Sounds like the timer in the relay is not working correctly. Barry is right that the later relay has a temp sensor built into the relay itself and does not depend on the sensor in the head. However, the temp sensor only controls the length of time the light stays on and not how long the relay operates. The relay should power the plugs for about 30-40 seconds independent of the glow plug light.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13