A Captain reports on two Rejected Takeoffs (RTO); he and his First Officer experienced for the same flight in a DHC-8; due to 'Auto feather not armed'. Lack of clarification about the 'Pre' and 'Post' Mod Auto feather system and Power Lever Angle (PLA) differences contributed to problem.

Narrative

After a 2 hour and 20 minute Maintenance delay had taken place for a previous Rejected Takeoff (RTO); another RTO occurred for the same reason. The Auto feather did not arm; the outside temperature was 24 degrees C. After being cleared for takeoff; the brakes were held as the Power Levers were advanced to 50%; I then released the brakes and applied power to about 90%. The First Officer noted that the Auto feather did arm and everything was normal except for the Takeoff Power was now at 92%. He adjusted the power to 90% and the Auto feather disengaged. He called 'REJECT; Auto feather Not Armed' and we RTO'd with a top speed of 35 KTS. After calling Maintenance; we returned to the gate and the Flight was canceled. Further discussion with Maintenance Control revealed there are actually two mods available within the Auto feather System. The older Mod; that our DHC-8 has installed on it; has a higher Power Lever Angle (PLA); than the newer Mod with lower temperatures. I read section 9-49 of the Pilot's Handbook to the Supervisor and he insisted that this Mod (Pre-Mod) was different than what we have been informed of in the DHC-8. I feel further clarification between the Pilot group and the Maintenance department is needed for the 'Pre' and 'Post' Mod Auto feather Systems after this situation.