, and twisted pairs (data bus).

Question 51

What will all trucks that have one or more on-board computers have and what does it do?

Answer 51

Will also have a network that allows the computers to communicate with each other.
- This network allows for scan tool communications.

Question 52

What does the network that allows truck on-board computers consist of?

Answer 52

Typically this network consists of two wires that are connected to the various modules and the data connector in parallel.

Question 53

What is bussing

Answer 53

The process of pulsing voltage to send signals is called multiplexing or bussing.

Question 54

Why are the bus network wires continuously twisted?

Answer 54

To help resist radio frequency interference (RFI) and electromagnetic interference (EMI) from entering this system.
- This method of sharing data among modules helps eliminate redundant wiring to the modules that need the same sensor information.