No image, Mag Safe ok, USB only works in 10-15sec, power ok

Hi.

I hope someone can help me.

My Apple Cinema Display won't turn on.

When Mag Safe is connected the fan turns on and the USB works for about 10-15sec. The USB turns off and then it turns on for 10-15sec. Keeps looping.

It won't be recognized by iMac or powerbook. It still can charge the powerbook.

I removed the LCD and disconnected all cables except the power cables from the logic board. The boards looks like new, no heat damage to see.

I measured the voltages on the Logic Board an they are all ok (i use the Apple Technicians Guide for help).

I then connected the USB and plugged in an memory stick and it works fine from the iMac. One by one I started to connect cables to the logic board. This to locate where the problem occur.

The fan runs full speed even with sensor 1+2 connected.

When connecting the Cinema Display LVDS LCD Cable MB382LL (LCD Function Interface cable) it went fine until it was connected to the Display. Then the problem reoccur. I tested the LCD Function Interface cable and it seems ok. The backlight cable was not connected to the logic board during test.

The miniDP cable was not connected to the LCD during test. So I don’t think the problem is the all in one cable.

Could it be a short circuit in the logic board or in the LCD?

How to test the LCD for short circuit?

Any suggestions?

In advance thanks.

Dan