Found it! You don't use the diagnosis box at all. You ground a terminal on the other side of the truck:
http://m.yotatech.com/showthread.php?t=169161
Looks like I have two codes, which are both speed sensors. Can't find anything about it in Haynes but I'll figure it out.
I've found plenty of posts diagnosing the overdrive light go the second gens but none for the first. I connect the terminals for diagnosis mode and I only get the CEL blinking. The overdrive light blinks when my tranny seems to refuse to downshift to first. It does this in "fits" and it happens...
