Wow, it only took 5 years to let us know what the problem was! Although PS leaks have resulted in many, many alternator replacements in this forum, you might be interested in knowing that PS fluid almost never actually damages the alternator, but it DOES cause poor electrical connection at the brushes. This results in erratic current output to the battery, and gets worse with heat.

The Nippon alternator in these cars is nearly bullet-proof; bearings are superb, rectifier bridge/regulator is very durable, and windings are over-designed. All these components are unaffected by reasonably clean oil, as in PS fluid, so long as it didn't attract enough debris to cause severe overheating from blocked cooling passages. As long as the brushes & commutator have not been damaged by severe arcing, a simple disassembly, cleaning and, if necessary, light polishing of the commutator rings is all that is necessary. No special tools are required. If it needs new brushes, those require soldering a few wires together--no big deal. Might save you $200 or more.

 

I've had the same problem with my 2008 gx470 changed the alternator and battery NOV and for the last 2 weeks It started with shut down and restart the car that works sometime then disconnect the battery and get better connect, I feel it's a ground/connect problem and still a day or 2 later same problem today I took connects off alternator and inspected to find the little plastic that protected from heat to ground was cracked on bottom and connect was tight but had play did not replace but put on so the connect had no play so far no problem but I'll give it 2 days and I am optimistic

 

Check out this post


https://www.clublexus.com/forums/ls-1st-and-2nd-gen-1990-2000/828619-negative-grounding-issues.html

 

I had the exact same problem with my 1999 GS 400 a year ago after replacing two alternators thinking it was defected. But it was not, it is actually your RPM gauge requires to be reset to idle at a higher RPM so the battery can be charged. It was frustrated till I get an electrical diagnostic at Lexus. The battery charge when you're driving but at red lights and traffic, it idle too low that the alternator doesn't send out enough current to charge the battery. I know its a little late on the reply but anyone who is having the same problem, would check with Lexus first. If your Lexus is idling below 600 RPM, the car is not charging.