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Hello! I have a 1980 300D (non-turbo) with 107,000 original miles on it. A few weeks ago while driving, I came to a stop light and upon trying to accelerate away from it, found that my transmission had stopped engaging completely. It does not engage in Reverse or Drive or any other gear at all. Just feels like it's in neutreal whenever I shift it into any gear.

When I got it home, I looked it over and found that the vacuum hose connected to the nipple on the transmission vacuum modulator cap had broken off and the hose was just hanging there. I found a new modulator cap and replaced it, thinking that would solve my issue, however, no luck. I have checked all the vacuum lines, and everything looks good and there is good vacuum. The drive shaft is attached and intact everywhere, and the shift linkage seems to be working properly as I can hear it engaging gears in the transmission. Is there any thing else I can look for or try/do before taking it into a mechanic for a new transmission? It's hard to believe that the transmission would have failed internally at such low miles. Really hoping I am just missing something small. Any help would be greatly appreciated! Thanks!
 

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It has fluid and levels look good. As far as I can tell I cannot see any metal particles in it or anything. It's a good red color. Not brown or pink.
 

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1982 300D Turbodiesel (US Spec)
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Sorry for the late response.

After ruling the possibility of it being low on fluid, I think it might be an internal transmission problem or something rare like the valve body coming loose.

Regarding the fluid, I don’t have any particular recommendation other than using a fluid compatible with dextron-II. You don’t want to use a fluid with detergents in it.

These transmissions are not very susceptible to brands. For as long as it is compatible, you should not have problems. This is not like a Honda or Chrysler…
 

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Any DEX-II compatible fluid will work.
Your symptom points to a failed oil pump, stuck open pump relief or broken input shaft. If it were a valve or clutch/brake issue you would still have either forward or reverse working.

Does it make any noise at all?
 
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