On my 93, it seems to be timed, as near as I can tell. I installed an manual glow relay, and it has helped me manually glow the plugs when/if I need to.
I just replaced the factory once since it failed with a switch that runs to a button in the cab. Have to cycle it manually everytime but it is nice controlling it during the winter and summer, depending on conditions. Welded to factory glow plug bracket so it is still removable.