Transmission is suddenly acting funky.

I drive a 2013 Ford Fiesta. I recently replaced the CV axle on the driver's side. I also did a transmission drain and fill using the motorcraft dual clutch transmission fluid to refill the transmission. Car has been fine for weeks until a particularly rainy day, yesterday on December 24th in California. The car seemed fine in the morning but as my trip progressed, I started to experience weird shifts on second and third gear. Like I lose power and then I get a high rpm, then it would shutter and go back into regular motion. I also had an issue to where I am stopping and the car would continue rolling for a bit until I really pressed the brakes down. This stopped when I parked for my doctor's appointment and the car was fine again. But tonight, I took the car out to do some work. It was ok at first but maybe and hour or two into my driving, the problems started happen again. The same odd shifting patterns, loss of power during shifting, same rolling further ahead a bit unless I really press the brake pedal down. If someone could give me some advice, i'd gladly appreciate it.

It's the water entry part that has me concerned...

• this may be a loose wire connection or when the CV was changed, perhaps a nicked wire in the speed sensor and/or ABS harnesses - crushed wiring can also cause this...due to replacement part or removal of the old one can damage the wires.

Did you disconnect the battery? This High-RPM issue may be due to low power. Not necessarily the voltage - but available current thru a corroded or loose connection point - drops the voltage due to power trying to flow across it. Check the connections of all those sensors in and around that Left axle - especially the speed ones at the steering knuckles where the CV splines meet as well as the seating of the TCM connector.

In essence - go back and review all the spots on the car you worked on - and make sure all those parts are seated, and any signs of corrosion or wet gaskets are a sign of water entry wreaking havoc with your electronics. Even a lose bolt or the castle nut on the axle to hub - can make a vibration show up and the resonation it makes, causes interference with the sensors "listening" for tone off the ABS wheel teeth used by the system - any sort of fault will eventually set a Check Engine Light so start looking as a code-scanner to use for looking thru your DTC connector and start keeping one in the car when this acts up.

You will need to use the scanner to find these codes...and look at the I/M monitors to see in any of them show NOT READY - if you do, that sub system is possibly causing the faulty shifting.

So what's the least and worst case scenario? Should I just take it to the dealer or do I keep working on it myself? The way the car behaves, I honestly think some water may have messed up a sensor but, I don't know the dct well enough to really make an estimated guess.

If you are not sure - then take it to the dealer - but one you trust. There have been a rash of "warranty" claims that become out of pocket expenses because the dealer and the Maker (aka FORD) can't or won't work with each other and or with you altogether to fix the problem. You get stuck with the bill for all those tests and attempts to replace or repair parts that really don't need repair or replacement - but have to be paid for and it's costs are usually passed onto you.

This shift "dropout" or loss of power can be electrically-caused by a faulty alternator not being able to keep up with demand, which would set (turn on) or make your dash lights like BATTERY symbol or Check Engine lite up - but that doesn't seem to be the problem - it may be a weak connection to the TCM you just have to physically unplug and reseat the connection - then do a relearn by just driving it.

It's why I mention to simply go back and re-check all the steps you did to replace the CV joint so you didn't overlook something and now have to scramble and maybe even get it towed to a dealer - just to fix a loose bolt or a wire that got pinched or broke during the work. Even the Serpentine belt slipping on the alternator - can cause the dropout because the TCM demands power the system can only provide instantaneously when both the battery and alternator can supply the power needed, then the battery recoups the charge while you drive and the alternator takes care of the rest.

If you do find something is broken or loose and you can't seem to repair it, at least when you take it to the dealer you can explain the problem and have them repair the broken part - they don't have to know the whole story - but if you had problems during the repair - they will see them too and may question you about it.