Morris Minor Owners Club

A bit of advice please regards Positive earth wiring?.

I have nicked the nice old Motorola radio out of my daughters Minor to put in my Alfa 1750, but I have noticed that it is designed for a Positive Earth car, like the Minor as you would expect.

Exactly what does this mean, for instance, if I wire the positive live into it, but use a negative earth, will it cause a short circuit, blow up the world, will the music play backwards, or will just work as normal?

Equally I will at some point need to wire up a 'normal' negative earth Radio/CD unit into the positive earth Minor, what are the issues with that little conundram?. I don't fancy changing the whole wiring system over but I noticed one post on here that seemed to hint i might have too

Hello Vm,

radios, like many, if not all, electronic devices do not like being connected with the wrong polarity and doing so damages them.

The best practical chouice is to change the vehicle polarity, your Alfa being much more difficult than the Minor, I would suspect.


Alec

Thanks Alec, it doesn't sound good then. Changing the polarity is not an option for either, certainly not the Alfa. I suppose if I isolate it from the car I could wire it with a positive to the casing, but if it touches anything metallic it could get interesting quite quickly couldn't it

So if the idea of putting it in the Alfa is out of the window, what's the score with fitting a modern unit in the Minor, are the casings on them 'live' but negative, or are they isolated within themselves?.

Hello Vm,

that's the point, the metal case is the return path so needs to be isolated then it can be connect to suit the radio's polarity. So in the case of a positive earth radio in a negative earth car, the 'live' positive connects to the radio case and the earth negative to the line connection. Also remember that the aerial will need to be isolated, as the outer sheath and mounting connects to the radio case.


Alec

Yes I see, it gets worse doesn't it, I hadn't thought about the Aerial.

It doesn't sound to be very practical to fit a modern unit into the Minor without altering the polarity then. Is this the case, or am I missing something?.

But then Minors are easily changable from +ve to -ve earth.

Yup, changing the Minor to Neg earth is very, very easy - and free. Here's how - http://www.mmoc.org.uk/technicaltips/Ne ... Earth.html

Brilliant, that sounds easy enough, the other good news is the Motorola can be modified to negative earth by just switching four wires across inside, it's labelled to make it easy for nuppit's like me!.

So we are all happy, Daughter can have her FM/CD mega base whoppa thingy and I can have pretty little period radio in mine now.

I love it when a plan comes together

I had a motorola to fix in fifi and it had a switch on the back that allowed you to choose polarity. But the thing was dead in the end so we ended up switching fifi's polarity...