E4OD TC will not lock up when cold

Working on my 1989 F-250 with a 7.3 International diesel that I've owned since 1991. A couple of years ago I noticed that my torque converter was not locking up when it was cold. At first it had to be real cold (under 20 degrees F) before I noticed it. Later it got to where it wouldn't lock up at 40 degrees. Well last year that E4OD died and I installed a rebuilt one. Transmission shifts great but alas the TC still will not lock up when cold. Now at 45 degrees or so I will have to go 2 or 3 miles before it locks up. As before, after it finally locks up I have no more problems until the truck gets cold again. At first it would be better when I plugged in my block heater. Now that doesn't seem to help as much. As per advice from the rebuilder I replaced the computer and I replaced the Tach sensor and speed sensor. No help! Any ideas???

I think thats the way it should be. you might have a temp sensor going bad. they shouldnt lock up until closed loop

My 90 f150 has a 4.9 with the e4od and it has been that way since new. It takes a couple of miles to warm up and lock up. My 05 seems to do the same thing.