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???

That's normal operation on most automatics. The PCM will keep the converter unlocked until the transmission fluid warms up to a certain temp. Tranny fluid can be too cold as well as too hot.

Thanks for the reply, as stated I have owned this truck for 15 years and this problem never existed until a couple of years ago. I live in central NC and it never gets real cold here.
You know, it's also possible that it's not a torque converter lock up problem at all. It may be that it's not going into overdrive until it warms up. What happens is I have to run about 2500 RPM to go 55 MPH and after it warms up I tach only about 1900 RPM's to go 55 MPH. The transmission shifts but it doesn't shift right.
What do you think??

The e4od has an oil temp sensor, which is mounted on the solenoid body, inside the pan. It sends info to the control module, and alters the shift points to compensate for the thicker, cold fluid. Most shops would not normally change this during overhaul. This may be your problem, and there are also solenoids, and control valves that affect the shift into od, and convertor lockup. You say the trans doesnt shift right, did they install a new convertor during the rebuild, and has it shifted funny since you got it back? I would take it back to them and let them look at it.