OT funny thing

[Gary]

I have a car Fiesta 1.3 Endura 2001 ? 2002. It is fitted with a Ford CD car radio. Up until two days ago the car radio worked fine (and it still does ). Here's the problem; ? about 18 months ago the rear screen heater stopped working. I looked at it and couldn't see anything obvious. I checked the connectors and the switch and the fuses. put to one side to do when I could fit it in.

On Saturday I got the workshop manual out and the multimeter. 10 minutes late I locate the problem, the relay which is mounted up behind the fuse box appears to not work. Upon examination of the relay the high current terminal has overheated and gone open circuit. Cleaned up and swapped that relay for another one of the same type mounted in the same fuse box checked circuits and it's okay.

Later that day I go to Tesco's and radio isn't working very well, low signal. I think to myself that's funny! Just out of interest I turn off the rear screen heater. The radio now works. So I turn the heat back on again, and the radio goes hissy, turn it off and the radio is fine.

I've rechecked that the rear screen circuits are all intact and not arcing or anything but it got me beat (that's not hard). Any ideas ?

[whisky-dave]

The drain on the battery from the working heater is causing the radio probl= ems, maybe the car battery is on it's way out, have you tried measuring the= voltage when the heaters on. Clucthcing at straws perhaps =20 I'd blame the cold weather ;-)

[Gary]

problems, maybe the car battery is on it's way out, have you tried measuring the voltage when the heaters on. Clucthcing at straws perhaps

full voltage. it is as if the aerial amp is being turned off. it is prosibly an earth problem but it is not obvious where to look.

thanks

[Brian Gaff]

What was the other relay being used for that he nicked though, kind of makes you wonder.... Some cars do use the screen heaters as an aerial. Ther eis usually though chokes in the circuit to stop problems with the apparent earthing of the other end of the heater!

Brian Brian

[Brian Gaff]

Vehicles are indeed strange things..

[Dave Plowman (News)]

Does it use the rear screen element as an aerial? If so perhaps the amplifier/isolator unit is not being powered correctly after your repair.

[John Rumm]

Or perhaps the original relay damage was caused by failure of the choke that isolated the element's power supply from its aerial function, so now when powered, you are in effect grounding your aerial as well?

[Paul Herber]

A capacitor will also be needed to isolate any DC from the aerial input.

[news.virginmedia.com]

I suspect it uses the rear heater element (tracks on the window, like my Honda had) as the aerial - and the original relay may have an inductor or similar on one or both of the "heat" terminals to prevent the radio signal being shorted by the car's electrical system - when the heater comes on one end of the element is effectively earthed, so you lose most of the signal. You may need to get an *exactly match* for the relay you took out, or you could look at adding an inductor in the lead from relay to element?

Dave H, (the other one)

[Harry Bloomfield]

Gary formulated on Tuesday :

Assuming as many do today - the rear screen heater is used as the antenna for the radio and the printed tracks on the glass are damaged, the poor tracks will generate masses of interference when the screen heater is turned on.

[Fredxx]

Why? We're talking 12V here, not a voltage that is going to jump some distance.

Is there is a bad connection, then it would still show when the heater is turned off.

[John Rumm]

Yup, although there is a fair chance the radio will have that internally.

[Dave Plowman (News)]

The installations I've seen that use the rear screen as an aerial have an amp mounted close to it - which I assume removes all the nasties before sending it off to the head unit.

[Jonathan]

We have that problem on an ancient Skoda which does have a separate radio aerial.

Jonathan

[Graham.]

That's what I thought too. I presume there isn't a conventional aerial on this car so it's a good guess that the heated rear window is providing this function. If so, there will be a couple of RF chokes to isolate the screen at RF, the only problem is that I would expect these chokes to be near the screen, not under the dash or in the engine compartment.

Incidentally I had a Civic with that arrangement for a while, I hated the car for various reasons, but the radio reception on AM and FM was superb.

[Harry Bloomfield]

I can assure you I do not what I am talking about.

[charles]

when I think about it, I had that problem on my no-so-ancient Citröen (only

10½ years). This had a separate roof aerial at the rear of the car. Replacing the aerial cured the problem. It had broken internally - probably because it got bent rather severely when I carried sheets of plywood on the roof bars.

[Andrew Gabriel]

A common problem with older Fiestas is that the earth return from the rear heater (and rear wiper if you have one) is via the boot lid hinges, and they go high impedance. Connecting an earthing strap between the boot lid and the car body fixes this. Don't know if it's related to your aerial problem though.

[The Natural Philosopher]

standard input is either a coil to ground, which would smoke on DC, or a low value kerpuffitor, which will block it.

[SteveW]

You used to be able to buy one as an add on. It was a grey, plastic tube with red ends and a bracket, plus flying leads for connecting the screen power and the aerial to. About the size of an old style ignition coil. Mine worked very well in my Sierra.

SteveW