It's been a lovely trip so far, driving the '81 300 td down the east coast from boston visiting friends, biking the overseas highway to Key West and then back up to New Orleans until....
I was on the highway in central florida and the transmission starts slipping. I knew I had a bit of a leak so I wasn't too surprised, it had been quite a while since I topped it up and I find it amazingly difficult to read the fluid level after driving it on the highway, I find that it needs to sit and idle for a good 15 minutes before i can make heads or tales of the dipstick. So I top it up and get on my way the the slipping goes away after another 10 miles or so and then starts coming back after another 75 or 100. I top it up again and then the same, more slipping after 50 or so miles. Now I look underneath the car and see some carnage: The entire underside of the beast is covered with transmission fluid, when I stop it's dripping off the exhaust and undercarriage and even the rear suspension, in fact as I keep going I notice a fine mist of it covering the back window. I keep topping up and keep slipping until I can make it to civilization and I spent last night in a rest area with the car up on jack stands (yes, I travel with jack stands) trying to figure out the location of the leak. I even drained it and dropped the pan to figure out if the gasket was totally hosed. I cleaned everything up with simple green dried it off and idled it forever and there were a couple of small drips coming from the driver's side, one in the middle and one in the back driver's side corner. The rate of these drips could in no way account for the massive fluid loss I experienced though, it must be a leak that kicks in only when driving somehow. My intuition is that it's mostly toward the back of the transmission, there is often a lot of fluid dripping off the mount (may just be blowing back). It looks like there's an electrical sensor next to where the drive shaft comes in and the bolt that holds it in has drips of red coming off of it. Is it possible to leak from there?
I'm still 200 miles from NOLA but I'm going to try and limp in there and find an indy on monday. I really love this car and have done lots of work on it and it would be a shame to leave it down here. Any thoughts would be appreciated, help!
thanks!
-=e=-

Check the cooler lines, it could be blowing back.
Since you have jackstands, I'd put it on the stands, chock the front wheels well, put it in gear and spin it up to speed to see if you can see fluid coming out.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Sounds like cooler lines to me also. Make sure the banjo bolts are tight, and wipe them all down and try what Kerry said.

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Hmmmm... Not cooler lines... My current brainstorm has to do with engine overheating. I had a simultaneous problem with the adjuster on the alternator becoming loose that kept recurring as I was driving. I figured that the rise in coolant temperature I was seeing was the result of the transmission slipping but now I'm wondering if it's the the other way around. Could the transmission slip in 4th as a result of overheated coolant raising it's temperature? It certainly seems like that's the case as I mickey moused a fix for the alternator bracket, kept an eye on the transmission fluid level and drove the 200 miles to nola without a hitch (or a substantial leak). Now I'm thinking that the fluid level was fine and it started spewing out after I overfilled it thinking that the slippage was due to fluid loss. It's always slipped a little in 4th but only when it's failed to shift down and I floor it at 40 mph or so. Perhaps this is all due to my own dumbassery (likely) but any thoughts about an overheating transmission causing slippage would be appreciated, I haven't found much on the subject by searching....
a million thanks.

More than likely it is way over full which is causing foaming, then boiling, then slipping, then trans fluid making its way out a vent. Take it to a good mechanic and say... please make this right... and while your in there turn up my trans pressure just a hair please

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1996 s10 zr2 with a 6.5 turbo diesel
1996 c2500 suburban with a 6.5 turbo diesel
1981 300sd with a turbo 617
Monte carlo figure 8 car