Model A1225 / Mid 2007 and Early 2008 / 2.4, 2.8, or 3.06 GHz Core 2 Duo processor

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Revision of iMac Mid 2007 strats up screen goes black (LED 4)

Hi All Repairmen,

Can you help me solve this issue ?

After reapplying thé CPU and GPU heatsinks, thé CPU fan runs wild, after a few seconden when after starting the iMac.

LED 4 (LCD) goes OFF after 2 minutes approximately.

I did already reset the SMC and PRAM.

Will reinstalling MacOSX on a new harddisk and original disks help ?

I will be glad hearing from you.

Update

When I ran ASD 3S116 the GPU Proximity Sensor test gives an error TEST FAILED.

Is it possible that the heatsink assembly is broken ?

Another tought , I applied a thermal pad over the two memorychips in one peace.

Maybe this way the GPU Proximity Sensor can't generate a readout ?

Update

When started I have video signal on the screen. After 2 minutes it goes black. The LED 4 goes OFF.

I checked the thermal sensors and they are installed correctly.

Maybe the LCD thermal sensor is disconnected or is covered with dust ?

I blew out a lot of dust under the plastic wrapping on the backside of the LCD panel.

Maybe you can help me further ?

Update

New developments, replaced the GPU Heatsink with an other videocard ATI HD 2600 Pro:

Problem with ASD testing: GPU 0 Die Test 1

Error: Sensor reading above alarm point.

Block Image

Replace the ATI HD 2600 Pro videochip ?

Answered! View the answer I have this problem too

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Chosen Solution

LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.

The run away fan is most likely being caused by a disconnected thermal sensor. The sensor on the GPU (back of logic board are the most common ones to miss.

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