A tractor of that age would be positive earth, your battery leads should have plus and minus stamped on them, also the battery will also have plus and minus on it, the plus is always larger diameter, the positive cable should be earthed on to the tractor somewhere and the negative should be attached to the solenoid. To be sure it hasn't been changed, take the wires off the Dynamo, connect the battery up as I said and get a piece of insulated wire long enough to run from the battery to the Dynamo, connect one end to the negative terminal on the battery and rub the other end on the smaller of the two terminals on the dynamo for around 5 seconds, this will polarise the Dynamo, then refit the wires to the Dynamo, then remove the wire from the battery.
Parahandy, February 2016
