What are the signs that glow plugs may be wearing out?

rough starts/no starts/blinking glow plug light/blown glow plug fuse/....
Why is there something that you feel is not right?

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

Never had the indicator light blink, but I have had the light stay off when I turned the key to the preheat position. If I cycled it to off and back right away it would light as it does normally, and the car would start, but a little rougher than usual. This was usually a signal that one plug was out. If cycling the key did not work, in my 1982 240D that means more than one plug is gone. Good luck, hope this helps, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Every once in a while the only sign is that the car will not start at all. The glow plug light is supposed to blink if there is a failure, but it doesn't seem to make any difference on my 1981 SD, just gets very hard to start all of the sudden.

Every once in a while my GP don't work on my '79 300SD. The fix is to pull off the smaller cable connector on the GP relay and put it on and pull it off a few times. The connectors are old and slightly green so once in a while they go open.

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