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I have put a cutoff switch into the earth side of the lead. I will only need this is an emergency, but i can also it as an anti-theft device.

Now the problem, how do I set something up so the stero will not lose all the settings. I did think of a very low fuse in the earth side but I tried a 1 amp, but the engine keeps running when the switch is off.

Any other ideas?

Paul

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You will either need to isolate the radio fully so it has its own earth (which may be difficult) or put the isolator on the live side with a separate unswitched feed to your radio.

I did think of putting it in the live line, but this would not kill the engine when turned off. Running an earth to the radio would not be a problem, so that might be the way to go.

Paul

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A proper FIA switch would solve the engine running-on problem.

Most radios have a constant live and switched live supplies, i'd take the constant live straight to the battery, with a fuse.

If you keep the switch in the earth line, you'll have to earth the radio to the battery seperately.

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The reason cutting the live will not kill the engine is the altanator keeps outputting power with the engine running and keeps it going. By cutting the earth it kills the engine.

Paul

Not sure I see why this is any different as surely both alternator and fuel cut of share a common earth (the engine block)?

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Not sure I see why this is any different as surely both alternator and fuel cut of share a common earth (the engine block)?

I have just the one earth from the battery to the gear box, done know why, just that way when I got it. So cutting out the earth is easy and it stops/turns all the power off, stoping the engine. I can get it all working as I want it. Just need to do a new earth from the battery to the stero, easy but will just take me some time, I could just reture the stero every time :blink: .

Paul

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Please excuse if being thick, and I would be first to acknowledge my electrickery skills not high, but wouldn't it be better to kill the +ve rather than the earth since that would completely cut the supply.

If you cut the earth and somehow another earth was created by intention or accident then the whole system is immediately live. If on the other hand you kill the power supply then surely there is nothing that can complicate it.

Or am I missing some fundamental "truth" here?

Best wishes

Malcy

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the danger you have here is the extra earth you fit to the stereo will still act as a vehicle earth as the radio casing is metal and the antenna connection is usually earthed to the chassis (as well as the stereo itself possibly touching the chassis)

so with the switch off the truck will still run as the earth link will be created from the battery to the stereo then on thru the antenna braiding into the chassis

and you'll most likely melt lots of wiring should u accidentally try to start it with the big switch off as the above connection will _try_ to carry the current from the starter motor

the extra link can be fused to stop it melting should u try to start it, but the fuel stop solenoid uses less power than the stereo so if the fuse is big enough to run that its big enough to keep the solenoid open

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Paul, i understand what your saying about it "running on" if you kill the live. When putting a kill switch on the trialer the ALT would have enough power to supply the INJ pump for a short time, solved by putting the ALT wire on the battery side of the kill switch.

looks like you will have to run a single earth wire to the radio to keep its memory.

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Well in the end Idid not run another earth to the stero, Iwill just retune it if the fuse pops. I ran a short wire across the back of the cutoff fitted with a 3amp fuse. I need to get some smaller ones. But I think a 1amp should blow when needed, but stay ok to save the streo settings.

Paul

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The easy answer here is to do it properly!

Use an FIA switch with engine cutout & alternator protection contacts and cut the main +VE supply with a small fuse to keep the radio & alarm (if fitted) working. An FIA switch costs between £20-£25 and comes with full fitting instructions. They are easy to fit as apart from interupting the main +VE supply all you need to do is cut the fuel solenoid or coil supply (open when off contacts) and take the alternator output to a balast resistor (using the closed when off contacts) The resistor comes with the kit.

Just isolating the battery using a simple on/off battery switch will not kill the engine but it will harm the alternator and likewise, cutting the earth means the (fused) supply for the radio will support other circuits until it is overloaded.

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