E250 charging system problem
 

I have a 1995 E250 van with 5.8 motor. Having a problem with the charging system. When first started in the morning the indicator shows that system is charging . It then gradually drops down as the idle drops to virtually no voltage at all. Revving engine produces no change. When put into drive with foot on brake to hold van in place, revving to about 1000 rpm will show normal charging. Normal charging can also be achieved under acceleration from a stop or when kept under load. At cruising speed charging drops back off. Lights, heater fan, wipers work fine when under same load conditions but not when discharging. Belt tension and condition seem fine. Any advice would be greatly appreciated. Thanks Mike

Nevermind