i have a 1994 aerostar ext. awd with a digital speedometer. after driving about 40 min. at speeds around 60 the speed drops to zero and jumps around. the trans. downshifts when this happens.if anyone has had this problem let me know.

 

Try this. Disconnect the main alternator output. Then roadtest the vehicle. The van will be on battery power. If this fixed it you probably have a bad diode in the alternator. Ive seen the same problem on other ford products