Chrysler Repair: 90 new yorker electronic transmission not shifting

Question
I changed the filter in my 1990 new Yorker, now it doesn't shift. The filter is securely in place. What's wrong?

Answer
Hi Mike,
That sounds like it is working in 2nd gear only which is called limp-in mode and it does that when the trans control module has noticed a fault which it seeks to minimize any damage from by cutting the power to the gear changing mechanisms until it can be remedied. To find out what the fault is you need to get to a shop that has a plug in code reader and have that device read out the two digit fault code number(s). The plug is under the dash at the far left end and is blue and has 6 pins. Be sure the codes are read from that plug as engine fault codes are read from a different plug in the engine compartment and you don't want to confuse those two sets of codes, which are both in 2-digit numbers. Let me know what codes you get. I can't understand what would have caused this to happen from changing the filter, assuming that you used the correct amount of fluid and it was Mopar ATF type 7176 or equivalent.
When we have fault number(s) we can then decide what to do.
The same fuse that powers the engine via its shutdown relay also powers the trans thru its shutdown relay, so if the engine runs then that power for the trans should be available to operate it. About the only simple reason for no shifting would be if the trans shutdown relay were not working. That relay #6 is in the outside-most row of 3 relays in the power distribution box, and is the middle relay in that row. You could try substituting relay #2 just to verify that the #6 relay is working. If that doesn't restore shifting then you could temporarily jump from the front to the rear pin of the relay socket which will power the transmission electronics and observe how it is shifting. But that should not be done accept to test as there would still be the problem which the trans controller is saying should be fixed to prevent further damage.
Please read the PS (below) and respond to it.
Thanks,
Roland