Unless you have the probe in the wrong place, it means you have no cycle, just 12V on the signal line, probably indicating an electronic failure.
Steve Brotherton will know far more than I.
Check you connections, but if you indeed have 100% all the time, the electronics aren't working.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!