ignition switch

try this:
1. First, insert the ignition key and turn the ignition switch to the “On” position (Do not start the engine). Leave the switch on for 11 minutes. The security light may flash or be on steady during this 11 minute period. Make sure the light is no longer on or flashing before proceeding to #2 .
2. Turn the ignition switch off for 30 seconds.
3. Turn the ignition switch to the “On” position (as in step #1) for 11 minutes again.
4. Turn the ignition switch off for another 30 seconds.
5. Turn the ignition switch to the “On” position for 11 minutes a third time.
6. Turn the ignition switch off for 30 seconds one final time.
7. Turn the ignition switch to the “On” position one final time and wait 30 seconds.
8. Attempt to start the engine. If the engine starts and runs normally, the relearn is complete. If the engine does not start, check for trouble codes and address them before attempting the relearn procedure again.

mine is a 91...will this work on it or is it for obd 2(later model diagnostics)

the early 4L80s had lots of issues with the wiring harness for the trans internal and external if you have a small bolt that goes throught the center of your plug in the harness for your trans then it is early and needs updated I am an ase cert. trans builder at the same shop for 15 years....I can get you the parts if needed or do total rebuild... the wiring kit comes with a new updated internal harness and a new plug and pigtail for the outside that has to be cut and connected

I'm not sure if this will help my truck said the same type of thing yours does and it ended up being the park neutral position sensor. it acted the same way as yours

yep 91 was the first year,what did you unplug the trans plug or the MLPS( manual lever position sensor)

no the engine should not even crank with the manual lever position sensor (mlps) unplugged, sounds like you got a wiring gremlin

yes you definately have a wiring problem the van should not start with the switch unpluged. check to make sure you didnt pinch the harness when you installed the tranny. check ecm connecters for corrosion or water intrusion. if you dont find anything I would suspect the ecm? when you clear the codes do they come right back? or do you have to drive it for them to reapear? sounds like the ecm is in "limp in" mode because it dont know what gear you have selected?

Oh sorry I thought you said you changed the in an earlier post but I see I read it wrong. Does it start in second after you clear the codes or only after the code sets? Does third and overdrive work? Torque converter clutch work?