I had a very similiar problem with my RK 24 where the glow plug light on the dash would not illuminate and the glow plugs would not work (no preheat cycle).
I cleaned the contacts at the preheat relay (middle relay in a group of three located on the r/h side of engine above the oil filter) and the contacts on the ignition switch. It has worked fine ever since.
The schematic that you need is for the TYM T254 series. Don't remember the full number but is the TYM version of an RK 24.