Chrysler Repair: Auto transmission fault codes: 95 minivan

Question
My 1995 Plymouth Voyager shifts 123 until it trys to go to 4th and then it jerks 3times-- every time- but wont go to to 4th gear.I have tried changing the solenoid 2 times with the exact same results.It also falls down to 2nd sometimes and I have to turn the key off to kill the fire and turn it right back on and then it will go back to 3rd.But it never goes to 4th. Please help if you can. I thought some kind of pressure switch. If it is that could you tell me where it is located. Thank you very much.

Answer
Hi Larry,
Throwing parts at a transmission situation is the wrong approach.
The best way to diagnose the problem is to use the on-board fault code system to get a two-digit fault code number that would give a cause for the problem. You do that by means of a plug-in fault code reader that plugs into a blue 8-pin socket located at the far left end of the dash on the lower side (called a Chrysler data link connector). The vehicle uses the OBD-I code system so the reader has to be capable of reading those code numbers (the OBD-II system began in '96 model year).
A nationwide auto parts store such as Autozone may do a free readout of the codes. An independent shop will usually do that readout for around $40.
But be sure to get the transmission fault codes (which are different from the engine fault codes which are also two-digit). Ask for the numbers, what they mean, what they recommend. Then get back to me with what you found and we'll compare notes with the shop manual.
When the trans drops into 2nd gear and won't shift up, that is called "limp-in mode" and is designed to minimize damage from driving in all the gears until the issue is resolved. It also means that there will be a fault code to help diagnose the problem.
Please read the PS (below) and respond to it.
Thanks,
Roland