Lockup 904 question

Many years ago I had an 88 d100, was the biggest POS I have ever owned, to which I attribute to being a 1st year Holley TBI setup you all know what they say about buying a vehicle that's the first to have _______ on it.
I spent more "good money after bad" trying to make that truck get me more than 8mpg when I didn't have a major tailwind behind me..... even my 79, full time 4wd ramcharger and my D300 dually weren't as thirsty as that 88 was ....

Among all that money spent was a swap from a 904/998/999 which was stock/original to the truck, to a 518 with OD. My original trans had a sending unit facing the back of the truck out of the trans case, that was a computer control for the LU. It looked just like a coolant temp sending unit from all outward appearance. .I don't remember my 89 diplomat having the same. Though it was carbureted. That may have been the difference in whether a given vehicle of the time had it or didn't.