I've only got 299,000 miles on my 83 300CD transmission. It shifts solid from 1st to 2nd, slips a tad on the shift from 2nd to 3rd under medium/light load, slips on the shift from 3rd to 4th with light load. I let up on the accelorator when it is ready to upshift to the next gear to try to eliminate the slip during those shifts.

I'd like to get another 300,000 miles out of this tranny but I doubt that will happen. I've have the bowden wire adjusted about how I like it and will probably have my indie adjust the modulator when I have the tranny serviced to see if he can firm up the 2-3 and 3-4 shifts.
I put a quart of trans-x about 40 miles ago to clean out any varnish before I have the tranny serviced. Yes, I drained some fluid out first so as to keep the level exactly correct.

1) My first question is, how long should I let the trans-x work before I have the transmission serviced?
2) Any other suggestions on what to do to help with the weak shifts?
3) Any suggestions on how to extend the life of this transmission, I'd like to get my moneys worth out of it before a rebuild
I'm mostly kidding on getting much more out of this tranny and I know at some point I'll have it rebuilt or replaced. I'm not sure if I'll rebuild it the next few thousand miles or wait until it gets worse.
I guess when it is time to rebuild, I'll see if my indy can recommend a trans. shop that he would trust if he won't do the job himself. This car has been taken car of by the same mechanic since new and I don't trust just anyone to tinker with it.
Sorry for the long message here. Just trying to get some input from other owners who know and love these cars.
Thanks, Tom

If I remember correctly trans X says to drain the fluid after 100 miles. It says right on the bottle. Just use cheap regular atf and change it every 20k-30k. I wouldn't waste money on Mobil 1 for a 300k trans. When you rebuild it then use Mobil 1.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Your trans sounds a lot like mine.

If your 2/3 valves are working well and shunting high vac at closed throttle, you can up the line pressure and adjust the vac valce to firm up shifts withouth haveing neck snaping downshifts(important for not driving one crazy). Otherwise, it's just worn out, probibly the seals dried up in the clutch drum, not holding fluid pressure. Your description sounds almost exactly like mine, and it's been just this way for 50K miles or so. It'll last, adjust your settings and driving style, and start looking for a replacement.

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

What does it cost to have a trans. rebuilt ?? or is better to buy a remanufactored one?? Dennis

Considering the age and mileage on the car, I'd say buy a used one that was working before it was pulled -- much cheaper than buying a rebuilt one or even rebuilding yours by yourself. That's what I did on my 500SEL which has even much fewer miles.

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver

Before I bought my 300TD I asked a fellow I know at a local shop what a replacement (rebuilt) trans would cost, installed. He quoted $1,800.00.
I was concerned about what I thought was slipping. (No power, early shifts)After buying it I found a problem with the Bowden cable and now it shifts crisply at higher rpm.
Good Luck,
Ron