Identification Of Noise Source

WARNING: This page is about a different car, the 2005 Toyota 4Runner. However, it is still accessible from the selected car via links, so may be relevant.

Radio Description

Radio frequency band
1. Radio Broadcasts use the radio frequency bands shown in the table below.
  
  Fig 1: Radio Frequency Band Chart
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
Service area
1. The service areas of the AM and FM broadcasts are vastly different. Even if an AM broadcast has clear reception, an FM broadcast originating in the same location may not be received at all.
  The service area of FM stereo broadcasts is small, and static as well as interference (noise) easily enters the signal.
  
  Fig 2: Identifying Service Areas Of AM And FM Broadcasts
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
Radio reception problems
HINT:

In addition to static, other problems such as "phasing", "multipath", and "fade out" exist. These problems are not caused by electrical noise, but by the radio signal propagation method itself.
1. Phasing
  The AM frequency band reflects off the earth's ionosphere at night. When this occurs, the reflected signal may interfere with the direct signal sent by the same transmitter. This phenomenon is known as "phasing".
  
  Fig 3: Identifying Phasing
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
2. Multipath
  A radio signal can sometimes be reflected by an obstruction in its path. When this occurs, the reflected signal may interfere with the direct signal sent from the transmitter. This phenomenon is known as "Multipath".
  
  Fig 4: Identifying Multipath
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
3. Fade out
  FM radio frequency is higher than AM. Therefore, it is more likely to be reflected by large obstructions such as tall buildings or mountains. For this reason, the FM signal will gradually weaken or disappear when the vehicle is behind such obstructions. This phenomenon is known as "fade out".
  
  Fig 5: Identifying Fade-out
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

Noise problem

It is very important for a technician to understand the specifics of the noise problem. To diagnose the symptom, use the table below.

NOISE OCCURRENCE CONDITION - PRESUMABLE CAUSE

Radio Frequency	Noise occurrence condition	Presumable cause
AM	Noise occurs in a specified area	Foreign noise
AM	Noise occurs when listening to an intermittent broadcast	An identical program transmitted from multiple towers can cause noise where the signals overlap
AM	Noise occurs only at night	Music beat from a far-off broadcast
FM	Noise occurs while driving in a specified area	Multipath or phasing noise resulting from a change in FM frequency

HINT:

If the noise does not fall into any category in the table above, determine the cause using "Radio reception problems" above. Refer to the multipath and phasing sections.