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Tutorials Diagnostic LEDs and Test Pads for iMac (Retina 5K, 27-inch, 2019)

Jimmie

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Diagnostic LEDs and Test Pads for iMac (Retina 5K, 27-inch, 2019)

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Warning: HIGH VOLTAGE. Be extremely careful when working inside the computer while power is applied and the system is energized. Avoid touching the logic board or power supply while the computer is plugged in. Be very careful not to touch tools to logic board components other than the test pads.
After unplugging the computer from the electrical outlet, wait two minutes before removing the display panel, disconnecting modules, or substituting cables and components. This will allow

the power supply and logic board time to discharge.

The iMac (Retina 5K, 27-inch, 2019) model has five diagnostic LEDs (1) and two pairs of test pads (2 and 3) that can help you troubleshoot the computer without removing the logic board.
The coin battery, located on the back of the logic board, provides power to the real-time clock (RTC) and nonvolatile random-access memory (NVRAM) when the computer is not connected to an AC power source. The RTC maintains the date and time, while the NVRAM stores information such as speaker volume, screen resolution, startup disk selection, and recent kernel panics. The coin battery is designed to last several years and does not normally require replacement. However, if there are issues with the functions listed above, the RTC and NVRAM may need to be reset or the coin battery may need to be replaced.
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1-Diagnostic LEDs
See the full description of LED behaviors at the bottom of this article.

2-Coin Battery Voltage Test Pads
  • Shut down and unplug the iMac. Allow approximately two minutes for the power supply to discharge.
  • Measure the coin battery voltage by using a voltmeter set for DC. Place the probes on the pads. If the voltage is 2.7 volts DC or less, then the coin battery should be replaced.
  • (–) negative (same as chassis ground)
  • (+) positive
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3-Real-Time Clock (RTC) Reset Pads iMac (Retina 5K, 27-inch, 2019)
  • Shut down and unplug the iMac. Allow approximately two minutes for the power supply to discharge.
  • Reset the RTC by shorting the pads in location C, shown below. Use the tip of a flat-blade screwdriver to touch both pads at the same time.
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Caution: Do not make contact with any of the surrounding components or traces while performing this procedure. Some of the smaller components can be broken off very easily. Physically damaged boards warrant a logic board replacement.
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Diagnostic LEDs

LED 1
Indicates that the trickle voltage from the power supply has been detected by the 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 the computer has been shut down correctly, LED 1 behavior may differ:
  • If a startup event is scheduled in System Preferences/Energy Saver, then LED 1 will stay on after a correct shutdown.
  • If no startup event is scheduled in System Preferences/Energy Saver, then LED 1 will turn off and will stay off as long as the power cord is kept connected and an AC power source is present. Disconnecting the power cord and plugging it back in will turn this LED back on, even if the 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 the AC receptacle.
  • If the power supply board is faulty.
LED 2
  • Indicates that the computer is turned on. This LED will be on as long as the computer is turned on (but is not asleep) and the power supply and voltage regulators are working correctly.
LED 3
  • Indicates that the logic board and GPU are communicating. This LED will be on when the CPU is communicating properly with the GPU. If LEDs 1 and 2 are on and LED 3 is off, then the backup battery (on the back of the logic board) may need to be reseated or the logic board may need replacement.
LED 4
  • Indicates that the logic board and LCD panel are communicating. This LED will be on when the computer is turned on and a video signal is being generated. If LED 4 is on and there is no image on the display, then the LCD panel or the cables between the LCD and logic board might be installed incorrectly or need replacement.
LED 5
  • Indicates that the logic board and LCD panel are communicating. This LED is on when the computer is turned on, a video signal is being generated, and the LCD Panel is signaling to turn on the backlight. If LED 5 is ON and there is no image on the display, then the LCD backlight or circuitry on the logic board may be malfunctioning. The LCD panel or the cables or the logic board may need replacement.
LED Startup Sequence

LED 1 = Power is available.
If no LED is visible:
  • Disconnect the power cord from the computer and wait 15 seconds to reset the power supply and LED status. Reconnect the power cord and check the LED status again.
  • Verify the AC source.
  • Verify that a known-good power cord is connected.
  • Verify the cable connection between the AC inlet and the power supply.
  • Verify the cable connection between the power supply and the logic board.
  • Verify the power supply.
LED 1 + LED 2 = Power is available and the system is turned on.
If the second LED is not visible when the power button is pressed:
  • Verify that the power button is connected to the power supply.
  • Verify power button functionality.
  • Verify the cable connection between the power supply and the logic board.
  • Verify the power supply.
  • Verify the logic board.
LED 1 + LED 2 + LED 3 = Power is available, the system is turned on, and the GPU was found.
If the third LED is not visible after the system is turned on:

Verify whether the fan is running when turned on (reset SMC and NVRAM, verify backup battery voltage for proper startup).
If the fan is not heard, go to the "No Startup" troubleshooting flow.

LED 1 + LED 2 + LED 3 + LED 4 = Power is available, the system is turned on, the logic board is communicating with the GPU, and the internal LCD was found.
If the fourth LED is not visible after the system is turned on:
  • Verify the embedded DisplayPort cable (eDP) connections between the LCD panel and the logic board.
  • Inspect the LCD display cables for cable damage.
  • Verify external video functionality and proceed according to the result:
  • If an external display works, verify/replace the LCD panel.
  • If an external display does not work, verify/replace the logic board.
LED 1 + LED 2 + LED 3 + LED 4 + LED 5 = Power is available, system is turned on, GPU is working, logic board is communicating with the LCD panel, and the LCD panel has turned on the LCD backlight.
If the fifth LED is not visible after the computer is turned on:
  • Verify the eDP cable and backlight cable connections between the LCD panel and the logic board.
  • Inspect the LCD display cables for cable damage.
  • Using a flashlight, check the following areas on the panel:
  1. Look in the top left corner of the panel to see if there is a dim Apple logo (the Apple menu logo).
  2. Look in the center of the panel to see if there is a login screen.
  3. Look at the bottom of the panel to see if there is a Dock visible.
  4. If the following items are visible, there may be something wrong with the display cables or the logic board. Proceed to the next step.
  5. Replace the LCD panel, check that all the LEDs are visible, and that the backlight functions. If the computer does not function after replacing the LCD panel, replace the logic board.
End of content. Hope this article is helpful.
 

AEVSE

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Do you have an idea when LED1 is not working LED 2 and 3 are working. LED4 +5 are not working.
The fan is spinning, I already changed the battery because the voltage was 2.8V.
 

miepstei

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Do you have an idea when LED1 is not working LED 2 and 3 are working. LED4 +5 are not working.
The fan is spinning, I already changed the battery because the voltage was 2.8V.
Hi - did you manage to figure this out?
 

kuly-s

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Hi - did you manage to figure this out? same problem - after a power surge? What is the problem?
 
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