The resistance type gauge you are likely using has a brass strip on the back of the movement that contacts the pod it is mounted in. That is your electric gauge(s) ground path to the gauge board, the gauge board is grounded to the dash sheet metal with the gauge board mounting studs completing the electrical circuit. The gas gauge sender is grounded on the gas tank by the flange mounting screws and the temp gauge sender by thread contact with the cylinder head, the amp meter is ungrounded. I also add a wire from one of the gauge board studs to another place on the frame or side sheet metal to be sure in case the dash sheet metal is poorly grounded (corrosion, rust, etc). You do not need the cables.
The gas and temp gauges are not polarity sensitive, they will work properly with either pos or neg ground system. The amp gauge is polarity sensitive only in reading the correct direction of current flow. Amp meter correction is simply swapping input and output terminal wires. The wiring on the gas & temp gauges need to be connected to the dedicated terminals for voltage input and sender which is the same for either pos or neg gnd.
Look at the back of your instrument panel. On both the gas & temp gauge (SW) the terminal on your right is the power input from an ignition switched source and the left terminal is out to the sender and from there to the mounting gnd.
Now look at the rough schematic I made up above for someone. This is the internal electrical circuit of the SW resistant movement gauge used by Case in that era and most likely the one you have now. The schematic is a front view, you will be looking at the terminals from the back. The 12v terminal is the input voltage from a switched source. The sender terminal is current out to the sender. The case ground is the internal brass strip in contact with the pod.
Both the input and sender terminals, as well as, the internal wiring have to be insulated from the case ground.
Assuming you are wired to the right terminals and don't have other tractor wiring issues, your input terminal or input internal wiring to the 12v or sender coil on one, or more, of the 3 gauges may be grounded causing a dead short.
To check this out: Disconnect all wiring to the gauge board, remove lamps, (the gnd strip is internal and will remain connected). Now connect up the switched input to the gas gauge 12v terminal, battery connected, switch on power to prove the terminal and internal wiring insulation to the coils (it is possible to have a shorted coil but the short would have to be very near the input wire). If the sender terminal is grounded to the pod it will not cause your problem, the gauge pointer will simply deflect pass the full mark or Hi temp mark on respective gauges.
Whether your problem reoccurs or not disconnect the gas gauge wiring and proceed to do the same thing to the temp gauge and amp meter in the same order. This will isolate which instrument is dead shorted to gnd, if any are.
You can do the same thing with a VOM on the resistance scale but it is less conclusive with a partial gnd then an operational check. Have all wiring disconnected, check resistance of the individual terminals to a good gnd (board mounting stud). I don't remember the resistance of these coils w/o hunting it up but it be a over 100 ohms. A zero resistance reading will definitely be a short.
I have prior instrumentation experience and have repaired several of these Case boards. Roger does outstanding work, I know that because I bought one from him, checked it out, and installed it in my 310 as replacement for one I had done myself because his craftmanship is far superior to mine. You're not going to find better quality restoration anywhere period. Having said that it is not real uncommon to get a defect like this in a new instrument board, mostly due to shipping & handling or on site installation error. It is very common for the customer to over tighten the terminal fasteners or rotate the terminal stud with the outboard nut causing a short.
Joe
