TomTom screen dims when connected to USB power: solution for TomTom 50 & 60 (and probably others)
I've seen others suggesting that this is due to the car USB PSU lacking sufficient current capacity, with the consequent voltage drop causing the dimmed screen. This absolutely wasn't the cause in my case as I'd built a multi-USB output PSU capable of delivering several amps, but the problem was still there just the same. Funny thing was that the GO 50 in our other car, fed from a 5V LIDL cigarette lighter adapter (which originally powered a dashcam that packed up years ago) behaved perfectly, and when I connected the GO 60, it too worked perfectly (more of which later).
I'd also read that the display is actually DESIGNED to dim when the TomTom is connected to a computer, as computer USB connections typically won't supply more than around 500mA which may not be enough to charge a depleted TomTom battery, as well as allowing data transfer, but a dimmed screen may reduce the current demand enough to allow the TomTom to connect and be updated.
Upon opening up the LIDL PSU and taking a detailed look, I discovered that the USB data pins (2 & 3 on the connector) were actually linked together - not unreasonably, as clearly they're not needed for data transfer in the car. My home-built car USB power supply used a 4 way USB stack salvaged from an old PC motherboard, and where I'd cut around the PCB holding the connectors I'd effectively isolated all the data tracks. When I added simple solder bridges between pins 2 and 3 on each of the USB connectors, everything worked perfectly!
The question remaining is: why should this work, i.e. how does the TomTom distinguish a PC USB from a non-PC USB (like a power supply)? Well, it's perfectly possible that when the TomTom is powered up, it looks for any signals on data pins 2 and 3, and if it sees ANY activity it assumes it's a computer trying to communicate. My in-car 5V PSU is switchmode, and my oscilloscope showed noise clearly present on the open-circuit data wires, not surprising as they acted as an efficient aerial along the full length of the cable. Perhaps this activity is enough for the TomTom to decide it's connected to a computer, dim the display and try to connect, which of course won't happen because it isn't.
So if you have the same problem, consider opening up the PSU and if they're unconnected to anything, linking out the USB pins 2 and 3. Of course you do so at entirely your own risk (don't complain to me if your Lamborghini spontaneously combusts), but you could be in the happy state I am right now!