My HP Pavillion dv6700 is doing the exact same thing. Power on and it shuts down in 1 second; no LED codes, no beeps, no POST, no display - nothing. I can actually tear it down completely but have no way of determining whether it's the mobo or CPU. Fan begins to spin for about 1/2 second so I'm assuming power is okay (maybe - I know that doesn't measure voltage, etc.) Also took hard disk out, hooked it up to USB/SATA adapter and can see, read and write to disk. I noticed that the Power & Battery LEDs on front edge will light, but NOT the Drive LED.
Also, repair manual mentions 'Internal Display Switch' in or near the lid hinge. I was wondering if this switch was stuck CLOSED, so I tore down but can't identify it. Repair manual NOT clear on this at all.
Anybody?