Why do i Have to go use eather to start my diesel truck

Those trucks have glow plug system that preheats the combustion chamber to make them fire up easier, specially when it's cold. When the ignition key is turned on you are to wait about five seconds before cranking the engine. So what I am saying is that the glow plugs might not be coming on even thought the indicator lights act like it's working. Back in 2001 we had a new Ford Diesel do that. Started hard in the Summer and refused to start without ether when cooler weather came in September. Warranty fixed ours but you may have to get a diesel mechanic to test yours. Probably nothing more than a relay or fuse. I doubt all the glow plugs would burn out at once.