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I've got an Audi TT MK2 2.0 TFSI which I'm preparing for track use. I want to fit a battery isolator switch as it will be sat not being used for long periods of time plus I think it's sensible for safety too in the event of a bad crash to be able to quickly turn off all the electrics. I bought by a switch like this one but off eBay:

https://www.halfords.com/motoring/battery-maintenance/battery-accessories/halfords-battery-isolator-switch-hef390-869552.html

The car battery is in the boot and the negative cable runs from the negative terminal of the battery and bolts to the chassis under where the back seats used to be. So I unbolted the cable from the chassis and bolted it to one of the terminals on the switch instead and then ran another piece of battery cable from the other terminal on the switch to the chassis. So the negative cable runs to the exact same place it just has the switch on it now.

That all seemed to be good, it was turning the electrics on and off just like I wanted but then when I went to start the car there was a pop sound and all the electrics died. I took the switch back off and put everything back to how it was pre switch and the car came back to life again.

I thought maybe I just had a dodgy switch that couldn't handle the amps or something as it was just a cheap one off eBay so I bought another off Amazon that said it was rated up to 400amps. Fitted this, went to start the car and that one popped too.

I took one of the popped switches apart to see what was going on inside and see what I had blown but all looks absolutely fine inside. I thought maybe there was a fuse in there or something but there isn't. It's a very basic device there are just the two terminals and a piece of metal that is held in place by a spring. When you put the key in it presses the spring down causes the piece of metal to make contact with the two terminals and complete the circuit. The piece of metal was still intact, no brakes, burns or anything the whole thing looked totally clean inside. Once they've popped though they will not work again which seems odd since I can see it is making a circuit still.

Anyone have any idea what's going on and where I might be going wrong?

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  • Is the new wire from the switch to the earth thick enough? And intact as well? It sounds like too many amps to crank the engine so must have burnt something out - can you bolt the switch down? Or take a picture of the insides? – RemarkLima May 18 at 22:47
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Ahh ok so I finally found out the problem. So from looking inside one of the supposedly 'blown' ones it just didn't make sense that it wouldn't work as nothing looked like it had been fried. So I connected it back up again without putting the case back on and just used a plastic clamp to compress the spring down and complete the circuit. Low and behold it worked fine!

I think what happened was because I was mounting it in quite a tight location I had to yank it around loads to get it in place and screw it down to hold it in position. I think this bent it out of shape a bit and stopped it working. I've now mounted it somewhere where it sits easily and it's working fine :-)

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OK, a quick Google and thinking back to my race car, those switches are usually switched on the live side with some additional wiring.

There's an article here: Battery Isolator switch

Then I was thinking about my wife's classic which has a screw isolator on the negative earth strap like this:

https://smile.amazon.co.uk/dp/B07J34S5DH/ref=cm_sw_r_em_apa_i_.hXWEbCMZRJ4B

If you want to isolate the earth I think you will need a screw type isolation switch.

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