Resetting the PMU on the Logic Board

The PMU (Power Management Unit) is a microcontroller chip that controls all power functions for this computer. The PMU is a computer within a computer. Its function is to:

  • tell the computer to turn on, turn off, sleep, wake, idle, etc.
  • manage system resets from various commands.
  • maintain parameter RAM (PRAM).
  • manage the real-time clock.

Important: Be very careful when handling the logic board. The PMU is very sensitive and touching the circuitry on the logic board can cause the PMU to crash. If the PMU crashes and is not reset, the battery life goes from about Ave years to about two days.

Many system problems can be resolved by resetting the PMU chip. When you have a computer that fails to power up, follow this procedure before replacing any modules:

  1. Disconnect the power adapter from the computer and check the battery in the battery holder. The battery should read 3.3 to 3.7 volts. If the battery does not have the minimum voltage, replace it, wait ten seconds, and then reset the PMU (refer to the next step). If the battery is good, go to the next step.
  2. Press the PMU reset switch (S1) once on the logic board. Make sure the connector J4 on the video riser card and the video riser card is installed in its connector, then proceed to step 3.

Warning: Do not press the PMU reset switch a second time because it could affect the PMU chip. S1 is located on the logic board, under the video card, near the edge of the logic board at the end opposite the video ports.

Pmu Reset Switch
  1. Wait ten seconds before connecting the power (adapter) and turning on the computer. If the computer turns on, go to the next step. If the computer does not turn on, there is something else wrong with the computer.
  2. Use Apple Hardware Test to check the computer.

Important: This procedure resets the computer's PRAM. After resetting the PMU, be sure to reset the time, date, and other system parameter settings.

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