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[ 1. Power supply's fan is turning.
  2. The only load on the power supply is the motherboard.
  3. Abnormal voltage under the load of the motherboard. ]


Either:
* Power supply is faulty.
* Motherboard is faulty in a way that is overloading the power supply (but not affecting the ability of the power supply to drive its fan).


What Now?

We will now make some resistance measurements.
You will need a digital multimeter, and the knowledge of how to use it to make resistance measurements.

IMPORTANT:

Do NOT use the continuity setting on your multimeter.  Use the resistance setting.

1. Turn off the power supply.

2. Disconnect both P8 and P9 from the motherboard.




3. Wait a couple of minutes for the motherboard capacitors to discharge.  (We do not want any residual voltage to affect the resistance measurements that you are about to make.)


4. On the motherboard, measure the resistance between the ground line and the +5 volt line.

On the many good IBM 5150/5155/5160 motherboards that I have, this measurement ranges in value from about 200 ohms up to about 1300 ohms. If your motherboard measures zero or a few ohms, then there is definitely a short-circuit on the +5 volt line. In which case, click HERE

5. On the motherboard, measure the resistance between the ground line and the -5 volt line.

On the many good IBM 5150/5155/5160 motherboards that I have, this measurement is so high that it it is beyond the capability of my multimeter. Initially, you may see a reading of some sort that slowly rises (as capacitors charge) but in the end, expect to see no reading. If your motherboard measures zero or a few ohms, then there is definitely a short-circuit on the -5 volt line. In which case, click HERE

6. On the motherboard, measure the resistance between the ground line and the +12 volt line.

On the many good IBM 5150/5155/5160 motherboards that I have, this measurement is so high that it it is beyond the capability of my multimeter. Initially, you may see a reading of some sort that slowly rises (as capacitors charge) but in the end, expect to see no reading. If your motherboard measures zero or a few ohms, then there is definitely a short-circuit on the +12 volt line. In which case, click HERE

7. On the motherboard, measure the resistance between the ground line and the -12 volt line.

On the many good IBM 5150/5155/5160 motherboards that I have, this measurement is so high that it it is beyond the capability of my multimeter. Initially, you may see a reading of some sort that slowly rises (as capacitors charge) but in the end, expect to see no reading. If your motherboard measures zero or a few ohms, then there is definitely a short-circuit on the -12 volt line. In which case, click HERE

So you did not measure any shorts (or partial shorts) on the motherboard's power lines.

CONCLUSION:

Because:
1. there are no motherboard shorts, and
2. because with the motherboard connected to the power supply, the voltages are incorrect,
the logical deduction is that the power supply is faulty (even though the power supply's fan is turning).