start up problems

The diagnostic LEDs to the right of the RAM slots (this may be covered by a piece of black electrical tape).

LED Startup Sequence

LED #1 = Power available.

If no LED is visible:

• Verify AC source

• Verify known-good AC cable is connected

• Verify cable connection between AC inlet and power supply

• Verify cable connection between power supply and logic board

• Verify power supply

LED #1 + LED #2 = Power available, and system is powered on.

If second LED is not visible when power button is pressed:

• Verify power button connection to logic board

• Verify power button functionality

• Verify cable connection between power supply and logic board

• Verify power supply

• Verify logic board

LED #1 + LED #2 + LED #3 = Power available, system is powered on, and video card found.

If third LED is not visible after power on:

• Verify that MXM video card is seated properly

• Verify logic board

If no lights are coming on you need to replace your power supply. A trickle charge is sent from the power supply to the first LED. If the first LED is not lit the power supply is not working. If the power supply is weak and can't sustain the system, the LED may be lit but unusually goes out with attempts to power on the machine.

Power up is the hardest thing on any of the components in a computer - RAM, CPU, etc... are pushed to their max and pulling more than operational current. The symptoms you are giving are of a weak, near dead power supply.

It is going to give off all the proper voltages on the pin out, as it is weak - not dead. The only thing it can't do is pass a current load draw test. You could do that by installing it in another iMac to see if it gives off the same symptoms or you could replace the PSU.