What was found
AIRCRAFT WAS CLEARED FOR TAKE OFF. TOOK THE RUNWAY, LINED UP, CAME TO A STOP & ADVANCED POWER LEVERS TO TAKE OFF. AS NR 2 ENGINE SPOOLED UP, THERE WAS AN INDICATION FOR THE S-DUCT THAT WILL ILLUMINATE WHILE ENGINE SPOOLS TO MATCH THROTTLE LEVERS POSITION. LIGHT DID NOT EXTINGUISH & TAKEOFF WAS ABORTED JUST AFTER BREAK RELEASE. AIRCRAFT ACCELERATED TO A TOP SPEED OF ABOUT 20 KNOTS, THRUST REVERSERS DEPLOYED & NO BRAKING ACTION REQUIRED. THE PROBLEM WAS A SWITCH ON THE S-DUCT DOOR WAS OUT OF ADJUST, WHEN POWER ADVANCED, BELIEVE THE SUCTION CREATED JUST ENOUGH PRESSURE TO MOVE THE SWITCH ON THE S-DUCT DOOR, ILLUMINATING THE ANNUNCIATOR. MAINTENANCE ADJUSTED THE SWITCH AND THE AIRCRAFT WAS RETURNED TO SERVICE WITHOUT ANY FURTHER INCIDENT.