Identification Of Noise Source

RADIO WAVE BAND
The radio wave bands used in radio broadcasting are as follows:

Fig 1: Identifying Radio Wave Band
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
SERVICE AREA
1. There is a great difference in the size of the service areas for AM and FM broadcasting. Sometimes an FM stereo broadcast cannot be received even though AM can be received very clearly.
  FM stereo has the smaller service area, and it also picks up static and other types of interference ("noise") easily.
Fig 2: Locating Service Area Of Radio Band
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
RECEPTION PROBLEMS
HINT:

Besides the problem of static, there are other problems, such as "phasing", "multipath" and "fade out". These problems are caused not by electrical noise but by the nature of the radio waves themselves.

Fig 3: Identifying Phasing Of Radio Band
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
1. Phasing
  Besides electrical interference, AM broadcasts are also susceptible to other types of interference, especially at night. This is because AM radio waves bounce off the ionosphere at night. These radio waves then interfere with the signals that reach the vehicle's antenna directly from the same transmitter. This type of interference is called "phasing".
  
  Fig 4: Identifying Multipath Of Radio Band
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
  
  Fig 5: Identifying Fade Out Of Radio Band
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
2. Multipath
  Interference caused by reflection of radio waves against obstructions is called "Multipath". Multipath occurs when radio signals emitted from the broadcast transmitter antenna are reflected against tall buildings or mountains and interfere with other signals which are to be received directly.
3. Fade Out
  FM radio wave tends to be reflected against obstructions such as tall buildings or mountains because FM frequency is higher than AM. For this reason, FM signals often seem to gradually disappear or fade away as the vehicle goes behind those obstructions. This phenomenon is called "fade out".
NOISE PROBLEMS
1. It is very important for technicians to clearly understand a customer's claim about noise. Use the following table to diagnose the phenomena.
  
  Fig 6: Identifying Noise Problems
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
HINT:

If the condition when the noise occurs does not meet any of the above, find out the cause based on "Reception Problems". Refer to the description about multipath and phasing mentioned previously.

Identify the conditions under which the noise occurs, and check the noise filter on the relevant part.

SYMPTOM CHART

Conditions under which noise occurs	Noise type
Noise increases when the accelerator pedal is depressed, but stops when the engine is stopped.	Generator noise
Noise occurs during A/C or heater operation.	Blower motor noise
Noise occurs when the vehicle accelerates rapidly on an unpaved road or after the ignition switch is turned on.	Fuel pump noise
Noise occurs when the horn switch is pressed and released or when pressed and held.	Horn noise
Quiet noise occurs while the engine is running, but stops when the engine is stopped.	Ignition noise
Noise occurs synchronously with the blink of the turn signal.	Flasher noise
Noise occurs during window washer operation.	Washer noise
Noise occurs while the engine is running, and continues even after the engine is stopped.	Water temperature sensor noise
Noise occurs during wiper operation.	Wiper noise

NOISE TYPE

Conditions under which noise occurs	Noise type
Noise occurs when the brake pedal is depressed.	Stop light switch noise
Others	Static electricity stored on the vehicle

Reference:

First ensure that the noise is not coming from the outside. Failure to do so makes noise source detection difficult and may lead to a misdiagnosis.
Noise should be removed in descending order of loudness.
Tuning the radio so that no station is received makes the noise more noticeable, making the recognition of the phenomenon easier.