Slam Buffington, first try what Apple suggest:"
Power-on LED illuminates when power button is pressed but fans do not spin (or spin only momentarily) and there is no boot tone or video
1. Reseat video card. (Make sure video card is fully inserted in connector and end of card is secured by the connector latch.)
2. Reset logic board. Refer to “Resetting the PMU on the Logic Board”.
3. Verify power supply cables are fully connected.
4. Reseat the processor
5. Replace the processor
6. Check for trickle voltage on the power supply connector. Refer to “Power Supply
Verification” in this chapter. If verification fails, replace power supply.
7. Replace logic board.
Power Supply Verification
To power on, the computer’s logic board requires a “trickle” power of +5V. If the system fails to power on, first reset the PMU. Then follow the procedure outlined below to determine whether the problem is related to the power supply.
Note: To verify the power supply, you need a volt meter.
1. Remove the power cord from the computer.
2. Open the computer, lay it on its side with the access side facing up.
3. Remove the front inlet fan assembly.
4. Remove the black plastic cap covering the 24-pin power supply cable connector (the P1 or largest connector). The cap fits tightly over the connector. To remove it, rock the cap gently forward and backward as you lift up.
Note: This is a top view of the connector when it is plugged into the logic board.
5. Press the release latch on the connector and disconnect the cable from the logic board.
6. Plug a known good power cord into the computer. Do not turn on the computer.
7. On the power supply cable connector: connect the black lead of the volt meter to pin 23 and connect the red lead of the volt meter to pin 1.
Note: This is a bottom view of the connector when it is unplugged and facing up.
The volt meter should measure approximately +5V. If you do not get a reading of +5V, recheck the volt meter connections and measure the voltage again. If the voltage is still not present, replace the power supply.
Power Supply P1 Connector
Pin # Signal Pin# Signal
1 +5Vstb 13 GND
2 GND 14 Power ON
3 FANtach 15 GND
4 GND 16 EMPTY
5 EMPTY 17 GND
6 GND 18 RTNaud (GND)
7 +12Vaud (12V2) 19 GND
8 GND 20 +5V sense
9 +3.3V sense 21 GND
10 GND 22 -12V
11 GND 23 GND
12 EMPTY 24 EMPTY
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 five years to about two days.
Note: For the location of the PMU reset button, see “Logic Board Diagram” in the Views chapter.
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 cord and check the battery in the battery holder. The battery should read 3.3 to 3.7 volts. If the battery is bad, replace it, wait ten seconds, and then proceed to step 2. If the battery is good, go directly to step
2. Press the PMU reset button once and then proceed to step 3. Do not press the PMU reset button a second time because it could crash the PMU chip.
3. Wait ten seconds before connecting the power cord and powering on the computer. If the computer does not power on, there is something else wrong with it.
Note: The above procedure resets the computer’s PRAM. After resetting the PMU, be sure to reset the time, date, and other system parameter settings."
Hope this helps, good luck.