Radio Antenna Open Circuit - Radio or Antenna Problem?
 

I have perfect TV reception, but no FM. Have had the A8 in bits trying to find a fault in the antenna wiring, but nothing obvious.

VAG COM gives an 00856 Radio Antenna Open Circuit fault. Does this indicate an internal problem with the RNS unit itself rather than the antenna amplifier or antenna wiring?

Hi, it would be extremely rare for the RNS unit itself to be the cause of this fault, and indeed I personally doubt the rear antennas will be the cause either.

In our experience, we have seen a few cars with faulty FM, but perfect TV.

The 'interesting' thing is that they all use the same aerials on the rear screen.

They are fed through a diversity switcher which lives at the bottom of the D pillar trim, on the passenger side (UK).

To access it, you will need to the trim covering the D pillar.

You will see 4 aerials feeding into it, and a number of connections out. 4 of the outputs will go to your TV tuner in the boot. 2 other connections will supply the FM radio at the front of the car. There is a non-coax power connector too.

If i were you, I'd remove this unit and inspect the FM connections thoroughly. Typically, the tiny coax connections on it fail, and sometimes even break away from the board. It is probably possible with a little skill to resolder it back together though, which will save some expense.

Photo attached for reference....

Thanks Ian, I have had the seats out and set eyes on that thing already. In fact, twiddling the antenna input did cause the hiss to change. I've done plenty soldering over the years, so I'll get the seats out again and get that little box opened up and inspected.

I'll let you know how I get on.

I eventually got to uncovering this box again and found that the main antenna connection (gold/brass thing) had become unsoldered. I managed to solder the little pin back down so I have reception again. The only problem that remains is a little bit of crackle and I think that is because the little wings (earth I guess) at the side of the connector were also soldered onto the board, but this is impossible to fix well with a conventional soldering iron. I don't know how it would have been done in the first place.

So this will do for the moment, but I'll try to pick up one of the diversity boxes in due course.

Thanks for your help Ian.