I tried a search, but nothing came up. new member, 2008 3 with 2.3l. P0778 code, after warm up, few miles drive, 5th gear off throttle, check engine light and AT come on. Hard down shift to 3rd, then the lights stay on but it seems to drive reasonably ok around town with firmer shifts. I hookup a Snapon Solus Ultra diagnostic scanner, and do a functional test while driving. It shows no readings and still drives the same. I stop and clear the codes and switch to Data Display and do another test drive. The car drives perfectly and the transmission shifts perfectly. The scanner is on the line pressure B% setting, and I get varying readings at the 4-5 and 5-4 shift moments depending on throttle and speed.
I disconnect the scanner while still driving, and it continues to perform perfectly, smooth shifts all through the range up and down, no check engine or AT lights for miles.
I stop and turn off the car and wait for a bit, start the car up and go again without Scanner hooked up. Very quickly the Check engine and AT are back on and the hard shifts etc. are as before. Rehook the scanner on the Data Dsiplay and all is perfect again.
My question is What would the scanner in Data Display be doing to the TCM or the ECU that aren't happening without it? Apparently all the wiring and the solenoids are in functioning condition, no mechanical problems. or physical wiring shorts or opens, since it can function properly.