Before I take the car into the mechanic I wanted to check with you guys to see what your thoughts are on my problem. I have a 1985 W123 300TD that spontaneously decided to leak out all of its transmission fluid in a couple of days while I left it sitting in the back yard. Leak seems to be coming from between the engine and the transmission... Any ideas on what it might be and what do you think the mechanic will charge? I'll see if Hauer's German Auto Repair can take the wagon next week.

Sounds like the front seal on the trans.

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MC Professor

1981 300D non-Turbo
with unknown 617. motor and 711.311 transmission

The transmission has to be taken off. It can leak from the front seal (most likely) or from the oil pump gasket (less likely). A failure of the gasket results in a slow leak, so a faulty front seal is more likely.

Only replacing the front seal is not very difficult. Replacing the gasket of the oil pump is time-consuming, as the old seal has to be scraped off very carefully, so that no old gasket parts fall into the transmission.

The bearing of the oil pump (just behind the seal) has to be good too. It can be replaced, but the pump has to come out. You can also replace the seals in the entire pump, but it is not necessary.

The mechanic will charge labour costs for removing and installing the transmission.

I recall that the rubber that holds the lines can degrade. Once that happens, there is metal on metal contact and the tubes get pierced??
Not sure if this is your issue, but thought I'd add this.

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1985 300d Turbo Diesel

+1 on the cooler line fatigue. Check the mounts for the lines and see if you have a work spot.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

It turned out to be the transmission oil cooler lines. I purchased 2 lines and it was amazingly simple to change them out. After that I went ahead and changed out the transmission oil with full synthetic ATF from wallyworld. I also including the stuff in the torque converter and put it a new filter and pan gasket. I want to thank everyone for all of their great help!