The glow plug indicator lamp on my New Holland TC35 Boomer would occasionally come on, and now it just stays on. According to the helpful threads in this forum, the cause is a faulty glow plug timer unit.. On turning ignition to "on" the dash light illuminates and pulses on and off as the timer switches from constant on (start) to afterglow (a series of shorter pulses). There is 12V output to the blue wire that coincides with the indicator lamp. With relay and timer disconnected I have checked that the continuity of all wire paths are good.
Google "glow plug timer s81nl connector diagram pdf". The results include a variety of images for how to wire it up/circuit diagrams for it. I would make a little test setup to verify it's operation, before wiring it up to the tractor.. -Yellow is the wire that activates the timer -black is connected to ground blue is a switched ground for a timer bulb the bulb receives 12v at the same time the yellow wire activates the timer.