So - Did I just cook my transmission or ???? (1980 300D Non Turbo 285K)
Three days ago everything ran fine but I had the glow plug light flashing at me. Three out of the five glow plugs took a dive on me so I went to replace them all this morning after the car sat for three days. Engine started fine. No more flashing light. Been there-Done that every few years forever.
Got in the car to drive it around and the transmission is slipping BIG TIME. Will go OK until you add some power, then it has a hard time getting from one gear to the next. I can hold it in “L” and it holds that one fine, but who wants to drive anywhere at 15mph? Come home after a total drive of about ¾ of a mile and find my fluid not even on the stick. Ooops. Add 1/3 of a quart and it’s at the top of the full line. Drive it again, same thing. So - I’m thinking I knocked off a vacuum line somewhere. Check all those (and the only 1 to the trans.) and all seems well. Put the suction gun to the line and it holds vacuum. Go down below and the seal to the modulator valve (red one) seems ok because the line/cap is sucked to it. I can hear it leak down when I wiggle the cap off. Get that off and turn the key in a half turn to see if that helps. Drive it. No help. Turn the key back a half turn from the original spot. Drive it. No help.
Any ideas ????? Change the modulator valve for 40.00 before I buy a 2K trans for sure. Any other thoughts? Thanks.

Make sure you don't have any massive fluid leaks. Vac leaks will cause hard shifts, not slips. If your trans shifted normally before and then suddenly started slipping you either don't have enough fluid in it or the clutch packs have worn out completely in the course of a very short time. If your fluid just all of a sudden went away then you're leaking somewhere. Get under there and look. Mayhap you had a mishap and curbed it and busted the lines or the pan. Who knows.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)

Could you have taken off the bowden cable at the throttle linkage? I have an 84 300 turbo so I don't know your throttle linkage arrangement. I have forgotten mine in the past. Eric.

LOVE‘N IT !!! Tranny isn’t blown after all !! Leave it to a fellow veggie burner to save my day !!! No, it’s not the bowden cable. I don’t have one. The only thing running to and from my tranny is the speedo cable and the single vacuum line to the modulator valve. But after seeing your post, I wanted to go check things again one more time before I drowned my self in Crown & 7. Popped the hood. Looked. No cables or other linkage running to the trans just as I’d thought (after 25 years, one would think I‘d know for sure every nut and bolt on this thing, but I guess not). Then took a look at the linkage a bit closer. BINGO. There’s a short little linkage that’s off, running to some gizmo on the back of the IP between the IP and the oil filter can. It’s connected to some vacuum control valve look’n thingy (love these tech terms ??) and that thingy has a vacuum line running up to the valve on top of the valve cover that controls vacuum to the modulator valve.
After a moment of dance and jubilation, two things came to mind. First - The plastic piece that’s supposed to hold that linkage on has to be cheaper than a new transmission. Second - I owe someone a beer !! (PM to follow. Check yer box.) I hooked it back up. Drove around the block. Shifts like new !! THANK YOU !!!

i love happy endings. sniff

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Grey '91 350SDL 214k Dad's car
Beige '81 240D 4 Speed 254k SOLD
Blue '82 300D 225k SOLD
White '95 E300D 46k SOLD
Blue '87 190D 2.5 Turbo 315k SOLD
Brown '80 240D 4 Speed 716k SOLD
Beige '80 300D N/A 119k SOLD
Blue '85 300D Model 186k T-Boned

I love it when endings don't include a $2,000.00 transmission overhaul.
YIPPIE !!!