Lolypop hi! I do not understand your proposal to check the problem.
In my measurements, all with original cables garmin and different gps, all working fine, I got the following results from the hidden USB Device menu:
1. In "USB Device": Power Cable and in "USB ID": 1A - 1.43 V (power connected)
2. USB Device: Serial Host (GTM36 and 25 cable) and USB ID:1A - 1.64 V (power connected)
3. USB Device: Type-B USB (USB PC cable) and USB ID:500mA - 2.25 V (PC connected)