Service manual

Troubleshooting 3-7
If Power Problem Occurs at Power-Up
If the system has a power problem on a cold start, the PCM LEDs are not valid until
after DCOK_SENSE has been asserted. The cause is one of the following:
• Broken system fan
• Broken CPU fan
• Power supplied to the system is out of tolerance (a power supply could be
broken and the system could still power up)
• PCM failure
• Interlock failure
• Wire problems
• Temperature problem (unlikely)
Recommended Order for Troubleshooting Failure at Power-Up
1. Check to see if any CPU fan or system fan is not spinning. Fans can fail by not
spinning and/or not putting out the tachometer output necessary as input to the
PCM comparator that checks the fans. (See steps 4 and 5.) Replace broken fan.
2. Replace the PCM.
3. Sequentially remove CPUs and try to power up after you remove a CPU. If the
system powers up, the last CPU you removed had a fan failure.
4. Check the output of the power supplies. See Section 4.1 for locations of +5 and
+3.43 volt output pins. If the output is above or below the threshold, replace the
faulty power supply.
5. Check the output of each system fan with a voltmeter. Probe the middle of three
outputs of the fans with the positive lead of the meter and ground the other
probe. The meter should read 2.5 volts to 3 volts. If a fan’s output is out of this
range, replace the fan.
NOTE: You will have to disable the interlocks to check the voltages in step 5.
You will have only 10 seconds to measure them. There is a 10-second delay
before the PCM turns off the power.
The PCM must sense a change in Vaux (auxiliary voltage) to start the power
supplies. Pressing the On button has no effect if the machine halted because of
a failure in the power system. The power supplies must be unplugged and
plugged back in for the On button to work.