IBM 6150 Hardware Maintenance Manual page 272

Step 9
1. Set the system unit power switch to Off.
2. Disconnect the power connector from one of the fixed-disk drives,
diskette drives, or the cooling fan.
3. Be sure the power has been off 30 seconds, then set the system unit
power switch to On.
4. Wait 15 seconds, then answer the following question.
bJD
IS THE POWER-ON INDICATOR ON?
NO
If
all of the drives and the cooling fan have not been disconnected,
repeat this procedure.
If
all of the drives and the cooling fan have
been disconnected, exchange the power distribution cable, then go
to "Step 17" on page 3-8800-17.
YES
The last FRU disconnected is the failing FRU. Exchange the
failing drive or cooling fan, then go to "Step 17" on
page 3-8800-17.
(9/86) Problem Isolation Charts
3-8800-9