Zetor with 398 hours.....generatorproblemes

Hallo zetorfans,
i have a 4718 built in 1978 with original 398 hours in top conditions ,technik is top but i have one big probleme.
The genarator works , but the polarity has changed!
There are -15 volts at the generatorpin D to the ground!
Also the generatorpin M works with negative volts.
I want to know,how the change the polarity of the field in the generator or what other thing could be wrong in the electric?
I also search for a instruction manual for the 4718.

Thank you
rex

Is it possible the wires got switched over?
New or reconditioned alternator is not so expensive, and easy to fit.