9.5 volts at the glow plug sounds a little low. The 20 amp draw is OK so maybe the problem is a weak battery. It would be interesting to see what happens if you leave the battery on trickle charge overnight and check in the morning.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970