Identification Of Noise Source

WARNING: This page is about a different car, the 2007 Lexus GX 470. However, it is still accessible from the selected car via links, so may be relevant.

RADIO WAVE BAND
1. Radio frequency band
  1. Radio broadcasts use the radio frequency bands shown below.
    
    Fig 1: Identifying Radio Frequency Bands
    Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
SERVICE AREA
1. The broadcast range for AM and FM is very different. Sometimes an AM broadcast can be received very clearly but FM stereo cannot. FM stereo has the smallest service area, and is prone to picking up static and other types of interference (for example, noise).
  
  Fig 2: Identifying Service Areas Of AM And FM Broadcasts
  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 by the nature of the radio waves themselves, not by electrical noise.
1. AM broadcasts are susceptible to electrical interference called phasing. Occurring only at night, phasing is the interference created when a vehicle receives 2 radio wave signals from the same transmitter. One signal is reflected off of the ionosphere and the other signal is received directly from the transmitter.
  
  Fig 3: Identifying Radio Phasing
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
2. Multipath is a type of interference created when a vehicle receives 2 radio wave signals from the same transmitter. One signal is reflected off of buildings or mountains and the other signal is received directly from the transmitter.
  
  Fig 4: Identifying Service Area For Multipath
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
3. Fade out is caused by objects (buildings, mountains, etc.) that deflect away part of a signal, resulting in a weaker signal when the object is between the transmitter and vehicle. High frequency radio waves, such as FM broadcasts, are easily deflected by obstructions. Low frequency radio waves, such as AM broadcasts, are much more difficult to deflect.
  
  Fig 5: Identifying Service Area For Fade Out
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

NOISE PROBLEMS

Technicians must have a clear understanding about the noise problems of the customer's vehicle. Use the following table to diagnose the problems.

RADIO WAVE REFERENCE TABLE

Radio Wave	Condition in Which Noise Occurs	Presumable Cause
AM	Noise occurs in a specific area	Strong possibility of foreign noise
AM	Noise occurs when broadcasting is faint	Same program may be broadcasted from various local stations If the program is the same, one of those may be tuned in
AM	Noise occurs only at night	Strong possibility of beats from distant broadcasting
FM	Noise occurs at a specific place during driving	Strong possibility of multipath noise and fading noise caused by changes of FM frequency

HINT:

When the noise does not match the examples above, refer to step 3, "Reception Problems". Check the descriptions about phasing and multipath.