There is a 17k-ohm resistor tied between ground and the normally unused pin of the mini-USB connector. This is how the Garmin detects if you are using a Garmin auto charger. Generic chargers will charge, but the Garmin will go into disk mode rather than navigation mode.
Garmin Nuvi GPS Power Connector pinout
Wiring diagram pinout to place Garmin Nuvi, Oregon and GPSMAP 62 GPS in recharge mode.
Standard 5 pin mini-USB connector has pin X connect to ground (pin 4) through a 200K ohm resistor (Note: The normal USB connector has only 4 pins, the mini-USB has 5 pins). To place Nuvi into charging mode, the 200K ohm resistor must be changed to 17.3K ohm. (also successful with 18k and 22K ohm resistor from Radio Shack, also a cheap 1/8W, 5%, 15k ohm resistor will work just fine!)
Nuvi behavior depends on mini-USB plug connection. (IMHO) it is like this:
pin 4 not connected – data cable, nuvi connects to PC as removable drive, navigation NOT possible.
pin 4 & 5 short-circuited – USB supply 500mA, possible navigation,
pin 4 & 5 connected through 18kOhm resistor – Garmin supply 1A, possible navigation.
pin 4 & 5 connected through 22kOhm resistor – Garmin supply 500mA, possible navigation.
I personally modified a cable that connected pin X to pin 4 (shorted), and verified the 500 mA charge behavior as noted above.
I simply waded up some aluminum foil and stuck it in the connector so that pins x and 4 were shorted. Works.
Nuvi requires 1000mA for simultaneous charging + GPS operation [many ‘generic’ 12VDC-to-USB adapters output a maximum 500mA]
Garmin Oregon requires a resistor of at least 47K ohm for running in external power supply mode (100K ohm also works). Smaller values are working, too, but Oregon then first shows a message that an unsupported accessory was found.
On Nuvi 670 add 18k resistor between pins 4 & 5 – and it doesn’t go into mass storage mode.
pin 4 not connected – data cable, GPSMAP 62 connects to PC as removable drive.
If it fails because it is connected to your car battery, it switches to external power supply mode after several secondes.
pin 4 & 5 connected through 47 k Ohm resistor – GPSMAP 62 switches to external power supply mode immediately without waiting.