lights on my 300D are dim and kinda yellow casting. Is the regulator adjustable?
Any other ideas?

No, it either is working properly, failing, or failed.

Check the battery for voltage both with the engine on and off.

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83 SD

84 CD

Clean the connectors and ground points.

Sixto
87 300D

Pull the regulator (it's part of the alternator and can be removed without removing the alternator -- depending on the year of your 300D, maybe) and check for worn brushes.

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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

LOL what is the Voltage.
Harbor Freight sells inexpensive (about $6) Volt-Ohm Meters/Mulitimeters.

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84 300D, 82 Volvo 244Gl Diesel

here's a page at dieselgiant that has a good walkthrough with pics of the regulator -
Mercedes Voltage Regulator Install

as said before, get a multimeter and check voltage first to see where you are at first.

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1981 240D ... 265k
1977 240D ... 265k
1977 240D ... 250k

Sometimes they go on sale for as cheap as two bucks. They aren't accurate like a $500 Fluke, but they are good enough for the work we do on these cars.

Get a meter and diagnose the problem before you go throwing parts at it.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

if you or someone you know gets a magazine that has harbor freight coupons in it... every couple months the "free item" coupon is a multimeter!

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1981 240D ... 265k
1977 240D ... 265k
1977 240D ... 250k