charging system on 1990 Dodge D150

i can only get a negative charge alt checks good batt good new regulator new starter new amp guage i'm stumped
and it is actually a 1987 model

Asked by for the 1990 Dodge D150
either the alt or regulator. how did you check them to determine they were good??
have a shop check it to confirm.

i had them checked at a shop the alt is putting out 12,9 and the diodes are good it is a new regulator and i ground the paint off of it and the firewall so it gets a good ground i can make the guage show right if i reverse the wires on it ie:hot wire to neg side of guage.i have checked every wire i can find even straight wired from the alt to the batt with the same results it has be buffaloed
What did it end up being? We are having the same problem. Replaced voltage regulator, Battery, then Alternator, in that order. Still nothing. Ammeter was out so we replaced it and it shows discharge.