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My first Garmin had the same type of behaviour as the new one also does, which is not a bug, but a Garmin specific festure: When the device is connected via a standard USB cable (to a computer or to a USB charger), it assumes it is in data mode. The device does not charge nor does the TMC work. The latter says, that the Antenna is not connected. However, there are only two cables going from the bulky 12V car adapter to the Mini USB plug. it took me a while to figure out the following:
The device consumes quite some power; especially when it is operating and charging at the same time. Add TMC functionality and it sucks 500+mA from the cable. However the USB 2.0 specifications allow only max. 500mA and the USB controller in your computer will probably get damaged by this current drain. Therefore the device needs to distinguish between data connection to a computer and power connection.
For that purpose, Garmin uses the ID pin 4, which usually is not needed, in a proprietary and special manner:
| Pin | Signal | Garmin data mode | Garmin use with power supply |
|---|---|---|---|
| 1 | +5V | +5V | +5V |
| 2 | D- | D- | short D- and D+ |
| 3 | D+ | D+ | short D+ and D- |
| 4 | ID | Not connected: data cable | short circuit with pin 5: USB supply 500mA 15-18kOhm connected to Pin 5: 1A ca. 22kOhm connected to Pin 5: 500mA |
| 5 | GND | GND | GND |
The short circuit bridge/resistor is built into the Garmin Mini USB plug.
So when you wnat to use another power supply, use 12V (plus 5V converter) right from your car's iginition line or want to have another solution and don't want to use original equipment, make sure to make the above modifications to the plug for full functionality.
