Hi, welcome to Just Answer.
Various late model BMW's end up with reception problems, below is a bullitin BMW put out to the technicians to further help issolate them. Take a look at it and let me know if you need to dig deeper into the system. Typically I have found bad radio grounds, faulty rear windsheilds, and faulty amplifiers causing the problem on the 325i's.
Many of the problems which occur with a car and not with a home stereo system stem from the differences between home and car antennas. A home radio/TV antenna, which is considerably larger, is usually mounted on the highest point of the roof. It is often multi-directional for tuning in the best possible reception. And a home antenna always remains in one place.
On the other hand, the antenna in a car is much smaller. It sits close to the ground rather than being high in the air. And it is virtually always in motion.
Given the difficult job car antennas are asked to perform, should problems with AM/FM reception be encountered by customers, it is most likely not the fault of the BMW radio.
The stations the customer is able to receive will depend largely upon signal strength. This varies depending upon the time of day, the season and other factors.
AM radio waves generally deliver a powerful, continuous signal over more than a one-hundred-mile radius from their point of origin - even in mountainous or inner-city areas. This is because AM waves are dispersed as ground waves which follow the curvature of the earth and space waves which actually bounce off the upper ionosphere, creating a downward route to the automobile, no matter where it is driven.
This factor is much better at night and also tends to be better in winter than in summer.
AM signal transmission problems:
The problem with AM reception is that it is highly susceptible to electrical interference from sources such as power lines, electrical storms - even from nearby vehicles.
In the United States, due to the large number of stations, many smaller AM stations are required to sign off or reduce power sharply at sunset in order to reduce interference with distant stations.
For AM signals, the single most important factor for good reception is the time of day.
AM signals almost always get absorbed by the diffusion layer of the ionosphere during daylight hours. As a result, all AM signals received during midday hours will arrive by ground wave, making reception of signals over a few hundred miles away unusual in daylight. At night however, the ionosphere reflects AM signals ("skywave"), making it possible for signals to be heard at much greater distances.
FM reception - which can include the car's local weatherband station - is usually better in sound quality than AM reception. However, unlike AM waves, FM signals are weaker, delivering reception for only about forty to fifty miles under even the best conditions.
FM signals transmission problems:
With FM signals, several problems can occur as a result of the way in which these signals travel and their relative lack of strength.
"Dead Spots" - The first of these problems is called a "dead spot". If a direct FM wave and a reflected FM wave reach the car antenna at the same time, they will cancel each other out.
"Multipath" - The second problem specific to FM signals is called "multipath". This is similar to a dead spot in that two stations are fighting for the same general frequency. A "multipath" is an area in which a reflected FM signal is occupying a frequency very close to that of a direct signal. If the reflected wave is stronger than the direct wave, the result will be a fluttering sound as the car passes through that area. This happens very often in inner-city areas.
"Fading" - The third problem often encountered with FM signals is called "fading". "Fading" occurs as the car is leaving the effective reception range of the FM station. What is heard in this case is the signal becoming weak and fuzzy. Because the range of most FM stations is only about forty miles, fading may be experienced quite often during long trips - and even right in a motorist's own neighborhood - if a particular FM station is broadcasting forty miles away or more.
"Station Swapping" - Another problem often heard when listening to an FM signal is called "station swapping". An FM receiver is designed to search for and lock onto the strongest signal in any area.
However, if there are two stations in a given area that are broadcasting on very close frequencies, the radio may "swap" back and forth between these two stations, depending on which signal is stronger.
Stereo broadcasts have an effective range of only about forty miles - unlike mono broadcasts, whose effective range is usually about fifty miles.
This means that if someone is listening to FM stereo broadcast, and the car is thirty miles away from the transmitter, interference may be experienced. With a mono broadcast, on the other hand, such interference will not be heard until the car is approximately forty to fifty miles away from the transmitter.
The weatherband is a special feature on some BMW radios.
It is designed as a special convenience for the drivers as they travel from one place to another. During a trip, the weatherband will automatically seek out the strongest weather broadcast in any given area.
When it finds a signal strong enough to lock onto, it automatically does so.
Effective radio diagnosis starts with screening the complaint. When a customer complains about a radio problem, it is very important to complaints (as explained above) prior to turning the car in for diagnosing and repairing.
Once all pertinent information relating to the complaint has been gathered, refer to the following steps:
Jump your antenna inputs from your antenna amplifier to a stock (replacement) rear window or another vehicle's rear window and retest to see if the defroster still causes the reception loss - if yes, the fault is in the car - if no, replace the rear window.
Yes, you sure can...just post back here. Be sure to check the rear windshild for dirt or damage.