IF a diesel cranks, and fuel is delivered at the correct time as controlled by the injector pump and the engine has compression and the temprature is sufficient in the cylinder, it must start. Which one are you missing?. The fuel solenoid relay is a notorious failure point, and does not pull the fuel stop solenoid. The solenoid has two windings, the pull winding, and the hold winding. The pull winding is powered by the fuel relay and it duration of 1 second is initiated by a ground that comes out of the display unit for 1 second. Do you hear the Clack of the relay and solenoid. Did you measure the 1 second 12 volt pulse on the fuel stop solenoid on the front of the injector pump? The other winding is held by the key buss to 12 volts continuously. This is a low current winding and when you turn the key to the off position, this makes the fuel solenoid relax and the spring pulls the rack in the injector pump back. The engine dies for lack of fuel. The glow plug system is a high current draw system that energizes the 4 glow plugs and pulls nearly 70 amps for about 15 seconds in the automatically initiated cycle that is controlled by the pre heat relay. This is the 70 amp rated relay on the firewall, usually in the center position of the three relays on the firewall. You can measure the 12 volts nominal on the glow plug buss. It also has a post glow function, that continues to heat the combustion chambers for 15 seconds AFTER the engine has started. This is not reflected in the "glow" lamp on the dash, but it occurs. If the failure to start is occuring soon after you shut it off, it is NOT likely the pre heat circuits and relay. Because there is a temprature sensor that prevents glowing if the temp is high enough to not need it. Such as when it has been running and you shut it off and attempt to start again. My money is on the fuel stop relay. Second guess would be the fuel solenoid. But the relay is notorious for failure. They are a dime a dozen on Amazon or E bay. Or at your favorite auto parts store.
