There's a TSB out on this. Service bulletin DDAF54_30-U-2107A April 27 1998.

Alternator lamp comes on but only very dimly. Check altenator output.

I guess a malfunctioning alternator can cause this to happen. It applies to all vehicles with ABS. Maybe the following link will work:

http://library.alldatapro.com/alldata/LIB~C31925~R0~OD~N/0/56775861/62706861/62706888/62706889/34853741/34850750/42063452/82241336

Kuan