I suspect one is giving me fits.

Thanks

Don[/QUOTE]

One failure mode of a really bad GP is shorting, when the wire element inside touches the case where it shouldn't. I have seen this, it blows fuses. If you have blown more than one fuse you might have one shorting plug, and testing with an Ohmmeter may not be useful in identifying the bad one. Change em all! The adage "leave well enough alone" may not be applicable here, and can invite more problems, if one is bad then others are going to fail and in my experience its a good plan to replace them all when one or two have failed, and if you expect cold weather where you live, Fall is the right time to check for any bad ones. With as many daily drivers using GP's as you have, GP problems are bound to surface!

Be warned however, if the GP's are old, I mean over 3 to 5 years old and have seen lots of use then you might find that the pencil type GPs tend to get gunked up with carbon and they won't come out easily. Do not yank on one if it is stuck, it may come apart on you (not a fun thing!). Some strong solvent like Lubro Moly is useful here with a twisting motion, unscrewing the plug from the gunk if possible (use the search button looking for glow plug) but I have heard of other recommendations).
In case you are interested, the resistance element in a glow plug is made from tungsten wire, the same material used in incandescent lamps. The Resistance ratio, hot to cold, is 14:1 (meaning 1 Ohm cold and the equivalent to 14 Ohms hot, hot being defined as around 1200 degrees). The elements in a glow plug never get as hot as the filament in a lamp, only dull red is necessary to help ignite the fuel, so a ratio around 3:1 is more likely, therefore 5 GP's would draw around 24 Amps with 14 V on them (engine running with alternator charging). Starting current is around 60 Amps for a few milliseconds anyway, dropping repidly as they heat up. This causes stress cracks on the fuse on the GP timer, so the first time one blows, look closely before you remove it, if it has a hairline crack, its a failure normally attributed to thermal cycling, a known failure mode seen in fuses.
Dieseldiehard
1971 220 (gas) 4-spd manual 106441
1979 300TD w/ ’85 turbo engine 295530
1983 300D 243280
1985 300TD 217300
1987 300D 258230

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!