yeesh, someone decided to make 'heating up gloplugs' real complicated.....
makes me appreicate the BX23S system I have. Turn key to 'gloplug', count 3 steamboats, start engine. Works every time, all year round.
basic math says a 0.5r gloplug will draw 24 amps at 12 volts, 3 will draw 72 amps. Like Tan, I'd bypass the 'mess'... get a good push button to power a 100A contactor/relay to supply battery power to the gloplugs. If you need to USE this tractor, it's the cheapest solution to get her running. If you have deep pockets, take to dealer and be sitting down when you get the bill. I suspect they'd replace EVERYTHING ...controller, ign switch, sensors, gloplugs maybe even a wiring harness or two. That could cost 2-3,000 $$ ?? unlike the $50 .
Had a better look at the scematic, lower right 'glow relay',looks EASY to bodge !. If you remove the connector going to the 'glow controller', you can ground the wire going to pin #4 of the glow relay. That should put power to the glowplugs. If that works,(put voltmeter on glowplug..should get 12V with relay on..) wire a $2 pushbuton between #4 and ground. Be nice to use 'spade lug' connectors instead of cutting the #4 wire, in case you decide to troubleshoot this properly ...all winter long.....