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I have a 13" 2011-model MacBook Pro running Lion that I connect to a Dell monitor.

The connection is made as follows: mini DisplayPort -> mini DisplayPort to DVI-D adapter -> DVI-D into monitor.

Everything works quite well graphically when I connect the laptop to the monitor, but it pretty much instantly causes the wifi to slow to a trickle.

This wifi trickling happens every time I connect to the external monitor. As soon as I unplug the connection to the external monitor, my wireless is restored to full power and I can connect to websites like this one.

Is there anything I can do to troubleshoot this very annoying problem?

Here's what happens when I switch from single monitor to dual while pinging 8.8.4.4:

64 bytes from 8.8.4.4: icmp_seq=14 ttl=56 time=30.070 ms
64 bytes from 8.8.4.4: icmp_seq=15 ttl=56 time=30.886 ms
64 bytes from 8.8.4.4: icmp_seq=16 ttl=56 time=31.343 ms
64 bytes from 8.8.4.4: icmp_seq=17 ttl=56 time=31.772 ms
64 bytes from 8.8.4.4: icmp_seq=18 ttl=56 time=30.662 ms
64 bytes from 8.8.4.4: icmp_seq=19 ttl=56 time=162.196 ms
Request timeout for icmp_seq 20
64 bytes from 8.8.4.4: icmp_seq=20 ttl=56 time=1071.753 ms
64 bytes from 8.8.4.4: icmp_seq=21 ttl=56 time=417.013 ms
64 bytes from 8.8.4.4: icmp_seq=22 ttl=56 time=478.626 ms
Request timeout for icmp_seq 24
64 bytes from 8.8.4.4: icmp_seq=25 ttl=56 time=367.057 ms
64 bytes from 8.8.4.4: icmp_seq=26 ttl=56 time=443.585 ms
Request timeout for icmp_seq 27
Request timeout for icmp_seq 28
64 bytes from 8.8.4.4: icmp_seq=29 ttl=56 time=365.072 ms

As you can see, soon as I switch to dual monitors, the return time drastically increases, sometimes timing out completely.

Any ideas?

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How does the wireless drop out? Does it turn off or does the connection simply drop? What does the WiFi menu bar icon look like (four bars, some black; for bars, all grey; or an empty quarter-circle)? –  CajunLuke Oct 18 '11 at 22:39
    
The WiFi menu bar icon still appears to be strong with all bars black. When I've turned WiFi off and then back on again, it sometimes restarts with an exclamation point in the middle before righting itself -- although never righting itself to the point where I get a proper internet connection. Sometimes I get data back from the internet very slowly -- most of the time I get no data. –  Sarhanis Oct 18 '11 at 22:43
    
Try ping 8.8.4.4 with the WiFi being strange and post the results as an edit to your question. –  CajunLuke Oct 18 '11 at 23:34
    
What make or brand of cable are you using? Did you try a different cable? What model Dell monitor (and resolution) are you using? I ended up using Griffin mini display port to HDMI for a Dell U2410 (at 1920 x 1200) because couldn't get mini display port to DVI-D to work. Apple store swapped out the adapters, without trouble. –  Jason S Oct 25 '11 at 0:36
1  
Holding Option while clicking the WiFi menu bar icon will give you a bit more info that might be useful. Does your RSSI (received signal strength indication) drop significantly, for example? –  gabedwrds Aug 11 '12 at 22:42

6 Answers 6

Looks like I accidentally stumbled across a solution: if I move my laptop an extra 30cms away from the monitor, the internet connection doesn't trickly away to nothing.

It's probably some kind of interference that is causing the problem, and it seems that keeping some distance between the monitor and the laptop is enough to resolve the issue.

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Switch your wireless access point to a different channel!

I had the exact same problem on a mid-2012 MacBook Pro with Mountain Lion. The Apple cable would give me errors on the external monitor, so I bought a 3rd party one that worked perfectly but killed my Wi-Fi connection. Switching the Wi-Fi channel from Auto to Channel 10 somehow solved this issue—works perfectly now! (Didn't figure this out myself, someone suggested it on some forum, but I don't remember who and where.)

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My 15" Macbook Pro Unibody used to drop all WiFi connections immediately when connected to my external monitor. I was ridiculous, I would have to disconnect my external monitor any time I needed to access the web. Fortunately, I found a solution.

Answer: Use an Apple-brand Mini-display-port to DVI adapter instead of a RocketFish brand adapter. Now I have wifi AND my external monitor, all at the same time.

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I have a similar problem. When I take my MacBook to the 1st floor of my house, the connection becomes very unreliable, extremely slow. If I try to disconnect and reconnect it fails to reconnect to the access point. The access point is placed in the basement, so the signal has to travel through 2 floors. Just as you, the wireless strength indicator indicates that I have a strong signal.

This used to work perfectly fine. But I can however see that there are now a lot more wireless networks (+10) than there used to be. So I assume that the problem is caused by interference from from my neighbors' wireless connection.

So perhaps if you can place the access point closer to your work desk, it could solve the problem.

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I had exactly the same issue with MBP Retina 13 and Dell 2408 DVI2mini-display and what helped here was to pull out the dvi-cable completely (as i tried to move away as far as possible from the monitor and then moved closer.) At the end I was 30-40cm away but with straight cable and now WiFi still works.

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Solution is: use original apple mini display port to DVI (or other) adapter to avoid losing wifi connection with macbook pro. (But this was not a problem before, so it seems to be some kind of software issue) Mars 30. 2013.

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