I own a Tz25Da that is getting difficult to start in the colder weather. On the TZ25 there is a light in the dash that stays on for 5 seconds and then goes out when you energize the glow plugs. As long as you hold the key in the glow plug position they are still energized. The New Holland manual states that in colder weather that extra time may be needed for cold weather starts but does not specify a max. time to keep plugs energized. I confirmed that I have voltage to all three glow plugs and checked the continuity after removing the brass jumper bar, everything seems to be as it should . Just looking for opinions for max. on time time so I don't fry them.