On a Caterpillar generator I have two diode blocks one Positive and one Negative, from the exciter there are six wires three go to the Positive diode block and three to the Negative diode block then from the rectified side of the diode block there is the Positive and Negative cable that go to the main rotor.
My question is how do you test these diodes? the older style were individual diodes which is straight forward but these are all together in a "block"
The other question I have is if the Positive and Negative cable going from the diodes to the main rotor were not marked because they were eg: burnt how do you find out which is Positive and Negative , what happens if they you got them crossed would you be 180deg out of phase?
Any info would be helpful, thank you.

Chayvo,

Each diode block is just 3 diodes sharing a common terminal at one end. Common anode for one polarity, common cathode for the other polarity. Test them like you test a normal diode. One lead on the common terminal, the other lead on each of the three wire terminals, one at a time. If any one diode is shorted or open, you have to replace the entire block of three.

With the burned DC wire question, I don't think it would make a difference.

Larry,
Thanks for that info its been a great help.

The DC wires definitely make a difference. If you get them backwards, you will blow your new diodes.
With the battery hooked up, not connected to the alternator, the plus should be plus and the minus should be minus on a meter.
One of them will also be grounded to the frame, usually the minus. There is not much positive ground stuff out there.
Chrysler used to do it back in the "happy days" era.

Greg,

The OP stated the Positive and Negative wires went to the rotor. In that case, I do not believe the polarity will make a difference. I base that on the assumption that the rotor is the DC Field and the stator is the output windings.

I suppose we may be talking about different wires

I don't think reversing the supply to the rotor would make any difference. All its doing is creating a magnet which is then driven past the stator windings.
My doubts are whether there may be more to the machine than the writer has outlined. Is there not a regulator?

If we are talking about the 2 wires going to the slip rings, you guys are right, I doubt it makes any difference. Sorry for any confusion