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Chapter 47

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  • 1. start
  • 2.
    • Prepare for ASE Electrical/Electronic Systems (A6) certification test content area “H” (Accessories Diagnosis and Repair).
    • Describe how AM and FM radio works.
    • Explain how to test speaker polarity.
    • Explain how to match speaker impedance.
    OBJECTIVES: After studying Chapter 47, the reader should be able to: Continued
  • 3.
    • Explain how crossovers work.
    • List causes and corrections of radio noise and interference.
    OBJECTIVES: After studying Chapter 47, the reader should be able to:
  • 4.
    • active crossover • alternator whine • amplitude modulation (AM)
    • bluetooth carrier wave • crossover
    • decibels (dB) electronically tuned receiver (ETR)
    • floating ground system • flutter • frequency • frequency modulation (FM)
    KEY TERMS: Continued
  • 5.
    • ground plane hertz • high-pass filter • impedance low-pass filter modulation
    • picket fencing • powerline capacitor radio choke • radio frequency (RF) • root-mean-square (RMS)
    KEY TERMS: Continued
  • 6.
    • satellite digital audio radio services (SDARS) • sideband operation • speakers • stiffening capacitor • subwoofer
    • total harmonic distortion (THD) • tweeter
    • voice recognition wavelength
    KEY TERMS:
  • 7.
    • The audio system of today’s vehicles is a complex combination of antenna system, receiver, amplifier, and speakers all designed to provide living room-type music reproduction while the vehicle is traveling in city traffic or at highway speed.
  • 8. AUDIO FUNDAMENTALS
    • There are two types of energy that affect audio systems.
    Continued
    • Electromagnetic energy or radio waves Antennas capture the radio waves which are then sent to the radio or receiver to be amplified.
    • Acoustical energy, usually called sound Radios and receivers amplify the radio wave signals and drive speakers which reproduce the original sound that was transmitted by radio waves.
    See Figure 47-1.
  • 9. Figure 47–1 Audio systems use both electromagnetic radio waves and sound waves to reproduce sound inside the vehicle. Continued
  • 10.
    • Electromagnetic energy moves through the air while acoustical energy moves air. Radio waves are measured in two ways: wavelength and frequency. A radio wave has a series of high points and low points. A wavelength is the time and distance between two consecutive points, either high or low, measured in meters. Frequency , also known as radio frequency , or RF , is the number of times a particular waveform repeats itself in a given amount of time and is measured in hertz . A signal with a frequency of one hertz is one radio wavelength per second. Radio frequencies are measured in kilohertz, thousands of wavelengths per second, and megahertz, millions of wavelengths per second. See Figure 47–2 and examples following.
    Continued
  • 11. Figure 47–2 The relationship among wavelength, frequency, and amplitude.
    • The higher the frequency, the shorter the wavelength.
    • The lower the frequency, the longer the wavelength.
    • AM radio frequencies range from 530, to 1,710 kilohertz.
    • FM radio frequencies range from 87.9 to 107.9 MHz.
    Longer wavelengths travel farther than shorter ones. Continued Lower frequencies provide better reception at longer distances.
  • 12.
    • Modulation The term used to describe when information is added to a constant frequency is modulation . The base radio frequency used for RF is called the carrier wave . A carrier is a radio wave that is changed to carry information. The two types of modulation are:
    Continued
    • Amplitude Modulation ( AM )
    • Frequency Modulation ( FM )
  • 13. Figure 47–3 The amplitude changes in AM broadcasting.
    • AM waves are radio waves that have amplitude that can be varied, transmitted, and detected by a receiver. Amplitude is the height of the wave as graphed on an oscilloscope.
    Continued
  • 14. Figure 47–4 The frequency changes in FM broadcasting and the amplitude remains constant.
    • FM waves are radio waves that have a frequency that can be varied, transmitted, and detected by a receiver. This type of modulation changes the number of cycles per second, or frequency, to carry the information.
    Continued
  • 15.
    • Radio Wave Transmission More than one signal can be carried by a radio wave. This process is called sideband operation . Sideband frequencies are measured in kilohertz. The amount of the signal above the assigned frequency is referred to as the upper sideband. The amount of the signal below the assigned frequency is called the lower sideband. This capability allows radio signals to carry stereo broadcasts. Stereo broadcasts use the upper sideband to carry one channel of the stereo signal, and the lower to carry the other channel. When the signal is decoded by the radio, these two signals become the right and left channels. See Figure 47–5.
    Continued
  • 16. Figure 47–5 Using upper and lower sidebands allows stereo to be broadcast. The receiver separates the signals to provide left and right channels. Continued
  • 17.
    • Noise Because radio waves are a form of electromagnetic energy, other forms of energy can impact them. FM Characteristics Because FM waves have a high RF and a short wavelength, they travel only a short distance. The waves cannot follow the shape of the earth but travel in a straight line from transmitter to receiver. FM waves travel the ionosphere into space and don’t reflect back to earth like AM waves. This causes an echo effect in the speakers. Flutter , or picket fencing as it is sometimes called, is caused by the blocking of part of the FM signal. This blocking causes a weakening of the signal resulting in only part of the signal getting to the antenna causing an on-again off-again radio sound. Flutter also occurs when the transmitter and the receiving antenna are far apart.
    Continued
  • 18. RADIOS AND RECEIVERS
    • The antenna receives the radio wave where it is converted into very weak, fluctuating electrical current. This current travels along the antenna lead-in to the radio, which amplifies the signal and sends the new signal to the speakers where it is converted into acoustical energy. Most late-model GM radios and receivers use five inputs:
    Continued
    • Power—constant 12-volt feed to keep the clock alive
    • Ground
    • Serial data—used to turn the unit on and off and provide other functions such as steering wheel control operation
    • Antenna input
    • Speaker outputs
  • 19.
    • Types of Antennas The typical radio electromagnetic energy from the broadcast antenna induces a signal in the antenna that is very small—only about 25 micro volts AC (0.000025 VAC) in strength.
    Continued
    • Slot antenna —The slot antenna is concealed in the roof of some plastic-body vehicles such as the U van. This antenna is surrounded by metal on a Mylar sheet.
    • Rear-window defogger grid antenna —This type of system uses the heating wires to receive the signals and special circuitry to separate the RF from the DC heater circuit.
    • Powered mast antenna —Powered mast antennas are controlled by the radio. When the radio is turned on, the antenna is raised, when the radio is shut off, the antenna is retracted. The antenna system consists of an antenna mast and a drive motor.
  • 20.
    • Fixed-mast antenna —The fixed-mast antenna offers the best overall performance currently available. The mast is simply a vertical rod. Mast antennas are typically located on the fender or rear quarter panel of the vehicle
    • Integrated antenna —This type of antenna is sandwiched in the windshield and an appliqué on the rear window glass. The antenna in the rear window is the primary antenna and receives both AM and FM signals. The secondary antenna is located in the front windshield typically on the passenger side of the vehicle. This antenna receives only FM signals.
    Continued See Figure 47-6.
  • 21. Figure 47–6 The five types of antennas used on General Motors vehicles include the slot antenna, fixed-mast antenna, rear-window defogger grid antenna, a powered mast antenna, and an integrated antenna. Continued
  • 22.
    • Antennas designed to pick up electromagnetic energy broadcast through the air to the transmitting antenna are usually one-half wavelength high, and the other half of the wavelength is the ground plane. This one-half wavelength in the ground plane is literally underground.
    What is a Ground Plane? For ideal reception, the receiving antenna should be the same wavelength as the signal. Because this is not practical, a design compromise uses the length of the antenna as one-fourth of the wavelength and the body of the vehicle as one-fourth of the wavelength. The body of the vehicle becomes the ground plane. Any faulty condition in the ground plane circuit will cause the ground plane to lose effectiveness, such as:
    • Loose or corroded battery cable terminals
    • Acid buildup on battery cables
    • Engine grounds with high resistance
    • Loss of antenna or audio system grounds
    • A defective generator causes an AC ripple exceeding 50 millivolts
  • 23. Figure 47–7 The ground plane is actually one-half of the antenna.
  • 24. ANTENNA DIAGNOSIS
    • The antenna collects all radio-frequency signals. AM radio operates best with as long an antenna as possible. FM reception is best when the antenna height is exactly 31 inches (79 cm). Most fixed-length antennas are exactly this height. The horizontal section of a windshield antenna is 31 inches (79 cm) long. A defective antenna will be most noticeable with AM radio reception.
    Continued
  • 25.
    • Antenna Testing If the antenna or lead-in cable is broken (open), FM reception will be heard but may be weak, and there will be no AM reception. An ohmmeter should read infinity between the center antenna lead and the antenna case.
    Figure 47–8 If all ohmmeter readings are satisfactory, the antenna is good. Continued For proper reception and lack of noise, the case of the antenna needs to be properly grounded to the vehicle body.
  • 26.
    • A common repair is to replace the mast of a power antenna. To help prevent the possibility of causing damage to the body or paint, cut a hole in a fender cover and place it over the antenna. If a wrench or tool slips during removal or installation, the body of the vehicle will be protected.
    The Hole in the Fender Cover Trick Figure 47–9 Cutting a small hole in a fender cover helps to protect the vehicle when replacing or servicing an antenna.
  • 27.
    • Power Antenna Testing and Service Most power antennas use a circuit breaker and a relay to power a reversible, permanent-magnet (PM) electric motor that moves a nylon cord attached to the antenna mast. Some vehicles have dash-mounted controls that regulate antenna mast height and/or operation, whereas many operate automatically when the radio is turned on and off. The power antenna assembly is usually mounted between the outer and inner front fender or in the rear quarter panel. The unit contains the motor, a spool for the cord, and upper- and lower-limit switches. The power antenna mast is tested in the same way as a fixed-mast antenna.
    Continued
  • 28.
    • Except in the case of cleaning or mast replacement, most power antennas are either replaced as a unit or repaired by specialty shops.
    Figure 47–10 A typical power antenna assembly. Note the braided ground wire used to make sure that the antenna has a good ground plane. All power antennas should be kept clean by wiping the mast with a soft cloth and lightly oiling with a light oil. Many problems can be prevented by making certain drain holes in the motor housing are not plugged with undercoating, leaves, or dirt.
  • 29. SPEAKERS
    • The purpose of a speaker is to reproduce original sound. Speakers are also called loudspeakers . The human ear is capable of hearing sounds from a very low frequency of 20 Hz (cycles per seconds) to as high as 20,000 Hz. No one speaker is capable of reproducing sound over such a wide frequency range. See Figure 47–11. Good-quality speakers are the key to a proper-sounding radio or sound system. Replacement speakers should be securely mounted and wired according to the correct polarity . See 47–12 and 47–13. All speakers used on the same radio or amplifier should have the same internal coil resistance, called impedance. If unequal-impedance speakers are used, sound quality may be reduced and serious damage to the radio may result. See 47–14.
    Continued
  • 30. Figure 47–11 Between 6 and 7 volts is applied to each speaker terminal and the audio amplifier increases the voltage on one terminal and at the same time decreases the voltage on the other terminal causing the speaker cone to move. The moving cone then moves air causing sound. Continued (a) (b)
  • 31. Figure 47–12 A typical automotive speaker with two terminals. The polarity of the speakers can be identified by looking at the wiring diagram in the service manual or by using a 1 1/2-volt battery to check. Continued
  • 32. Figure 47–13 A speaker polarity tester can be easily constructed using a 1 1/2-volt battery. To test a speaker, connect the positive (+) lead of the battery to one speaker terminal and the negative (-) lead to the other speaker terminal for just a second and observe the direction the speaker cone moves. The positive (+) terminal of the speaker is the terminal that causes the speaker cone to move away from the magnet when touched with the positive (+) battery lead. Continued
  • 33. Figure 47–14 (a) Two 4-ohm speakers connected in series result in a total impedance of 8 ohms. (b) Two 4-ohm speakers connected in parallel result in total impedance of 2 ohms. (b) Continued (a)
  • 34.
    • Impedance Matching All speakers should have the same impedance. If two 4-ohm speakers are being used for the rear and they are connected in parallel, the total impedance is 2 ohms.
    Continued The front speakers should also represent a 2-ohm load. Two front speakers —each 2 ohms Two rear speakers —each 8 ohms Solution: Connect the front speakers in series (connect the positive [+] of one speaker to the negative [2] of the other) for a total impedance of 4 ohms (2Ω + 2Ω = 4Ω ). Connect the two rear speakers in parallel (connect the positive [+] of each speaker together and the negative [  ] of each speaker together) for a total impedance of 4 ohms (8Ω / 2 = 4Ω).
  • 35.
    • Speaker Wiring Wire used for speakers should be as large (low AWG number) as practical to assure full power reaches speakers. Typical “speaker wire” is about 22 gauge (0.35 square millimeter). Tests by audio engineers have concluded that increasing wire to 14ga (2.0 square mm) or larger greatly increases sound quality. All wiring connections should be soldered after making certain that all speaker connections have the correct polarity. Be careful when installing additional audio equipment on a General Motors vehicle system that uses a two-wire speaker connection called a floating ground system . Other systems run only one power (hot) lead to each speaker and ground the other speaker lead to the body of the vehicle.
    Continued
  • 36. A floating ground means that all audio components such as the receiver head, amplifier, and speakers share a common ground that is above and not connected to the chassis ground. This arrangement helps prevent interference and static that could occur if these components were connected to a chassis (vehicle) ground. If the components are chassis grounded, there may be a difference in the voltage potential (voltage); this condition is called a ground loop. What is a “Floating Ground”? CAUTION: Regardless of radio speaker connections used, never operate any radio without the speakers connected, or the speaker driver section of the radio may be damaged as a result of the open circuit.
  • 37. SPEAKER TYPES
    • Tweeter A speaker designed to reproduce high-frequency sounds, usually between 4,000 Hz (4 kHz) and 20,000 Hz (20 kHz), a tweeter is very directional and is usually mounted on the inside door near the top, windshield “A” pillar. Midrange Designed and manufactured to best reproduce sounds in the middle of the human hearing range. Most people are sensitive to the sound produced by these midrange speakers. These speakers are also directional in that the listener can usually locate the source of the sound. Subwoofer A subwoofer , sometimes called a woofer , produces the lowest frequency of sounds, usually 125 Hz and lower.
    Continued
  • 38.
    • Speaker Frequency Response Frequency response is how a speaker responds to a range of frequencies. A typical frequency response for a midrange speaker may be 500 Hz to 4,000 Hz with a tolerance of 3 decibels ( dB ). A decibel is a measure of sound power, and it is the faintest sound a human can hear in the midband frequencies. The dB scale is not linear (straight line) but rather logarithmic, meaning that a small change in the dB reading results in a large change in volume of noise. An increase of 10 dB in sound pressure is equal to doubling the perceived volume. Therefore, a small difference in dB rating means a big difference in the sound volume of the speaker.
  • 39. CROSSOVERS
    • A crossover is designed to separate the frequency of a sound, such as low bass sounds, to a woofer designed to reproduce these low-frequency sound. Two types: passive and active. Passive Crossover A passive crossover does not use an external power source. It uses a coil and a capacitor to block frequencies a particular type of speaker can’t handle and allow those frequencies that can be handled to be applied to the speaker. This type of passive crossover is called a low - pass filter , because it passes (transfers) only the low-frequency sounds to the speaker and blocks all other frequencies. A high - pass filter is used to transfer higher frequency (over 100 Hz) to smaller speakers.
    Continued
  • 40. A bass blocker is a capacitor and coil assembly that effectively blocks low frequencies. A bass blocker is normally used to block low frequencies being sent to the smaller front speakers. Using a bass blocker allows the smaller front speakers to more efficiently reproduce the midrange and high-range frequency sound. What is a Bass Blocker?
  • 41. Figure 47–15 Crossovers are used in audio systems to send high-frequency sounds to the small (tweeter) speakers and the low-frequency sounds to larger (woofer) speakers.
    • Active Crossover Use of an external power source makes for superior performance in active crossovers , called electronic crossovers . These units include many powered filters and are considerably more expensive than passive crossovers.
    Two amplifiers are necessary to fully benefit from an active crossover. One amplifier for higher and midrange frequencies and the other for subwoofers. If you can afford two or more amps, consider using the electronic crossover. Continued
  • 42.
    • Battery power is often slow to respond. When the amp attempts to draw a large amount of current, the capacitor will try to stabilize voltage levels by discharging stored current as needed.
    A powerline capacitor , called a stiffening capacitor , refers to a large capacitor (abbreviated CAP) of 0.25 farad or larger connected to an amplifier power wire. The purpose and function of this capacitor is to provide the reserve energy to the amp to provide deep bass notes. Use a Powerline Capacitor if Upgrading a Sound System See the Powerline Usage Chart on Page 539 of your textbook. A rule of thumb is to connect a capacitor with a capacity of 1 farad for each 1,000 watts of amplifier power. Figure 47–16 Two capacitors connected in parallel provide the necessary current flow to power large subwoofer speakers.
  • 43. Figure 47–17 A powerline capacitor should be connected through the power wire to the amplifier as shown. When the amplifier requires more electrical power (watts) than the battery can supply, the capacitor will discharge into the amplifier and supply the necessary current for the fraction of a second it is needed by the amplifier. At other times when the capacitor is not needed, it draws current from the battery to keep it charged.
    • Capacitor Installation If the capacitor were connected to the circuit as shown, the capacitor would draw so much current that it would blow the inline fuse. To safely connect a large capacitor, it must be precharged .
    Continued
  • 44.
    • To precharge the capacitor:
    Continued Step #1 Connect the negative (  ) terminal of the capacitor to a good chassis ground. Step #2 Insert an automotive 12-V light bulb, such as a headlight or parking light, between the positive (+) terminal of the capacitor and the positive terminal of the battery. The light will light as the capacitor is being charged and then go out when the capacitor is fully charged. Step #3 Disconnect the light from the capacitor, then connect the power lead to the capacitor. The capacitor is now fully charged and ready to provide the extra power necessary to supplement battery power to the amplifier.
  • 45.
    • RMS power
    What Do Amp Specs Mean? Signal-to-Noise ratio This spec, measured in decibels (dB), compares the strength of the signal with the level of the background noise (hiss). A higher volume indicates less background noise (eg.105-dB rating is better than a 100-dB rating). Total harmonic distortion ( THD ) represents the amount of change of the signal as it is being amplified. The lower the number, the better the amplifier (for example, a 0.01% is better than 0.07% rating). THD Peak power RMS power at 2 ohms Peak power is the maximum wattage an amplifier can deliver in a short burst during a musical peak. Spec in watts indicating power the amp delivers into a 2-ohm speaker load, achieved by wiring two 4-ohm speakers in parallel or by using 2-ohm speakers. RMS means root-mean-square ; the rating indicating power the amp is capable of producing continuously.
  • 46. VOICE RECOGNITION
    • Parts and Operation Voice recognition is expanding. It allows the driver to perform tasks, such as locate an address in a navigation system by using voice commands rather than buttons. In the past, users had to say the exact words to make it work such as the following examples listed from an owner’s manual for a vehicle equipped with a voice-actuated navigation system:
    • “ Go home ” “ Repeat guidance ” “ Nearest ATM ” The problem with these simple voice commands was that the exact wording had to be spoken. Newer systems recognize speech patterns and take action based on learned patterns.
    Continued
  • 47.
    • Voice recognition can be used for the following functions:
    Figure 47–19 The voice command icon on the steering wheel of a Cadillac.
    • Navigation system operation
    • Sound system operation
    • Climate control system operation
    • Telephone dialing and other related functions
    Figure 47–18 Voice commands can be used to control many functions, including navigation systems, climate control, telephone, and radio.
  • 48.
    • Diagnosis and Service Voice recognition is usually incorporated into many functions of the vehicle. If a problem occurs:
    • Verify the customer complaint (concern). Check the owner’s manual or service information for the proper voice commands and verify that the system is not functioning correctly.
    • Check for any aftermarket accessories that may interfere or were converted to components used by the voice recognition system, such as remote start units, MP3 players, or any other electrical component.
    • Check for stored diagnostic trouble codes (DTCs) using a TECH 2 scan tool.
    • Follow the recommended troubleshooting procedures as stated in service information.
  • 49. BLUETOOTH
    • Operation Bluetooth is a radio frequency standard for short-range communications. The range of a typical Bluetooth device is 10 feet (3 m) and it operates in the ISM (industrial, scientific, and medical) band between 2.4000 and 2.4835 MHz.
    Continued Bluetooth is a wireless standard that works on two levels:
    • It provides physical communication using low power, requiring only about one milliwatt (1/1,000 of a watt) of electrical power making it suitable for use with small handheld or portable devices, such as an ear-mounted speaker/microphone.
    • It provides a standard protocol for how bits of data are sent and received
  • 50.
    • The Bluetooth standard is an advantage because it is wireless, low cost, and automatic. The automotive use of Bluetooth technology is in the operation of a cellular telephone being tied into the vehicle. The vehicle allows the use of hands-free telephone usage. A vehicle that is Bluetooth telephone equipped has the following components:
    Continued
    • A Bluetooth receiver can be built into the navigation or existing sound system.
    • A microphone allows the driver to use voice commands as well as telephone conversations from the vehicle to the cell via Bluetooth wireless connections.
  • 51.
    • Usually. In order to use two telephones, the second phone needs to be given a name. When both telephones enter the vehicle, check which one was recognized. Say “phone status” and the system will tell you to which telephone the system is responding. If not the one you want, simply say, “next phone” and it will move to the other one.
    The term Bluetooth was used by the early adopters of the standard and they named it for Harold Bluetooth, the king of Denmark in the late 900s. The king united Denmark and part of Norway into a single kingdom. Where Did Bluetooth Get Its Name? Can Two Bluetooth Telephones Be Used in a Vehicle?
  • 52.
    • Many cell phones are equipped with Bluetooth, which may allow the user to use an ear-mounted microphone and speaker.
    Figure 47–20 Bluetooth earpiece that contains a microphone and speaker unit that is paired to a cellular phone. The telephone has to be within 33 feet of the earpiece. If vehicle and cell phone are equipped with Bluetooth, the speaker and microphone can be used as a hands-free phone when the cell phone is in the vehicle. The cell phone can be activated in the vehicle by using voice commands.
  • 53. SATELLITE RADIO
    • Parts and Operation Satellite radio, also called Satellite Digital Audio Radio Services or SDARS uses satellites to broadcast high-quality radio. SDARS broadcasts on the S-band of 2.l320 GHz to 2.345 GHz. XM Radio Standard equipment or optional in most GM vehicles, using two satellites launched in 2001 called Rock (XM-2) and Roll (XM-1) in a geosynchronous orbit above North America. Two replacement satellites, Rhythm (XM-3) and Blues (XM-4) were launched in 2006.
    Continued
  • 54.
    • XM radio offers 150 channels of digital audio including:
    Continued
    • 68 commercial-free music channels
    • 33 news, sports, talk, and entertainment channels
    • 21 traffic and weather channels
    • Major-league baseball
    Reception Can be affected by tall buildings and mountains. To help ensure consistent reception, both SDARS providers:
    • Include in the radio itself a buffer circuit that can store several seconds of broadcasts to provide service when traveling out of a service area.
    • Provide land-based repeater stations in most cities. See Figure 47–21.
  • 55. Figure 47–21 SDARS uses satellites and repeater stations to broadcast radio.
  • 56.
    • Antenna To be able to receive satellite radio, the antenna needs to be able to receive signals from both the satellite, as well as repeater stations located in many large cities. There are various types and shapes of antenna, including those shown here.
    Figure 47–23 A shark-fin-type factory antenna used for both XM and OnStar ® . Continued Figure 47–22 An aftermarket XM radio antenna mounted on the rear deck lid. The deck lid acts as the ground plane for the antenna.
  • 57.
    • Most satellite radios contain temporary memory so the radio will continue to operate even if the signal from the satellite or repeater station is temporarily disrupted due to tall buildings or other objects. Diagnosis and Service The first step in any diagnosis is to verify the customer complaint (concern). If no satellite service is being received, first check with the customer to verify that the monthly service fee has been paid and the account is up to date. If poor reception is the cause, carefully check the antenna for damage or faults with the lead-in wire. For all other satellite radio fault problems, check service information for the exact tests and procedures. Always follow the factory recommended procedures.
  • 58. ESN means electronic serial number. This is necessary information to know when reviewing satellite radio subscriptions. Each radio has its own unique ESN. What Does ESN Mean?
  • 59. RADIO INTERFERENCE
    • Radio interference is caused by variations in voltage in the powerline or picked up by the antenna.
    Continued Figure 47–24 A radio choke and/or a capacitor can be installed in the power feed lead to any radio, amplifier, or equalizer. A “whine” increasing in frequency with engine speed is referred to as alternator whine . It is eliminated by installing a radio choke or a filter capacitor in the power feed wire to the radio.
  • 60. Figure 47–25 Many automobile manufacturers install a coaxial capacitor, like this one, in the power feed wire to the blower motor to eliminate interference caused by the blower motor.
    • Ignition noise is usually a raspy sound that varies with the speed of the engine. This noise is usually eliminated by the installation of a capacitor on the positive side of the ignition coil. The capacitor should be connected to the power feed wire to either the radio or the amplifier or both.
    The capacitor must be grounded. If a standard automotive condenser is unavailable, use a 470-µF 50-volt electrolytic capacitor. Continued
  • 61.
    • A radio choke , which is a coil of wire, can also be used to reduce or eliminate radio interference. Radio interference being picked up by the antenna can best be eliminated by stopping the source by making certain that all units containing a coil, such as electric motors, have a capacitor or diode attached to the power-side wire. Most radio interference complaints come when someone installs an amplifier, power booster, equalizer, or other radio accessory. A major cause of this interference is the variation in voltage through the ground circuit wires. To prevent or reduce this interference, make sure all ground connections are clean and tight . Placing a capacitor in the ground circuit also may be beneficial.
    Continued
  • 62.
    • Radio noise can be broadcast or caused by voltage variations in the power circuit to the radio. A capacitor and/or radio choke are most commonly used components. A “sniffer” can be used to locate the source of the radio noise. See Figure 47-26.
    When diagnosing sound system interference, try running separate 14-gauge wire(s) from the sound system power lead and ground to a battery outside of the vehicle. If the noise is still heard, the interference is not due to a generator diode or other source in the wiring of the vehicle. The Separate Battery Trick CAUTION: Amplifiers sold to boost the range or power of an antenna often increase the level of interference and radio noise to a level that disturbs the driver. Continued
  • 63. Figure 47–26 A “sniffer” can be made from an old antenna lead-in cable by removing about 3 inches of the outer shielding from the end. Plug the lead-in cable into the antenna input of the radio and tune the radio to a weak station. Move the end of the antenna wire around the vehicle dash area. The sniffer is used to locate components that may not be properly shielded or grounded and can cause radio interference through the case (housing) of the radio itself.
  • 64. AUDIO NOISE SYMPTOM CHART See the chart on Page 544 of your textbook.
  • 65. A radio failed to work in a vehicle that was outside during a thunderstorm. The technician checked the fuses and verified that power was reaching the radio. Then the technician noticed the antenna. It had been struck by lightning. Obviously, the high voltage from the lightning strike traveled to the radio receiver and damaged the circuits. Both the radio and the antenna were replaced to correct the problem. Lightning Damage Figure 47–27 The tip of an antenna that was struck by lightning.
  • 66.
    • To unlock the radio, the tech used the following steps (the code number being used is 4321).
    A customer replaced the battery in a GM vehicle and now the radio display shows “ LOC ”. This means the radio is locked with a customer code stored. The GM Security Radio Problem - Part 1 Thankfully, the owner had the security code. If the owner had lost the code, the tech would have to secure a scrambled factory backup code from the radio and then call a toll-free number to obtain another code. The code will only be given to authorized dealers or repair facilities. Step 1 — Press the “HR” (hour) button—“000” is displayed. Step 2 — Set the first two digits using the hour button—“4300” is displayed. Step 3 — Set the last two digits of the code using the “MIN” (minutes) button—“4321” is displayed. Step 4 — Press the AM-FM button to enter the code. The radio is unlocked and the clock displays “1:00.”
  • 67.
    • This display indicates that too many incorrect codes have been entered and the radio must be kept powered for one hour and the ignition turned on before any more attempts can be made.
    • This display means there is a customer’s code stored but the radio is unlocked.
    • This means there is no customer code stored and the radio is unlocked.
    • Means customer’s code has been entered once and the radio now is asking that the code be repeated to verify it was entered correctly the first time.
    Other displays and their meaning include: The GM Security Radio Problem - Part 2 “ InOP” “ SEC” “—” “ REP”
  • 68. SUMMARY
    • Radios receive AM (amplitude modulation) and FM (frequency modulation) signals broadcast through the air.
    • The radio antenna is used to induce a very small voltage signal as an input into the radio from the electromagnetic energy from the broadcast station.
    • AM requires an antenna whereas FM may be heard from a radio without an antenna.
    • Speakers reproduce the original sound, and the impedance of all speakers should be equally matched.
    Continued
  • 69. SUMMARY
    • Crossovers are used to block certain frequencies to allow each type of speaker to perform its job better. A low-pass filter is used to block high-frequency sounds being sent to large woofer speakers, and a high-pass filter blocks low-frequency sounds being sent to tweeters.
    • Radio interference can be caused by many different things, such as a defective generator (alternator), a fault in the ignition system, or a fault in a relay or solenoid.
    ( cont. )
  • 70. end