Troubleshooting no video issue

Hi all,

There are many threads on this specific issue, but alas not too many solutions.

Situation is as follows:

iMac as stated, no video. External monitor does work.

Nothing on lcd, not even very faint.

I have tried to replace the lcd, but no dice.

I have measured the contacts on the driver board that are supposed to drive the back light leds of the lcd and none of them have any voltage over them.

When measuring other components on the driver board, I measure 12V, so it's not dead.

I then delved into the service manual of this iMac and measured the voltage of the test pins on the motherboard. Pin 4 and 13 (to test lcd related functions) give 0 Volt, all other pins have the correct voltage.

According to the service manual I should replace GPU first and if that doesn't solve the problem, the motherboard.

Am I correct in presuming that the GPU is cleared since external monitor is working, or could this still be a GPU-related issue?

Funnily enough during the troubleshooting flow in the service manual they do use external video working/not working as a condition for diagnosing, but not to differentiate at the end between GPU and motherboard.

So to me it's unclear at the moment if my tests clear the GPU completely or not.

I also have to mention that of the 4 test leds only the first 2 light up. Nr 3 and 4 do not. Led nr 3 is supposed to light up when there is communication with the GPU. But since the test is performed without an lcd, possibly the led isn't on because of that?

Should it go on when I connect an external display (which works)?

My gut feeling says it's the motherboard that's faulty, but I don't really have the luxury to order both parts.

Any insight much appreciated.