Furmark changes Fan-Speed

[crizon]

Hi,

I've just bought a sapphire radeon HD4850 dual fan sink with latest Catalyst drivers. Now i wanted to test the GPU a bit and found furmark. I read that furmark stresses GPU's like no other program so i figured that when my card passes the stress test, I should be fine with all other applications.

The problem is: Somehow Furmark lowers the fan-speed dramatically, when it's running. I can hear the change very clearly. Furmark manages to get the GPU to >80° very quickly, but again the fan is running very slowly as soon as the test starts.
Same thing happens when i manually set the fan-speed to 100%. As soon as i start the furmark benchmark or stresstest the fan slows down. When the test is finished the fan instantly goes to 100% speed.
The GPU seems to manage other applications very well. After 1,5h of Crysis the temprature was at ~72° (and the fan doesn't even run at 100% all the time).

I obivously can't complete the test with that bug. Is that problem known?

[JeGX]

There's nothing in FurMark that can change the fan's speed. Maybe it's a driver issue or bug in the bios of your videocard. Have you posted on overclocker/hardware forums about this problem. I believe this problem is known. Let me know if you find the solution.

[rogalik]

My 4870's fan also slows down but a little, I think that too much stress is put on VRM and it can't provide enough power to the fan. It pwers the rest of the graphics card instead. BUT I'm not an electrician and can be wrong