This car is new to me but apparently has never had transmission issues. It has been sitting for at least 8 months. I have driven around town for 3 days without a problem and began a longer road trip today of about 45 miles. At mile 40 it started to down shifted from top gear into 3rd and I took an exit and let it sit for a little while and checked the fluid. The fluid was a little high but only by 4-8 ounces. It was shifting hard when I took it back on the road and when I went back to the interstate it began to slip. And began to slip to the point to where I could not go forward anymore. I felt the transmission fluid radiator which was very very hot. When it cooled down I was able to drive again for a few miles until it heated up again.

I checked the transmission drain bolt and was going to drain the fluid. I realized it was stripped and understood by that the transmission fluid probably has not been changed for awhile.

Question-do you think the transmission is failing because of a clogged filter or bad oil? My first step is to drop the transmission pan and change the filter and the oil.

Any advice would be welcome. What is the size of allen bolt that holds the pan?

Thanks

Will

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Thanks,
Will

1985 300D- 222k- (bad transmission)
1980 300D- (parts car)
1982 300SD- 242k