What is ecc relay?

It has to do with the tranny shifting. It also has about 4 different names - can't remember what Ford calls it but the parts catalog isn't listed the same as the label on the relay box.

I don't remember off the top of my head what exactly it does but when mine went out on a 90 f250 it made the tranny go into the emergency mode which makes it shift really strange and only runs in a few gears. Pulled it out and cleaned the contacts and it has worked ever since.