can u telll me whats the diff. between the 240 and the 300 series? is the car smaller in size? engine?

I will assume you are speaking about the W123 chassis 240D (went off the market in the USA after the 1983 model year) and the 300D (went out of production worldwide after the 1985 model year). These cars share the body and most accessories, but have some minor detail differences, other than the engine. The 240D is a 4 cylinder 2.4 liter Diesel. No factory turbo. The 300D is a 5 cylinder 3.0 liter Diesel, which was turbocharged from the model year 1981 on, I believe.

There are trim differences to distinguish them, which are really pretty trivial (such as the chrome strip on the 300D rear light molding, which is black rubber on the 240D). A few additional items as standard equipment (automatic transmission and automated climate control or air conditioning were standard on the 300D, or maybe a better term would be mandatory, while the 240D can be had with a 4-speed manual and a simpler, less costly to repair, manually set air conditioning system), but for the most part all body panels and suspension parts can be interchanged. Many engine parts are the same too just more of them on the 300D because of the extra cylinder. This is less so on the turbocharged 300Ds, but still items like starters and glow plugs are the same.

With the added weight of the larger engine and some additional complexity with the automatic tranny and climate controls, and possibly a larger Diesel fuel tank, comes a larger size wheel/tire combination on the 300D. Other than that, I think the cars are essentially the same.

The 300SD of the same era is a different story. This car was an "S" class, the larger MB sedan, which is a different chassis altogether. It had the same engine but is significantly larger and more luxurious. I hope this helps, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

the 240 is always a diesel series w115 or w123.
the 300 can be gas or diesel series w109, w123, w116, w126 and
earlier series whose numbers i dont remember.

they make a gas c240 now but i'm not sure its a real mercedes.

don