Diesel engines are actually very simple. They need 12 volts to spin the starter, run the lift pump on the fuel and open the fuel shut off switch. They need fuel to burn. There is no ignition system, the heat and the compression in the cylinder, along with the injection of the fuel under very high pressure (25,000 PSI+) cause the fuel to ignite. They need air flow through the air cleaner, which is not electric in any way and usually isn't the cause of an engine not starting. So when the diesel doesn't start, you;
1. Make sure the 12 volt battery can turn the engine over, open the fuel shut off switch and operate the pump to lift the fuel from the tank to the injector pump. The injector pump is gear driven off the engine camshaft so when the engine turns over, it compresses and injects the fuel.
2. Make sure the fuel is getting to the injector pump, which means the 12 volt fuel shut off is opening. The shut off has a strong magnet it in when the power is turned off (with the key), the magnet shuts the switch, closing the fuel flow, stopping the engine. When a fuel shut off switch goes bad, it won't open. Some models can be manually opened although on the 1025r, its not very easy since the shut off switch is incorporated into the injector pump. The fuel switch is the "SNAP" or loud click you hear on many tractors when you turn the key on to warm up the glow plugs. It's not as loud of a snap on the 1025r due to it being located in the injector pump (verses being a separate location like it was on earlier models), but you can still hear the "click" when the switch opens, which is the magnet being released with the current flow when the switch is powered.
3. Make sure fuel is getting through the fuel filters with the small under floor filter being a chronic issue and the fuel filter in the glass fuel bowl on the separator being the 2nd place to check for fuel. Also when checking the fuel separator, make sure the red ring in the bottom of the separator isn't floating in the fuel, as that indicates water in the diesel fuel and water doesn't burn very well, especially in large concentrations or quantities.
At 60 degrees, you shouldn't even need the glow plugs to ignite the fuel. They help make the process easier and faster, but if they aren't working, the engine will still usually start, it just takes some more cranking of the starter..............
If the battery is weak and isn't ;
-spinning the engine starter fast enough to start the engine
-opening the fuel shut off switch
-operating the fuel pump and getting fuel to the injector pump
-not operating the glow plugs
the engine usually won't start.........