If my memory serves me right, take the pos lead of a volt meter and connect to the pos battery terminal, then take the neg lead and connect to the POS terminal of the starter solenoid. The reading should be less than 1 volt with the circuit energized. Anything more indicates resistance in the circuit.

Hope this helps.

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"I was a dirty bird, Carol's not grungey - she's *****in" John Milner....American Graffiti

Good suggestion, quicker than mine.

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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

Maybe the Starter Brushes are worn out.
I think in the end the Starter will need to be pulled and if you want taken apart so that the parts can be cecked.
That is assuming the Electrical Connectoins are all OK and a good Battery.

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84 300D, 82 Volvo 244Gl Diesel

More data:
I have a digital voltmeter in the cigarette lighter. Key on voltage is 12+ volts. When it 'clicks' and starter doesn't spin, voltage drops to 10+ volts. I think this is another indicator that starter is getting power, just not spinning.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13