Transmission slips only on the highway going up an incline

It is possible that you are feeling the torque converter unlock, transmission slip, or engine hesitation.

You mentioned that the fluid was pretty dirty when you changed it. Has it ever been changed before? If the transmission is actually slipping, the replacement transmission fluid will become dark pretty quickly. Of course, if you just drained the fluid out of the pan, then there was dirty fluid still in the torque converter and transmission fluid cooler so you will get some darkening of the fluid from mixing the old and new.

The best way to properly diagnose the condition is for a technician to hook up transmission pressure gauges and take the car for a test drive to duplicate the condition and record pressures.

Yes I put in new fluid and Yes it darkened up VERY Quickly. I'm not wealthy otherwise I wouldn't try to fix the problem myself. Should I attempt to Flush the fluid somehow or just keep changing it out the way I am now? Is this a serious matter? If so then I guess I would have to bite the bullet and take it to lexus. Thanks Ray. Your advice is always appreciated.

Unfortunately, it does sound like the transmission is slipping. Slipping occurs when one or more of the friction surfaces is worn or burnt, or the pump is not developing sufficient pressure, or one of the passageways is clogged or restricted.

You can take it to a dealer or transmission shop to have the transmission flushed, although the process occasionally causes problems. I would try 2 or 3 more fluid changes until the automatic transmission fluid (ATF) is relatively clear and see what happens. It might be worth it to invest in a transmission pan gasket and drop the pan and check out the screen. If the screen is clogged, try flushing it with some ATF. Some metal particles in the screen are OK but a lot of metal particles means that something in the transmission is broken or badly worn. Dark sludge on the screen is an accumulation of friction material from the clutches and brakes - there should not be a lot of it. By the way, make sure you are using the ATF listed in the owner's manual.

Good luck!

Ray that response makes great sense. I have been using the correct ATF from Lexus. I considered dropping the pan to check the screen and magnets but like you said I will replace a few more times and see what happens. Here is what I feel is the key to my problem which can further narrow down what may be wrong. The transmission only slips while doing 75 mph AND going up a steep incline. The transmission runs fine during all other times. It doesn't slip at 80mph and it doesn't slip and 65mph. It also doesn't slip at 75mph unless going up an incline. Does this narrow the problem down at all ?? I was hoping to take a cross country road trip next month but dont know if it's such a good idea anymore.

Without taking the car for a road test, it's hard to say what is happening. If it is a continuous slip, then you have a problem. If the RPM jumps about

100 to 300 RPM and holds steady, then it sounds like the torque converter is unlocking, which isn't a problem. Hopefully, this is all that you are experiencing.

good luck!