a radio mystery solved

Post by **Dru** » Sun Feb 03, 2008 9:32 am

My radio started misbehaving a while ago; it would work OK for a while, then cut out and then come on again. Then it would only work on low volume, then it finally just kept cutting out and in again. V annoying.

I suspected there may be a poor connection in the power supply; pulled radio out, no obvious prob. Phoned the dealer who fitted the radio, to see about replacing it on warranty. They suggested I take it in. So I did.

Cutting to the chase.... the car has a master cut-off switch on the battery earth supply. The fuse for the radio is mounted on the earth line which connects to the cut-off switch. The fuse had blown, and the radio continued working through the earth on the aerial line. Only not working very well, of course.

Here endeth this lesson learned

Post by **Rob_Jennings** » Sun Feb 03, 2008 11:39 am

A fuse should never be placed in the earth line! That would leave the equipment live even if it failed in some way.

also if you have a cut off switch in the car the normal practice is to have a low amp fuse across the switch, so that things like radio etc will still operate, but if you were to try the starter the fuse would simply pop

Post by **bmcecosse** » Sun Feb 03, 2008 1:31 pm

And when it has blown - any further efforts to use any of the electrical equipment (with the main OFF) will also find a path through the radio and it's aeriel = burned out radio!

Post by **MoggyTech** » Sun Feb 03, 2008 3:09 pm

Is the Car Negative or Positive earth?

Post by **Dru** » Mon Feb 04, 2008 11:53 am

It's negative earth. The earth line from the radio connects to the battery side of the cut-off switch, so that the supply isn't interrupted by the isolator key being removed.

That's a very good point about the radio being permanently live; I'd better install a fuse on the supply side, hadn't I?

Post by **Kevin** » Mon Feb 04, 2008 1:00 pm

The earth line from the radio connects to the battery side of the cut-off switch,

the dealer who fitted the radio,

Why did they wire it like that as there are plenty of better earth points available.

Post by **Dru** » Mon Feb 04, 2008 3:09 pm

I don't want to second-guess their reasons, but as the radio is mounted in the passenger-side glove compartment, it's a short run to the battery earth lead. I suppose it makes sense to connect the radio earth on the battery side of the isolating switch so that there is a permanent circuit; otherwise the radio memory would be wiped every time the isolator key was removed.