ASUS M5A97 LE R2.0 won't start with 2 HD6870 in crossfire

[Simonj_93]

MOBO: ASUS M5A97 LE R2.0
CPU: AMD Phenom 2 965
RAM: Corsair 2x2x4GB
PSU: Chieftec 750W
GPU: 2x Radeon HD 6870

The system works fine with only one graphics card installed but when i shut it down and plug in the second card it won't even post.

The motherboard is brand new. The ram, the cpu and the graphics cards i bought from a friend. Each card works individually but not in crossfire.

I'm quite new to the crossfire thing so please help.

 

[Phillip Corcoran]

Could be the PSU - - wattage okay but may not have enough amps on the 12v rails for two 6870's.

See post from ko888 in this thread: http://www.tomshardware.co.uk/forum/346394-33-does-power-6870-crossfire

Check the amps rating for the +12v line on the rating plate of your PSU.

 

[Shaun o]

A phenom 2 965 requires a motherboard capable of providing 140W of power to the cpu socket when your 965 cpu is at full or heavy load.
One HD ATI 6870 will draw current under set conditions of.

Measured power consumption - 6870

Advertised TDP = 150W
System in IDLE = 173W
System Wattage with GPU in FULL Stress = 295W 12v amp required 34A
Difference (GPU load) = 122 W
Add average IDLE wattage ~ 19W
Subjective measured GPU power consumption = ~ 141 Watts

Then add what wattage all of the other bits of hardware consume or require in voltage and amps.
Hard drives ect, Dvd drives. Add in sound cards ect.

Either you do not have enough wattage, or the amount of amps stated on the Chieftec PSU for the 12v power rails is not high enough to drive two 6870 cards in a crossfire configuration due to a lack of Amps supplied on the 12v power rails of the psu.

Have a look to see what it states on the Psu the maximum amp output is from each 12 power rail.
Or the combined 12v power rail amp rating.

 

[Simonj_93]

But shouldn't the computer atleast start with 2 graphics cards in, even if the PSU isn't enough.
Because when i push the start button nothing happens.