85 300TD cold weather trans thump
 

I sometimes experience a clunk when I slow down from my trans. This only seems to occur when the temp. gets below 40 deg F. It used to happen often until I found a leak in one of my diaphrams under the dash. Could this be a vacuum problem or do you think it may be something else. I was thinking of a faulty modulator valve? Thanks.... PS all my locks work and I don't have the problem when the wather warms up..

I notice the same exact thing... All my vacuum system is fine, as far as I know. I actually think it has something to do with ATF. I had it changed last time and they used quaker state ATF, the variety that says its good for Euro cars (thats what the indy uses). I think maybe its a bit thicker and so is a bit harsher to shift. I am going to have it changed by the MB dealer soon to see if that makes any difference at all (the fluid has only 20K, but it seems a bit foamy lately, so I want to change it out).

Ill update if that makes any difference, once I have it taken care of.

I also notice that there is a bit of a clunk from the rear end sometimes, especially if feathering the throttle on and off while going real slow at low rpm. Dont know if its related, but it only really shows up in the cold as well.

JMH

In my case I'm not sure about the ATF fluid. It sometimes happens even after driving for a few hours which would lead me to believe theat the ATF fluid is plenty thinned out by then. I've had the same clunk in the rear now for the last 250k. It doesn't seem to get any worse though. PS I don't reccoment you go to the dealership, unless you have money to burn!!..

I have a mity vac. How do you set it up and whats the ideal reading to the transmission. Thanks

Turn the heater off by pushing the far right button on the control. If the condition improves, you have a vacuum leak in the heating system somewhere. If it doesn't look elsewhere. Could be flex disks but you need to exclude vacuum leaks. Ideally, you can tap in a Mity Vac and monitor the vacuum to the transmission as you drive.