Vehicle is an 82 300D turbo. The starter seems to be slow cranking and dies off after 10-12sec. Voltage drop occurs quickly from 13.5 to 12.5 then to 10.5 as it slows right down. I was taught starter draw becomes an issue if th volrtage drop is 9.5 or below. Most starter troubles I've ever had has been with units that either work or don't, never had one turn slow. A new battery and cables made no difference. It's a very voltage sensitive problem as it is much easier to turn over if the glow cycle is skipped

Worn brushes can cause the reported symptoms.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

If battery and wiring and brushes are good and cranks slow, then it could be binding from worn bushings/bearings, or partially shorted armature windings.

I had similar symptoms as yours for about a year on my 83 a few years ago and took the starter apart to find a worn top bushing, which caused the armature to collide with the field casing, which caused a partial short in the armature windings. That starter also drew excessive current per the ammeter test. After replacing the armature with a good used one and replacing both bushings/bearings, current draw dropped to normal levels and the starter was as good as new. You may want to start with testing the current draw , which is an easy test. I don't remember the numbers but if you search for my thread on the subject, the numbers are there.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I've had something similar on my 300sd. Id start at cheapest first. and i assume it would be similar to a 300d.

Id start with cleaning connections first. there is a 3-4gang black electrical box that a bunch of the battery/starter wires converge onto on the wheel-well there. try wire brushing every wire there and reassemble with a bit of dielectric grease. Id also just do one section at a time as to avoid mixing them up on reassembly.

Also check battery terminals for oxidation, clean and grease and re-tighten.

Then if that doesn't improve anything, then look into the starter IMO.

Its not been the case with my Mercedes but on my Toyota Tacoma once i had a battery crap out and act strangely. i think it was a bad cell, but i wouldn't start when cold but a charger and a little warmth during the day it wasn't an issued. Battery still tested good. New battery fixed it.

As above,

1. get battery tested (you never know)
2. clean all connections between the battery and starter as well as the engine chassis ground
3. Odds are its time for a new starter. The brushes wear down and make intermittent or poor contact.

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

Don't people replace brushes any more?