The saga continues...

Changed the fluid about 3 weeks ago, including the filter and torque converter. I have confidence the Valvoline Maxlife fluid is only going to help the transmission, but first gear only seemed to get more puny. The last week or so I had to be extremely gentle with it until I got past about 5 MPH or it would slip.

But, today I put it up in the air and checked transmission pressure... a full 5 bar! It appears to have the updated modulator on it with the plastic, slightly-adjustable cap - I wonder if the pressure wasn't checked and set correctly when that was swapped out some time in its past. Anyway, I corrected the pressure to approx. 3 bar (spec is 2.9) and it behaves much differently - including engagement of first. I only had time to take it down the street and back to make sure it would go through the gears, but I'm optimistic that it will improve first gear's behavior.

I noted that when the modulator is connected to vacuum pressure at the check port goes to near zero - why is this? Also, any theories why excess line pressure could cause apparent first gear slippage?

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'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".