High fan when external display attached

[Tex-Twil]

Hi,
I have a rMBP (15-inch, Late 2013) and when I attach an external monitor (1920x1200), the fans start to spin quite fast (4000 rpm on idle) which is quite annoying. The CPU itself is idle as well. 5 seconds after unplugging the external display, the fans drop back to 2000 rpm

Here are the temperatures with the external display connected

and here when it's disconnected:

Any idea what could cause that?
cheers

 

[Toutou]

An external display connected forces the MacBook to use the dedicated GPU (I assume from the readings that you have one), and even if you don't have one, the [integrated] GPU has to work harder to process more pixels, generating more heat...

But those higher temperatures aren't that high to trigger this kind of fan activity. Are you talking about an instant response? As in "you connect the display, the fans instantly jump to 4000 RPM"? Have you tried resetting the SMC?

 

[Tex-Twil]

An external display connected forces the MacBook to use the dedicated GPU (I assume from the readings that you have one), and even if you don't have one, the [integrated] GPU has to work harder to process more pixels, generating more heat...

But those higher temperatures aren't that high to trigger this kind of fan activity. Are you talking about an instant response? As in "you connect the display, the fans instantly jump to 4000 RPM"? Have you tried resetting the SMC?

Hi,
I understand that in theory the GPU could be more solicited but, as sou said, the temp readings don't seem to be too high. The fans do not "jump" instantly to 4000 rpm. It takes around 30-45 sec for them to start spinning. When I unplug the screen, they calm down after 5-10 sec

 

[v3nom]

Do you have the Nvidia GT 750M in your MacBook? If yes, the 750M isn't used most of the time and turned of. Attaching an external monitor will activate the 750M and produce more heat in your system.

 

[campyguy]

I've mentioned a minor part swap in a couple of other threads, depending on your set up, and I'll recommend it here. As an owner of the same Mac as the OP (with a dGPU), and with one or two Dell P2715Q displays attached and running, I switched my mDP>DP cable from the stock Dell cable to a Cable Matters cable and didn't see any difference. Then I switched to the only "certified" DP 1.2-compliant mDP>DP cable, made by Accell. I used to see the same temperature range as you have shown - my Mac runs MUCH cooler now, see my screen grab (using °C for a comparison) and you'll see for yourself. My Mac is also in clamshell mode, driving one of the Dells - I never have my fans rev since the switch to Accell cables...

FWIW, I used an expensive multimeter to determine that the Dell and Cable Matters cables were returning power back to the Mac on Pin 20, and my Apple TB and Accell cables were not returning power over Pin 20. DP cables should not be returning power back to Macs or PCs on Pin 20. Since switching to Accell DP-compliant cables on my Macs and PCs (I own a small company), ALL of our computers are running cooler...