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Modèle A1312 / Mid-2011 / processeur 2.7 & 3.1 GHz Core i5 ou 3.4 GHz Core i7, ID iMac12,2

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Why don't I have the correct voltages @ power supply or logic board?

I have purchased a new power supply and logic board for Imac EMC#: 2429, with still the same results. Led 1 lights, led 2 does not when power button is pressed. Power button checks good with continuity check. I have 120 volts into power supply. Pin 4 @ PSU is 12 volts, the only other voltage is 4 volts at Pin 6. Verified voltages @ Logic board, same pins measuring the same. When I press the power button I should be getting voltage on pin 11, and led 2 should light? Nothing, is the PSU, I purchased bad? Any answers will be appreciated!

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Let's just take a step back here. What caused you to buy a replacement power supply and a logic board in the first place

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@jbouvman first we’ll add some general information.

Resetting the System Management Controller (SMC)

The System Management Controller (SMC) is a chip on logic board that controls all power functions. If computer is experiencing any power issue, such as not starting up, not displaying video, sleep issues, or fan noise issues, resetting SMC may resolve it. To reset SMC on an iMac (27-inch, Mid 2011):

1. From the Apple () menu, choose Shut Down (or if the computer is not responding, hold power button for approximately ten seconds until it powers off).

2. Unplug the computer’s power cord.

3. Wait 15 seconds.

4. Attach the computer’s power cord, making sure power button is not being pressed.

Note: If you press the power button while inserting the power cord, the iMac will enter a mode in which the fans run at full speed.

5. Wait 5 seconds, then press the power button to turn on the computer.

Resetting Parameter RAM (PRAM)

PRAM stores certain system and device settings in a location that OS X can access quickly.

Exactly which settings are stored in the computer’s PRAM varies depending on the type of computer as well as the types of devices and drives connected. To reset PRAM:

1. Shut down the computer.

2. Locate the following keys on keyboard: Command, Option (Alt), P, and R. You will need to hold these keys down simultaneously in Step 4.

3. Press power button.

4. Immediately press and hold Command-Option-P-R keys.

Important: You must press this key combination before the gray screen appears.

5. Hold down keys until the computer restarts, and you hear the startup chime a second time.

6. Release keys.

LED Functions

LED #1

• Indicates that the trickle voltage from power supply has been detected by main logic board.

This LED will turn ON when you connect the iMac to a working AC power source. The LED will remain ON as long as the computer is ON or asleep.

• When computer has been correctly shutdown, the LED#1 behavior may differ:

• If a startup event is scheduled in System Preferences/Energy saver, LED#1 will stay ON after a correct shutdown,

• If no startup event is scheduled in System Preferences/Energy saver, LED#1 will turn OFF and will stay OFF as long as power cord is kept connected and AC power source is present.

Disconnecting the power cord and plugging it back will turn back this LED ON , even if computer is still off.

• After disconnecting and reconnecting the AC power source, this LED could remain OFF if the AC power source is missing or disconnected , if the logic board is disconnected from the power supply or from the AC receptacle, or if the power supply board is faulty.

LED #2

• Indicates that computer is turned on. This LED will be ON as long as computer is turned on (but is not asleep) and power supply and voltage regulators are working correctly.

LED #3

• Indicates that computer and video card are communicating. This LED will be ON when computer is communicating properly with video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the backup battery (on back of logic board) may need to be reseated, or the video card might be installed incorrectly or needs replacement.

LED #4

• Indicates that computer and LCD panel are communicating. This LED will be ON when computer is turned on and video signal is being generated. If LED#4 is ON and there is no image on display, then the LCD panel, the LED backlight board, or the cables between LCD and logic board or backlight board might be installed incorrectly, or need replacement.

Now a bit more in depth:

LED Startup Sequence

LED #1 = Power available.

If no LED is visible:

• First disconnect the power cord from computer, then reconnect it to reset the LED status, in case computer was correctly shutdown and AC was kept connected since

• Verify AC source

• Verify known-good power cord 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 the MXM video card is seated properly

• Verify if boot chime is present and fans are running when powered ON (reset SMC and PRAM, reseat or check backup battery if necessary for proper boot up):

If POST boot chime is not heard, go to No Startup symptom flow,

If POST boot chime is heard, go to No Video symptom flow.

LED #1 + LED #2 + LED #3 + LED #4 = Power available, system is powered on, video card

found, and internal LCD found.

If fourth LED is not visible after power on:

• Verify internal DisplayPort cable connections between LCD panel and logic board

• Inspect LCD display cables for cable damage

• Verify external video functionality, and according to result check the following items:

If external display works then verify/replace the LED backlight board

If external display works then verify/replace the LCD panel

If external display does not work verify/replace the logic board

Here is what you definitely want to check:

No Power

Unlikely cause: speakers

Quick Check

No Power

Symptoms

• No power

• No fan spin

• No startup sound

• No image on external display

• No hard drive or optical drive activity

• Caps Lock LED on wired keyboard doesn’t light when pressed.

Quick Check

1. Verify power source.

2. Verify power cord.

3. Listen closely for signs of activity from system including: rotating fans, hard drive or optical drive activity, startup sound, etc.

4. Disconnect all peripherals.

5. Reset SMC.

Deep Dive

1. Attempt normal startup. Press power button to start up computer. Does the computer exhibit any signs of power activity, such as fan or hard drive activity, startup sound or Caps Lock LED on wired keyboard?

Yes Go to step 2.

No Go to step 4.

2. Verify display functionality. Confirm whether a video signal appears on the display. Is a video image clearly visible on the display?

Yes Run Mac Resource Inspector (MRI) to obtain latest test results. Redirect to appropriate symptom.

No Go to step 3.

3. Verify video without backlight. On a display with dim or no backlight, shine an LED flashlight through the glass on the front of the display to check for video signal. Is any video visible with flashlight?

Yes Go to Backlight Issue / No Backlight symptom flow.

No Go to No Video symptom flow.

4. Disconnect power cord. Carefully inspect the connectors between AC inlet and power supply, then DC connectors and cables between power supply and logic board, for damage or poor connections.

Yes Reseat misconnected cable(s), or replace damaged one(s).

No Replace DC power cable.

Go to step 8.

5. Inspect power button and verify if it is properly connected to the logic board.

Yes Go to step 6.

No Reconnect power button.

6. Disconnect power button from logic board. Inspect cable and connector for damage. Using a multimeter (set as ohmmeter), verify continuity between the two pins of the power button when it is pressed.

Yes Power button is functioning correctly. Reconnect power button to logic board and go to step 7.

No Power button faulty. Replace rear housing (which includes power button).

7. Reconnect power cord. Press power button and verify if system turns ON. You may alternately check for diagnostic LED # 2 to illuminate when the power button is pressed. See the section on Diagnostic LEDs for more information.

Yes Power supply functioning. If computer still won’t boot, go to No Startup.

No Replace DC power cable.

Go to step 8.

8. Press power button and verify if computer turns ON.

Yes Issue solved with replaced DC power cable.

No Replace power supply.

Go to step 9.

9. Press power button and verify if computer turns ON.

Yes Issue solved with replaced power supply.

No Reinstall power supply and replace logic board.

If No Power symptom persists after all steps have been followed use minimum configuration troubleshooting to proceed. Try disconnecting hard drive, optical drive, AirPort card, SD card reader and Bluetooth/camera/sensor cable to determine if one of them is preventing the power supply from functioning.

Block Image


Pin 1 Ground

Pin 3 5V power for Hard Drive(s). Note: If present, the 12V power will come directly from the Power Supply to the Hard Drive thru the power harness cable.

Pin 4 Standby 12V power (permanent power coming from power supply and present as long as AC cable is connected, even if computer is off); corresponds to LED #1

Pin 6 Backlight Control pulse width modulated signal (from logic board to LED backlight board, to adjust the backlight level setting according to user setup)

Pin 11 12V Run-Mode power to logic board (coming from power supply, present as long as system is on or asleep); corresponds to LED #2

Pin12 Power On Request signal (from logic board to power supply when power button is pressed)

Pin 13 Backlight Enable (signal from logic board to Backlight Controller board, to enable backlight

Press power button and Verify if system turns ON. You may alternately check for presence of a 12V DC signal between pin 11 and pin 1 of logic board when power button is pressed.

Let us know what you get when you try this out or if you get stuck anywhere.

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Jerry sera éternellement reconnaissant.
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