This wasn't the first time I've drained the tranny fluid so I apporached it too casually. How difficult is it? Remove the drain ****, let the fluid (approximately 2 qts/1.9 L) out, replace the plug and fill. I've done this at least 3-4 other times.

This time, my drain pan was almost to the top which is 6 qt capacity, I didn'tthink much but now I estimate it at maybe 5 quarts. I was scratching my head why so much fluid was drained. I filled it the customary 2 qts and took it for a drive. Seemed fine.

Next day out, I'm sitting at a light and when I release the brake, the car didn't move...then it caught and shifted normally. This wasn't right and I knew it was related to my drain. I get home and check the dipstick and warm level is in the correct range.

I buy two more quarts and with the one left over, I have almost 3 quarts to do the procedure. This time I measured the fluid in the drain pan and it's 1 quart. I'm thinking sheep!, I hope I didn't ruin anything. A call to a local shop alleviated my fears as he relayed the tranny is near bomb proof and a few miles even low wouldn't have grave consequences.

I fill it up the 3 quarts and the cold level is fine now and next morning I put it in reverse and the RPM needle while in gear revs up to 1,200 RPM. WTF!!! I figure maybe with the low fluid, I need to let any air or fluid equalize again. I drive off and it's slipping only in first and R position and now N-D is becoming harsh. When warm, it's fine.

I dig into the service manual and read through the AT section and trouble shooting. The throttle cable can be linked to some issues so I look at the position. The book states 0-1mm tolerance. The indicator is at nearly 2.5mm. I adjust it and next morning, I drive it and again, it slips in R but not as bad and the P-R and N-D is now nicely smooth when engaged/shifted.

After a test drive, I pull the dipstick and find the fluid level when warm is still in the cold range...WTF. I pour in another 4 oz. and next morning I drive it again. Fluid levels when warm are approaching normal levels and I repeated a 4 oz. fill again. After 5 days is this, I finally got it this evening.

I'd been told that the tranny was more sensitive to overfill than under and and since it didn't trigger a CEL, the mechanic told me to just keep adding fluid till I got it right. Easy to understand but after my screw-up, I had no idea how much fluid I'd mis-calculated.

Had I accurately measured what came out into the drain pan and replaced it exactly the first time, I could have saved myself this trial and test period. I'm baffled why more drained out than past times.

 

It is always a good practice to measure how much is drained and pour back exact amount. You seriously underfilled and likely introduced air into the system. ATF functions as a hydraulic fluid, having air in ATF does not help tranny to generate pressure to shift gears.
Most of the ATF is in the torque convertor, some in tranny cooler, some in lines, some in oil restrainer. I got 7 quarts out on RX300 AWD when there a regular drain and fill only take out a little over 4 quarts. But I had to do 1) drop the pan, 2) take out the oil restrainer 3) disconnect cooler line and start the engine to get them out. Did you ever drop the pan and exam the filter and the oil restrainer?
Pictures and step I took here
https://www.clublexus.com/forums/sho...d.php?t=220061

 

Great write-up in the RX section.

I didn't drop the pan. Past drains, the fluid has still been reasonably clear/clean. (Done at most oil change intervals.) It was a really stoopid move on my part to not accurately measure what came out.

 

what I usually do is add fluid until the level comes between the low and high notches of the COLD area. Then I start the car, run the gears and check the fluid, adding if needed to get the level between the hi/low notches. Works everytime.

 

I recommend you to drop the pan next time. My tranny drain plug is clean, after I drop the pan, I found out that RX's tranny drain plug is on a relatively elevated position, metal shavings are collected mostly by three lower placed magnets (literally on the very bottom of the pan). Drain and fill will get rid of fine metal particles floating in the ATF, but not those already retained by magnets.