General Wiring and Troubleshooting Guide
General Wiring and Troubleshooting Guide for
PMT Vehicle Docks
Applies To All Powered Vehicle Docks and Cradles
Recommended Wiring Technique
1. PMT recommends hard-wiring all power adapters into the vehicles power
system. This means that if the adapter for your device or vehicle dock came
with a cigarette socket plug on it’s input cable, it should not be used.
Instead, cut the cigarette socket adapter end off of the cable, then strip the
bare wire and use crimp connectors to properly and reliably interface power
and ground into the vehicles power system.
2. When adding peripherals such as video systems and USB printers, etc to a vehicle docking station,
PMT recommends that all devices be connected to common sources for both power and ground
connections. It is also preferential to use a ground source that is connected directly to the negative
terminal on the vehicle battery. Refer to figure 1a on the following page.
Note: USB is susceptible to power and ground loops. This is due to active power coming from the
computer dock via the USB cable (+5v Red and 0v black).
If the peripheral (such as a printer or video system) has it’s own power supply there can be an offset
in the +5v and 0v on the USB coming out of the peripheral, and this can cause problems with
equipment and connectivity. To help minimize potential problems, PMT recommends wiring in a
dedicated ground wire from a suitable location on the vehicle dock (vacant screw hole), to the base
of the mount that it is connected to.
Figure 1a
Docking Station
USB cable
Power adaptor
USB Peripheral
(printer, scanner, etc)
Ground at
base of
- negative
+ positive
Fused negative
terminal strip
Fused positive
terminal strip
- negative
+ positive
Vehicle Battery (12vdc)
If you are unsure of potential ground and power loops within your vehicles wiring, and you are
having trouble with USB devices, follow the below procedure to identify and eliminate any
potential ground or power loop problems. Refer to figure 2a and 2b on the following page.
Step 1: Cut a USB extension cable (male to female) in half and strip both ends of the power and
ground wires (red and black) so that you can connect a volt meter to get readings. (The green and
white data lines are not needed for this procedure).
Step 2: Plug the male end of the cut extension cable into the vehicle dock (with computer docked)
being careful not to let the red positive wire touch any metal surface as it will be live with 5 volts.
Step 3: Connect the female end of the cut extension cable to the peripheral.
Step 4: Connect the volt meter to the leads to be tested and power up the system. Dock-computerperipheral
Step 5: Measure the DC voltage
1. Red to red (should be almost 0 volts)
2. Black to black (should be almost 0 volts)
3. Red to black on dock side (should be about 5 volts)
4. Red to black on peripheral side (should be almost 0 volts)
Step 6: If the voltages are not as above please check your power system for incorrect wiring and
possible power and ground loops.
Figure 2a
Male to female USB extension cable
Figure 2b
Female end to
Black -
Red +
Male end to
vehicle dock
Troubleshooting Continued
Another potential cause of USB related problems comes from improper or missing drivers installed on the
host computer. Check on the computers device manager section in the control panel in Windows based
computers for anything that is flagged with a yellow exclamation point. Try uninstalling then reinstalling the
correct driver for your device.
You may need to manually force your computer to check for updates through windows update to ensure
that all necessary drivers are installed.
USB Port Limitations:
A common miss-conception is that all USB ports are the same and can power any device. This is wrong
and can lead to a dead port if too many high current draw devices are connected to it. Currently the
circuit boards in vehicle docks can only support 250mA on USB2.0 and 500mA on USB 3.0 of current draw
per port which is not enough to charge many of todays high powered smart phones and tablets. There are
dedicated USB “charging” ports on many computers (see figure 3a below) and these are usually labeled
differently than normal ports, however vehicle docks are not meant to provide charging current. PMT
does offer solutions for this in the form of optional dedicated USB charging ports that are available as
accessories from us and charge at up to 1000mA.
Figure 3a
Non-charging USB port
Charging USB Port
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