JD 755 Compact electrical short

KDw · Aug 23, 2009

I have a short to ground on the 10 amp fuse that supplies power to the fuel pump, gauges, etc. It may be in the wrapped wiring harness, but would like suggestions in tracing down the problem. Upon moving that wiring just a little, I was able to get the right side of that fuse (to the switch) away from shorting to ground, but not the other side of the fuse. I have the diagram, but don't understand the connection to the 'thermo start' or how it works. The book shows no description of this part, other electrical parts and sensors or where I could locate the connection point on the tractor. Before splitting the wire wrap, I would like to find the physical connections to test the wire from point A to B, point B to C, etc.

JD755 · Aug 24, 2009

By saying you could remove the short from the switch side only, I assume the fuse is open or removed. If you did have an actual short on the switch side, I would expect a fusible link to have opened up. Do you still have 12 volts from the switch, with the fuse removed?

Be sure on the pump side of the fuse that you have a real (zero ohm) short, and not just the low resistance that can be from normal electrical components. The fuel shutoff solenoid is connected there and would show a somewhat low resistance constantly. You can test for that by removing the electrical connector from the solenoid. Same thing applies for the fuel pump also on that line.

The pump side of the fuse also provides voltage to pin 3 of the thermo start controller. This provides power for the temperature sensing circuit inside. The controller will provide a switched ground on pin 5 for the preheat light on the panel at start up. The controller has an additional higher current input (with fusible link) on pin 2 to provide voltage to the glow plugs through pin 4. The glow plugs will normally be kept powered on for a longer period of time than the preheat light has its ground connection. Pin 1 of the controller connects to ground. I would not expect pin 3 of the controller to have a low resistance like the solenoid and pump would.

I am a long time 755 owner and have all the factory manuals. As a retired aerospace electrical engineer I'm willing to provide whatever help I can.