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MG MGA - Electrics question

I have just tried to take a reading of my battery using a multi meter, by putting the negative probe on the horn fuse connection and the positive probe on a nearby earth point (positive earth). The meter registered just half a volt. I then realised that i had forgotten to switch on my battery isolation switch which isolates the positive terminal of the battery from earth. With the isolation switch back on I had a healthy reading, but why would I get a reading with it off?
I assume it can only be because the isolator switch is faulty in which case I need to buy another one. Or is there some thing else going on?
Thanks
Graham M V

Hi Graham,

Do you have any devices connected to a permanent live, for example a modern car radio.

Regardless, I wouldn't worry about it too much. Remember volts are a measure of electrical "pressure", current (amps) is the measure of "flow". I would guess the current you are measuring is tiny (micro amperes).

It is quite possible to find a volt or so as a result of galvanic corrosion between two dissimilar metals somewhere on the electrical path you measured.

Paddy
P Reardon

Thanks Paddy,
I dont have anything with permanent live, but I am sure you are right as my multimeter didnt record any current, just the small voltage. There is some continuity between the + battery terminal and an earth point with the isolator breaking the circuit, so pretty sure that it is the isolator switch which is not 100%, but as you say. it really is nothing too much to worry about. Thanks for your input
Graham M V

That's not an uncommon thing in electrical troubleshooting. You could be picking up induction somewhere in the circuit. To be nitpicky, remember you aren't measuring current unless the meter is connected in series with the circuit or you have an ammeter clamp around the conductor (and your meter is in the Ampere position). If you are worried about the condition of your isolator, disconnect it completely and use your meter to measure resistance in the open, then closed, position. In the closed position, I would expect it should read somewhere between .2 and .6 ohms. If you read high resistance then change it. In the open position, it should read "open" or "infinite". No circuit. If not, I'd replace it. In any case concerning your original worry I think you're just picking up a phantom voltage and you can ignore it.
Mark J Michalak

This thread was discussed between 11/11/2011 and 14/11/2011

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