The 737NG book says...
The SPEEDBRAKE DO NOT ARM light comes on when the automatic operation of the speedbrake system does not arm correctly. This occurs when the speedbrake lever is in the ARMED detent OR either thrust lever is in reverse, and one of these occurs:
Antiskid system inoperative
Auto speedbrake actuator not retracted
Disagreement between the wheel spin speeds and air/ground signals
Internal faults in the auto speedbrake module.
Have you looked at the Fault Isolation Manual?
Last edited by NSEU; 23rd September 2008 at 06:34.