Pressing the brake affects AM radio reception

coldax · Post by **coldax** » 03 Jun 2012, 13:59

Hey all - I just picked up a 2000 V70 SE, non-turbo automatic.
The stock HU SC-816 works fine, except for AM radio reception.
Depressing the brake pedal actually changes the reception - so let's say I am driving and listening to a station and it sounds good. Then I press the brakes and the station is consumed by static, as if I were passing near overhead power lines. Press the brakes again, back to normal. What's up with that?

I pulled the HU, and I see what looks like 2 antenna leads. I assume there is one in each of the wagon's rear side windows for diversity, correct?

One of the leads into the HU is wrapped in black tape that doesn't look stock like the other one. They are inserted all the way in. My next step is to peel back the tape and see if maybe it got sliced and there is a short or something.

Has anyone else experienced this phenomenon? Is the wagon radio reception usually worse than a sedan due to lack of antenna?

Thanks,
coldax

Post by **BEJinFbk** » 03 Jun 2012, 18:24

The window antennas have amplifiers near the rear windows.
The main antenna is on the left side and it's amplifier shares a
grounding point with the brake lights. It may be a poor ground.

Open the tail light access panel on that side, locate the grounding
point and be sure that it's clean and tight. Hopefully, that'll get it.

Post by **dcarlson12** » 03 Jun 2012, 19:17

Is this also the situation with S80s? My static is however only for the 730 am station/frequency...

Post by **BEJinFbk** » 03 Jun 2012, 22:23

Can't say for sure, but it's worth checking out. Many systems share grounding points.
You can check for your specific car pretty easily. Pull up the diagrams from this site
and have a look at the grounds for both systems. They'll show where to find them.

coldax · Post by **coldax** » 06 Jun 2012, 08:26

Thanks. That specific ground point looks clean and tight. Is there another one on the right side?

Post by **BEJinFbk** » 06 Jun 2012, 10:52

That's the side for the "sub" antenna of this two antenna system.
I'm not sure, but I think that the radio only uses the main antenna for AM.
Worth checking, though. I'd have a look just to be sure.

At this point, you may want to get a hold of a substitute antenna, and
just unplug both connectors at the back of the radio and see what you
get with the known good sub. It may not work as well as the built in,
but it'll help you with a direction on this problem.

Post by **jimmy57** » 06 Jun 2012, 13:22

Volvo had a service bulletin about am noise years back. It was to install dust caps on rear that has contact springs that touched the end of spindle. My recollection is that it was a reverse situation though. I think it was for AM static that built and applying brakes stopped it.
When the brake light switch closes it is a splice to lights, ABS, TCM, and shiftlock on that year model.
Could be something from one of those.
If the radio wiring has been hacked from previous aftermarket head installation then wiring issues there could be introducing the noise.

Post by **BEJinFbk** » 06 Jun 2012, 14:48

You mean the connections to the window themselves?
As for interference from other systems, that's why I'd
try a substitute antenna. Isolate and diagnose.

coldax · Post by **coldax** » 07 Jun 2012, 10:51

I'll be investigating this further over the weekend. Until then Life always seems to get in the way...

coldax · Post by **coldax** » 11 Jun 2012, 14:54

Here is an update for those still interested or who have suggestions:

The two antenna leads at the HU are fine.
A 'click' sound is heard through the speakers when pressing the brakes.
Pressing the washer fluid control produces static through the speakers.