(ET) Issue partially solved.

[Chris Zach <czach computer org>]

It's not the controller card or the diodes or the cruise control.

I have a double fault to ground on the tractor. Damn.

The clue is the arcing up by the deck. My guess is the middle motor wire 
from the deck to the motor has gone to ground. When I unplug the deck, 
the tractor works properly. When I plug it in, off or on, the tractor 
does the weird stuff.

The upshot of this is there is a *SECOND* ground somewhere on the 
tractor frame itself. Since it's affecting speeds 1-4, my guess is it's 
in the wire between the last resistor and the connection back to the 
motor circuits. Thus in speeds 1-4, the tractor is pulling power 
*AROUND* the resistor bank by going thu the ground, down to the tractor 
deck, and thru the tractor deck return wire.

This would also explain the problem in reverse: Remember that the E20 
reverses the armature, not the field. In this case, the reversed 
armature would put a counter voltage on the frame, which would basically 
short circuit the field and result in the motor trying to run only on 
it's compensation coils. Nowhere near enough, thus the CB-3 breaker trips.

Amazing. Hunting down the deck ground will be easy. Finding the one on 
the tractor will take more work. In addition I think I will make an LED 
circuit that will tap off the center two batteries to frame and will 
light up if there is ever again a ground fault.

Let that be a lesson to *everyone*

Chris