Unable to connect usb-c monitor if not connected to power.

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Unable to connect usb-c monitor if not connected to power.

I have a weird problem with two Macbook Pro 15" (2017 and 2019) and a Dell P2719HC.

When the MacBook is not connected to the power supply, I am unable to connect the monitor. I have to try connect-disconnect for a lot of times and if I am lucky, it connects.

What I noticed, though, is that when I connect the MacBook to the power supply the monitor works immediately!

As this happens with two different MacBooks and given the fact that the monitor's usb-c cable is working as a power supply, I think that MacBook can not understand if the cable I connect is a power source or a monitor and choose the power source in most times. On the other side, when the power supply is connected, MacBook has not to think if the new cable is power supply so it gives signal right away.

MacBook Pro 15”, macOS 10.14

Posted on Jul 10, 2019 11:10 PM

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Answer 1

Lindsay_B.

Community Specialist

Jul 11, 2019 1:58 PM in response to NikolasCGR

Hi, Nikolas,

I understand you're using an external display with your MacBook devices and are wondering what is causing the displays not to work if the MacBook isn't connected to power. I'm happy to help!

It sounds like the USB-C cable you connect the display with is being used both to transfer the data to the external display, as well as to power it. Since there is no issue whatsoever while the MacBook power is connected, and the exact situation occurs with two different computers, it seems to me like the display draws too much power for the MacBook batteries to support both their own functions on the computers in addition to the display.

To rectify this, you would need to connect your MacBook to a power source when using the external display. I'd like to share this resource with you that will help if you ever run into an issue like this again. Get help with video issues on external displays connected to your Mac

Cheers.

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Unable to connect usb-c monitor if not connected to power.

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