I just measured mine yesterday. There are actually 2 measurements you want to take.

First, disconnect the plug on the relay and measure the resistance from the connector to each glow plug. The resistance should be less than .5 ohm. This confirms you have a good low resistance connection to the glow plug from the relay.

Next, measure the resistance from the tip of the glow plug to the block of the engine. This should read .75 to 1.5 ohms each.

Mine were:

2 @ 0.9 ohms
1 @ 250 ohms
1 @ 800 ohms
1 @ 17k ohm

I have three bad glow plugs.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)