Freightliner has issued a recall to install a new vdc for all vehicles manufactured between may 26, 1998 and may 21, 1999.  The Field service modification number is SF199AB/SC199AB.   Some of the problems associated with faulty VDC units are incorrect voltermeter readings and odometer failures.

Also note that the voltmeter will also most likely need to be replaced.   Your Freightliner dealer may not be aware of this.   If your voltmeter does not read in the middle of the range, then you need a new voltmeter also (especially when you run the engine and generator at the same time).