The "quick and dirty" way to test the glow plugs is to pull the big plug off the pre-glow relay, and the pin numbers correspond to the glow plugs. Using a good digital multimeter, measure from each pin to ground. You should get a value around 0.7 ohms (zero point seven ohms). That is a rather low resistance number for some meters.

The resistance of all the plugs should be about the same. Any outliers are suspicious.

Also check the big strip fuse (80 amps) under the cover of the pre-glow relay. Remove it (two philips screws) and hold it in your hand. Flex it a little. There should be no cracks.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC