What Garmin model? You may need a null-modem adapter in line with the connecting cable.The Scanner was set to the manual specifications of 4800 bps. The Garmin tests were done in the open - I was not in a contained building. I purchased the Garmin unit when I purchased the scanner. There was nothing between the antenna and the satellites. The format was the default of DDD MM SS ss. It was giving me a county name for the nearest city name. Then it gave me a city name that was west of my location by 15 miles. Then when I keep responding NO button, it was say it could not location use another method.