I don't know if this any help, but I too have a TC33D with a glow plug light that acts like it has a mind of it's own. I have noticed that even with the light not operating that the timer still seems to be ok. I can hear an audible click when it cuts off the power to the glow plugs. I've learned to just listen for the click noise before I fire it up. May not be your problem, but I wanted to throw it out there just in case.