Voltage Problem, reverse problem

[orionambulance]

Hi everyone! great forums here, i read posts on a daily basis!

On with the situation. I have a 1998 Ford E 350 7.3 Powerstroke ambulance. I purchased the vehicle this week after it was sitting, unstarted for 1.5 years minimum. Truck starts up and got new batteries in. The guy i bought it from said it needs a new alternator. When started, it shows 16 volts and has a battery light on the dash. I took the alternator out and had it tested at autozone who said it tests fine. I cant figure it out. I inspected the battery cables as much as i can and everything seems in order. I tried grounding the alternator to metal and negative terminal hoping it would be a bad ground, no luck. I cant find an external voltage regulator, but i;m not sure where to look. Also, the truck stalls in reverse unless you give it steady throttle, unsure if its related.

Thanks in advance for any help!

David.