1220 Warning lamp comes on

[ford351c]

I have a 1220 that the second light from the left side comes on. The left most one is the battery. Sometimes it comes on right after i start the tractor others i can run it for 30-40 minutes. first question what is that light for? (paint worn off the three indicators lamps) i think the 3rd one from the left may be glow plug as it light up when i turn key to left for the glow plugs, then goes off when i turn key to the right. second question what may be turning that light on...oh some times its on very dim and others very bright