(Solution now found, I will add it into the replies below). The car will upshift most of the times ok, but downshifting it will clunk pretty hard and pretty much all the time especially at lower speeds. Like you're driving along at 10 mph with the car in 2nd gear and it has to downshift as you approach a stop sign

It's possible this is NOT the transmission, but maybe something else. I replaced the transmission mount 2 years ago and the front (or was that the rear) flex disc 7 years ago. I have it at a transmission shop now, they thought maybe the modulator was the problem so they replaced that, but that made no change. We've sort of ruled out any vacuum leak because if there was a leak, it'd clunk all the time.

Any thoughts?

1991 300d, 196k

If the clunk is from the rear, check the rear differential mounts. Bad ones will make a lot of noise on downshifts. Replacing mine made a world of difference.

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

My downshift clunks went away after adding shims to both my rear axles where they mount to the differential. Apparently a previous owner had lost the original and the axles had alot of slop.

__________________
1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660

I was getting nasty clunks on all shifts until recently. I replaced the brake booster hose because it's vacuum fittings were way past brittle. Suddenly got scary smooth shifts and basically no clunk. I still get it if I let it downshift itself into first, if I shift it manually into first it doesn't really do it, or if I take it out of gear when I'm approaching a stop it doesn't do it.

__________________

Kanthaka 4

Other ideas: worn axles, flex disc, or trailing arm bushings.

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

Definitely check the flex disks for play. My GF's c230K was clunking badly on downshifts, turned out to be a loose front flex disk.

__________________
2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

Check your vac again. It needs zero vac at upshift for no clunk and full vac at downshift for no clunk. If you unplug the vac line from the tranny, it should upshift fine but clunk badly at downshift.

The transmission shop just called. I haven't driven the car yet, but he said it shifts smooth now.

I told him when I left the car that maybe there's no "vacuum leak", but it's possible there's a "partial vacuum leak" somewhere that would cause this. I previously replaced as many rubber hoses as I could see, but it's possible there's something under the car that I can't see.

They found one of the vacuum hoses not plugged in 100% of the way that was causing the problem.

I don't know if this was mentioned previously.

I inadvertently disconnected the Vacuum Line to the Transmission Modulator and the up shifts were nearly the same but the down shifts were extremely harsh.

People have had the Trans Modulator Cap Crack or fall off.

I am not sure if a bad Trans Modulator would have the same symptoms you mentioned.

__________________
84 300D, 82 Volvo 244Gl Diesel

The transmission shop changed the modulator thinking it might be the problem, but it wasn't. Then they finally found that one of the vac lines on top of the engine wasn't fully in which was causing the problem.