IBM 3270 Maintenance Manual page 209

You may have a bad LED on your diskette drive.
If the diagnostic programs perform correctly but
the LED does not light, check for a minimum
voltage of 1.5 V dc on the diskette-drive logic
board when the spindle is turning .
• For a Type 1 diskette drive, check between
pins P9-1 and P9-2.
• For a Type 2 diskette drive, check between
pins J3-15 and J3-16.
Type 1
Tvpe2
J·3 Connector
1 24
DID THE VOLTAGE MEASURE AT LEAST
1.5 VDC?
YES Replace the Diskette Drive (see: Section 5,
"Removal/Replacement and Adjustments").
NO
3-600-10
Maintenance Information