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So the power supply's fan is turning (continuously).

All that proves is that part of the power supply is working.
Therefore, both the power supply and motherboard are still under suspicion.


What Now?

You will need a multimeter, and the knowledge of how to use it to make DC voltage measurements.

Whilst plugs P8 and P9 are still plugged into the motherboard, make the following five voltage measurements, then proceed to the 'What Next?' section.  An example of measuring the +5 volt line is pictured at here.

Plug	Pins	Colour	Function	Expected voltage measurement
P9	4/5/6 3	Red White	+5 volts -5 volts	Expected to be between + 4.8 volts and +5.25 volts    (source: IBM manual) Expected to be between - 4.6 volts and -5.5 volts    (source: IBM manual)
P8	4 3 1	Blue Yellow Orange	-12 volts +12 volts POWER GOOD	Expected to be between - 10.92 volts and -13.2 volts    (source: IBM manual) Expected to be between + 11.52 volts and +12.6 volts    (source: IBM manual) Expected to be over + 2.4 volts    (a TTL level of HIGH)

What Next?

Click here if all 5 lines (POWER GOOD, +5 volts, -5 volts, +12 volts, -12 volts) measured as expected.

If the 4 voltage supply lines (+5 volts, -5 volts, +12 volts, -12 volts) measured as expected (i.e. within tolerance) but the POWER GOOD line is lower than +2.4V, then the power supply must be faulty.

Click here if any of the 4 voltage lines are lower than expected.

Click here if any of the 4 voltage lines are higher than expected.