The speed shouldn't matter,sounds like you have a lead shorting out at that speed,that could be vibration that only reaches a peak then,or wind force making a lead touch the body/engine at that speed.Take it down a long slope and let it coast from 45ish and see if the light comes on around 50ish,if it does then the wind force is blowing a wire around,take it to 50 in a different gear (bypassing)the gear when it usually happens and suspect that it is vibration at certain revs causing it(still suspect a lead touching).Or have your alternator checked out,first options are free anyway.Good luck.P.S you shouldn't coast a vehicle this is only a scenario.
anon, September 2008
