Chrysler Repair: 96 grand voyager speedo jumping, inner fender, rust belt

Question
The problem came on slowly but is now always there. While driving the speedometer jumps erratically back and forth,sometimes between 10 and 90 mph.The problem seems to never occur during acceleration,only during part throttle and decel.If the cruise control is used at 70 mph, the trans jerks frequently,like it is being disengaged.The trans went into fail safe mode, 2nd gear,and I pulled the battery cable ,erased the memory to get the car home.The transmissiion was replaced 13 k miles ago,and we went back to the shop that did the work.The tech test drove the car,repaired the output speed sensor harness connector (per mopar service bulletin) and replaced the speed sensor,although he said he didnt think it was  defective.The problem continued,he hooked his scan tool up to find only the egr code that has been there 4 yrs.No trans module, body control module or other engine codes. He said the three operate on a common bus but isnt equipped to diagnose more of the problem.Any ideas on where the trouble might be? The mileage is 171k and we live in the rust belt.Could this be a corrosion problem? And the worst question; Will I be forced to go to the dealer?

Answer
Hi Jeff,
Before suspecting the transmission I would pursue the speed sensor wiring to make sure it hasn't been damaged. The output sensor of course runs the speedometer and also is necessary for the proper control of the trans. If you satisfy yourself that the sensor (new) and the wiring is solid, then unless you have a code that says there is something wrong with the data bus itself you are probably left with the gear in the trans that drives the sensor as the suspect. The shop that did the work would be the logical place for that evaluation and possible repair under warranty. The wires from the sensor go to pin 13 (light green/white) and pin 14 (dark blue/black) of the 40-pin  plug at the transmission controller which you will find on the right inner fender. My suggestion would be to measure the resistance between the sensor plug and the transmission plug pins and verify that it is close to 0 ohms (e.g. continuity) and also try flexing the cables/harness at both ends and along the way to see if that might reveal an intermittent open circuit in either wire.
If you think the trans shop's reader is not sophisticated enough to read all the codes (which is doubtful) then a free readout at Autozone parts store or a lower cost read at an independent shop (compared to the dealer) would be useful too.
Of course if the trans is driving normally and not going into limp mode, then your cluster may be the only cause of the problem with the speedo. But you seemed to say that was an issue too.
Roland
PS Please evaluate my answer, thanks.