Thanks for all your replies.
Based on Steve's suggestions I did some measurements
this evening. The following table shows the voltages I measured in
the different speeds with the transmission
in neutral. I later measured under load and the only major difference
was the armature voltage that caved in on heavy loads mainly in
speeds 1-3, which was expected.
Speed Armature Field Battery
1 29.8 34.5 37.5
2 30.6 34.4 37.3
3 31.6 34.4 37.3
4 33.6 33.4 37.0
5 35.5 33.2 37.0
6 35.5 11.7 36.7
7 34.7 8.1 36.7
8 34.4 5.5 36.5