Diesel tips. I just tonight solved this problem on my 3600 diesel, almost like yours. If battery good, take lower metal cover off of operator station. [check neutral safety switch] Disconnect two wires going to gearbox and put an ohm meter on the wires going to the gearbox. Shift to HI should read infinite ohms. SHift to LOW should read infinite ohms. SHift to NEUTRAL between HI and LOW. Should read zero of few ohms. if false, bypass with a jumper and attempt start, if starts, then replace switch, otherwise..[test ignition switch] disconnect battery pos terminal. remove thin nut and pull ignition switch down into view. Big fat wire is incoming positive and the blade it connects to is the big blade at middle or 12 o'clock so connect one ohm meter wire to this big post (you did disconnect battery right?). Connect other ohm meter wire bottom "some call it post 5" and is at 6 o clock and it only has continuity (zero ohms) when ignition switch in turned to spring loaded start position. with the ohm meter connected to big fat red post and post "5" turn to start position and look for zero ohmsTest to make sure this reads zero ohms when switch is turned to start position if still infinite ohms, replace ignition switch. Reconnect battery. We now will [test solenoid] Little wires going to solenoid are often connected, connect voltmeter to post of little wire and the other to ground. MAKE SURE GEARS IN NEUTRAL and measure voltage when ignition is in start position, engine will start or you see 10 or more volts or no volts. If no volts, this wire (which goes back to post "5" on on ignition switch) is bad. If you see volts, solenoid or starter needs more investigation and could be bad. My problem was start switch.
