Service Cables

Three extension cables (076-0897) are required when troubleshooting the iMac with the bottom housing open, as shown below. Important: Diagnostic service cables allow the unit to run while the bottom housing is open (as shown above). However, the computer cannot run for more than five minutes with the bottom housing open. If it is open longer, the CPU may overheat and become damaged.

Connect the cables as shown below.

A= power cable B= drive data cable C= blind mate connector

The PMU Chip

The PMU (Power Management Unit) is a microcontroller chip that controls all power functions for the computer. The PMU (location shown below) is a computer within a computer. It has memory, software, firmware, I/O, two crystals, and a CPU. 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: The PMU is very sensitive and touching the circuitry on the logic board can cause the PMU to crash. If the PMU crashes, the battery life goes from about five years to about two days if the PMU is not reset. Refer to the next topic, "Resetting the PMU on the Logic Board" for the procedure.

Many system problems can be resolved by resetting the PMU chip. The PMU reset button is located on the right side of the board, refer to the logic board diagram below.

Location of the PMU Button

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