Model A1311 / Late 2009 / 3.06 or 3.33 GHz Core 2 Duo processor

197 Questions View all

How to Troubleshoot Logic Board

I changed my power supply twice. One green LED lit even after Power button is pressed. I unplugged power button from logic board and ohm'ed the button. OL at rest and 1.8 ohms when depressed. Momentary switch, button is good. How can I test the logic board or can it even be tested? Or do I have to change parts until it works?

Really need help with this, can't afford to change parts hoping it will work.

Answered! View the answer I have this problem too

Is this a good question?

Score 0
Add a comment

1 Answer

Chosen Solution

herb, just to reiterate you only have LED #1 coming on.

LED Functions

LED #1

• Indicates that trickle voltage from power supply has been detected by logic board. This LED will remain ON whenever the iMac is connected to a working AC power source. The LED will remain on even when computer has been shut down or put to sleep. The LED will turn off only if AC power source is disconnected or power supply is faulty.

LED #2

• Indicates that logic board has detected power from power supply when computer is turned on. This LED will be ON when computer is turned on 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 video card might be installed incorrectly or need replacement. Note: LED #3 is not present on models with integrated graphics.

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 LCD panel, LED backlight board, or cables between might be installed incorrectly or need replacement.

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

Here are some test points you can check.

Logic Board Test Points

Test points, which can be used to verify proper power flow, are accessible on logic board when LCD panel is removed. All voltages given in Troubleshooting Symptom Charts assume that computer is plugged into a known-good power outlet with a known-good AC cable. Some guidelines for using test points:

• Warning: HIGH VOLTAGE: Use extreme caution when live testing!

• Do NOT lean over or touch the power supply area during live testing.

• Keep your fingers behind finger guards on test probes when measuring.

• Turn dial of voltmeter/multimeter to measure DC (direct current, usually indicated by a solid horizontal line over dashes). If your voltmeter requires a set voltage range, choose a DC range that includes the voltage you are measuring.

Block Image

Logic Board Test Point Function

Pin 1 Ground

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

Pin 12 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

• Connect black probe to ground. Connect red probe to test point and verify voltage.

Let us know what you find out.

Was this answer helpful?

Score 1
Add a comment

Add your answer

herb will be eternally grateful.
View Statistics:

Past 24 Hours: 0

Past 7 Days: 3

Past 30 Days: 15

All Time: 322