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Bavarian Soundwerks' Mobile Electronics Forum
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Old 04-21-2004, 09:32 AM   #1
325iBob
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Join Date: Oct 2003
Location: Littleton, Colorado
Posts: 31
My Ride: 01 325i Sedan Sport
AM Radio/Defroster Noise

If I turn on the rear window defroster while listening to AM radio, I get loud interference noise. The noise disappears when I turn off the defroster.

The dealer wants to replace the rear window, but that will take away my tint. They'll retint, but it won't match.

Does anyone know the solution to the interference noise problem?
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Old 04-21-2004, 10:24 AM   #2
bearing01
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Join Date: Mar 2004
Location: San Diego
Posts: 324
My Ride: 02-E46/86-E30
Replace window only if under warranty.

Hi,

The back window rear-defroster also contains the AM/FM radio antenna. The antenna is just printed conductor on top of the glass. The rear-defroster pushes current through the conductor to heat up the conductor and melt the ice/frost. If there is a problem with your antenna then the radio should be crappy for every AM station. The AM radio band is like 500kHz to 1600kHz. The FM band is 88MHz - 108MHz. If changing the antenna makes the distortion go away I will be very very suprised. The antenna is passive and does not create distortion. However, if you have bad / loose connections to the antenna at the window then that could promote a little distortion... but I don't think it would be enough to be heard. Sounds to me your defroster creates a distortion in the same frequency range as the AM frequency band.

Something to check:
1. Turn on the defroster and try all the AM channels. Are there any channels that do not pick up the defroster's distortion?
2. Try the same for the FM channels.

It is possible the FM band won't pick up the distortion even if the defroster distortion is over its frequencies. FM stands for Frequency modulation. AM stands for Amplitude Modulation. In your AM problem case... the AM signal coming in is very low like 10 micro volts. The desired signal is embedded in the peak amplitude of the incoming voltage. THerefore, if there is some distortion or noise like 1 milli-volt from the rear-defroster .. then it will swamp the 10 micro volt incoming AM signal. All you will hear is the Noise / Distortion from the defroster. FM, on the other hand, the desired signal (music) is not in the amplitude of the incoming signal but in how its frequency varies. The FM radio demodulator is more robust and will reject the noise/distortion more than the AM. That's why you may not hear the noise/distortion in the FM band.

Having said all that above... how to fix this? Although I have never worked with such a problem before,.... I would guess that the defroster has a filter at its output. This filter basically blocks any noise or distortion (it creates) in the DC current going out to the rear window. Somehow this filter either does not exist or you need to put one there. The filter is basically an Inductor (coil of wire) and a capacitor (usually blue or orange in color). If you install this yourself it is possible you can affect your antenna's performance in a bad way. If you choose Inductor/capacitor values that are too small then you will also filter out the AM and FM signals in off the antenna. Your radio won't pick up anything.

If I were you I'd do the following:
1. Look at the connections to the antenna/defroster. Make sure nothing looks coroded. If it is, clean it up.
2. If it is the filter in the defroster... let the garage take care of it if your car is under warranty.

If they have no clue what they're doing and they waste 100's of $ trying to figure this out... then its their loss and your inconvenience. If they want to charge you for the new window and labor then you're probably going to get ripped off. Ask them the root cause of the problem and tell them you want your old window installed (or money back) if it doesn't fix the problem... before you get the job done. Ask to talk to the mechanic and see if he truly knows what the problem is... or if this is just guess work in their part. An inductor and capacitor at radio shack will cost you about $1.
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