I'm having a tough time figuring out the source of radio noise in my 1997 S500. With the key in the number 1, or accessory position, the radio, C/D and cassette all work perfectly, but when the key is in the number 2, or run position, I have very bad noise with the radio bands even though the C/D and cassette still work perfectly. This happens whether the engine is running or not.

I have 2 factory radios and a new after market radio. All 3 have the same symptoms.

I also installed a new Hirschmann universal antenna but it made no difference.

I was wandering around on ebay looking at used amps but found a part called an "antenna amplifier noise supressor" listed and wondered if that could be the problem. Here's the listing: Mercedes W140 S320 98 99 Antenna Amplifier Noise Suppressor 1408202189 21048 | eBay

Does anyone know where this part is located in the car?

Thanks, Bob Short

__________________
2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire

Bob,
I looked up the part number in EPC. If you don't subscribe, you probably should. Any way in typical German that part is called a "parasitic stopper" and is located inside the left c-pillar. It also shows another with the part number A140 820 36 89 Couldn't see what the difference was though. And have you contacted Becker Radio Service back in New Jersey? They might be able to shed some light on this.
Anziani
'97 CL600 52K

Thanks for the extra info. I had asked a few of the eBay sellers where the part was located and one of them said it was inside the left c-pillar. I hate taking that thing off as from what I understand it's virtually impossible without breaking the attachment tabs off.

I'm replacing the original head unit with one from Axxera that has a slide-out 7" monitor that works great with the original factory amp. So I'm going to start by trying a little experiment. I'm wondering if I abandon the factory antenna cable and run a new one that goes straight from the radio to the antenna, taking the "parasitic stopper" out of the mix. I bought an inexpensive radio antenna at O'Rielly's that I'll plug into the new radio and see what happens. I'll report on how it comes out.

__________________
2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire

Just a thought from an electrical engineer:

There are several sources of noise from within a vehicle (as opposed to storms, power lines, neon signs, etc).

- spark ignition
- alternator
- tires rolling on the pavement
But you are at rest with the engine not running so that rules those out

The one thing that happens going between position I and II is that you power up most of the systems on the car. Motors are notorious for this kind of noise especially series dc motors with brushes like you have all over a car.

I would look at the blower motor. Maybe fuel pump but I think it only runs a few seconds if no rpm is detected.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech