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Had a student who wired up a cig. lighter on his Sparrowhawk dash. He picked up the ign. switch feed in, passed it through a 3 amp fuse, and to the cig lighter. When I transmitted on the radio at idle all was well, but to our supprise, when i transmitted on finals, the engine went dead. It took a while to realise that the engine fault was linked to the transmitter. The funny thing was as soon as the fuse was removed all was well, even with nothing plugged in to the socket. The antenna plate shared the same earth wire as the airframe, socket, digital dash and vhf radio. It seemed that a high frequency energy was making its way to the computer and shutting down the feed to the fuel pump relay. The socket will be wired with a twin cable straight to the battery.