2008 Lexus RX350. 120k. The vehicle died while driving after low battery light came on. Mechanic changed the alternator and battery.
On the way home, the trans started jumping out of gear, rpms revved high and dropped in gear.
Now it shifts erratically, slips, can't find gears.
How could replacing the battery and alternator lead to this?
Codes then came on P0776 Pressure Control Solenoid B Performance/Stuck off
P0741 Torque Converter Clutch Circuit Performance Stuck Off

Pretty sure the codes came on as a result of the erratic shifting and are not the actual problem.

Unless they somehow messed with the tranny neither the battery nor alternator will have any affect on your trans BUT by disconnecting power to the TCM (Trans control module) and ECM (engione control module) will cause the car to learn your driving technique/habits again... this usually takes under 100 miles to relearn so maybe just give it a little time. I would write down the codes, erase them, go drive, and see if everything resets here shortly. Not that this is your problem but at that milealge you should make sure you replace both the trans fluid as well as clean the trans filter it is not a tough or costly job

The car is pretty much undriveable with the slipping, banging into gear and erratic shifting.

Do the RX350's have issues with the ecu's/tcu's like other models?

Could be that a ground wire was left unhooked somewhere. My 2002 Highlander had similar behaviours after a valve cover gasket change, I let one of the small 10mm ground terminals unhooked. Was so bad I thought my trans was done for

no, not heard of issues with their ECM's or TCM's, occasionally have heard of tranny issues but most are the result of not changing the fluid regularly and at 120K yours is overdue for fluid and filter change. Several people have said their cars felt odd until it releearned their habits once again.

I changed the fluid and filter to rule that out. I think it is electrical somehow.

I pulled the battery and did not see any ground wires laying loose. I'll check around the alternator.

I think this is more than a relearn.

I didn't find any issues with the wiring. Maybe I'll try a junk yard ECU.

I put a new valve body, trans filter and 3x trans fluid change with WS fluid.
Fluid was black, probably original. Not metal on magnets.
This all made no improvement.

I still have a hard time believing that the trans went immediately after having the battery and alternator installed.
I am still thinking it is electrical.

in changing the fluid draining it only takes out a portion there's still quite a bit left in, IF you find that driving it and the TCM recalibrates to work better you may want to do another flush to get out the rest seeing how you said it was black. I still don't suspect it being an ECM or TCM but could be wrong.

Me either.
Of course anything is possible, like one in million chance on the day your alternator went out, transmission dies.

It doesn't add up. Lol

I will hazard a guess the alternator failure was a side effect
Like posted above, make absolutely sure your frame grounds and engine grounds are legit.

(1): What is the engine running voltage measured at the battery terminals?

​​​​​​(2): what is the engine running voltage with red probe of the meter on positive battery terminal and the black probe on the valve cover?

(3): what is the engine running voltage with red probe on the positive battery terminal and the black probe on a metal chassis ground? Can use anything unpainted metal on the chassis.

The voltage was the same across all measurements 14.0

Ok, I agree ECM, TCM probably not this issue as I have seen almost zero failures with them.

I checked the ground wire on top of the trans for voltage, also 14.0.

Who's next? Throw out some ideas.

Lets fix this together, LOL.

​​​Other than the transmission issue, is there anything else unusual that you've noticed?

Any dash idiot lights illuminated, scanner codes, or stuff not working(or glitchy) that was working before this happened?

PS, your running voltage is good.
You may see approx 14v when first starting the car and it may drop to approx 13.5 after running a while.

Nothing else is not working or glitchy.
I ordered a Bi-directional scanner.
Going to reset the trans TCU and reinput the Compensation Code.
I did find another driver that had an issue with the ECU on a 2008 Sienna, which is the same trans, so maybe the ECU is back on the table as a possible issue.

The Engine Ecu controls the transmission valve body solenoids on our cars.

Are you measuring any AC voltage at the battery terminals?

Just being thorough with a new alternator and transmission issues that immediately followed.

No a/c volts.