Transmission in limp mode.
A friend and I have changed out the ECU, transmission computer and ETAX relay on my 95 Chrysler LHS. Yesterday I replaced the input/output sensors in the transmission. I took it for a short run last night and an extended run tonight, approx 30 minutes for the ECU to acknowlege the new sensors. It did OK for a while. Shifted into all gears. Albeit a bit bumpy but wasn't dragging. When on the freeway, it bogged down again into 2nd gear. I pulled to the shoulder, turned the ign. Off/on again and got it home. The check engine codes are 12 and 55. Which I understand to be disconnect of ECU/battery and end codes. The check engine light has never illuminated since I've owned the car (12) years. I'm wondering if it's a trans. Pump problem, tranny solenoid, throttle position sensor or relay shorting out. This has me absolutely baffled. I did notice tonight smoke coming from under the hood. I checked the tranny fluid, fairly new fluid and filter change and noticed what looked like oil burning off the manifold. If the valve cover gaskets are leaking could that cause a shifting problem with the engine sucking air? My friend, an elder, 35 + years engineer/electrician retired from GE and he is also stumped. I also understand Chrysler is notorious for wiring issues. Any insight would be greatly appreciated. Thank you very much for your time.
I've done a lot of automotive electrical work including teaching it for nine years. Chrysler has some of the fewest wiring problems, especially when you consider there can be over a mile of wire in one vehicle. One of the things you didn't mention was checking for diagnostic fault codes. You'll need a scanner to access the EATX. Most code readers only access Engine Computers. Any codes that were set were erased when you unplugged the computer but now that it defaulted to "limp mode" again, any relevant codes should have set again.
There are some generalizations that might get you started. If you start the engine, shift to "drive", and it starts out in first gear like normal, up-shifts, then bangs back to second gear right after an up-shift, it is usually related to slippage in one of the clutch packs. This can be expected long before the mileage you listed. If you start the engine, then it starts out in second gear right away when you shift to "drive", it is generally sensor-related or there's some other electrical problem.
Most scanners that display live data will also show the "clutch volume index", (CVI). That is a set of four numbers that represent the volume of fluid it takes to apply each clutch pack. As the fiber plates wear down, it takes more and more fluid to fill in behind the pistons and apply those clutches. This is where the computer constantly learns and updates its shift schedules to maintain a nice crisp shift throughout the transmission's life. The downside is you don't have the six months to a year warning that wear is taking place like we did years ago. You go from shifting like a new transmission one day to slipping during an up-shift and going to limp mode the next day. The fault code will usually something along the lines of "gear ratio error. That is detected when the input shaft speed is not the same as the output shaft speed, factoring in the gear ratio when it's in first, second, or fourth gear.