Fresh new battery, but won't hold a charge.
Therefore, I suspect either alternator or voltage regulator has shot craps.
The Alternator passes "basic" voltage testing at elevated idle but I still suspect it is defective considering it is said to be a common problem with SC400's.

One internet site notes that VR is internal to alternator (so it would get replaced along with a new alternator install anyway).
Another internet site notes that VR is a separate device and mounted on one of the inner fenderwells.

Which is correct for a '93 SC400 ?

 

Im almost positive an internal regulator

 

its part of the alternator generally speaking. on the sc400 if you keep loosing alternators often you have a small leak on the power steering right above it, and the drops over time kill the alternator. something to look into before you install your new alternator, or you may be reinstalling another new one soon.
It doesn't sound like that alternator it sounds like your battery to be honest. even if its fresh if you discharge it fully even on accident it can damage it enough to not start a SC which is heavy on electronics anyways.

 

How long does the battery "not hold a charge"? PS fluid can leak onto the alternator and roast it, like Ali mentioned.

 

Well, liquid anything + electronics wont end well. Don't know what exactly the PS fluid roasts. Doesn't matter at the time cuz' you're gonna lose your alternator unless you stop that leak.

I do know that PS fluid could cause the belt on the alternator to slip...

 

True, but IMAO it would be a waste of time and/or money. The rectifiers is $40 alone. Imagine what a rebuild kit would cost. A new alternator would cost roughly $130. You end up putting that much into it anyways, including the 4 hours or so that the rebuild took you.

BUT if anybody wants "rebuilding the Alternator on a SC400" DIY, ClubLexus has that too. Here.

https://www.clublexus.com/forums/per...r-rebuild.html

 

i overflowed my power steering right on to the alternator and fried it. that was when i found out that the voltage regulator is built into the alternator, best thing to do is just replace the alternator and go from there. you could have a ground in your electrical system between the alternator and the battery but that's why you should always have your alternator tested before you replace it

 

OP Back again.
I hadn't checked postings for a while. Lots of replies. Thanks to all.

In my case, it WAS simply a defective VR. Since it was still the OEM alternator, and since there were no significant PS leaks to mess with, we elected to just replace the entire alternator (and internal VR). I could have just spent $50-$60 for a new VR, but that would have taken even more time to install AND disassemble AND repair, and you still have an alternator with questionable life left on bearings, etc.

FWIW, I'm a retired Electrical Engineer (and long-time gear-head). "Electrically", Mage is correct. Since PS fluid is basically an oil-based product, it is essentially an insulator and should not short anything inside the alternator if it gets inside, although it will make a gummy mess as it combines with dust & dirt. Want an example: The heat-transfer fluid that is filling the inside of every pad-mounted transformer you see is either mineral-oil or soybean-oil, and continuously exposed to as much as 34,500 volts. However, my reasoning was the same as SC400slide. After you spend the effort to pull off the alternator, why mess with old bearings, rectifier diodes, etc. Unless it is VERY new and you are certain the bearings and diodes are fine, just replace the entire alternator (including the internal VR along with it) and move on. It might waste a few bucks, but I would feel very dumb if I removed the alternator, disassemble it, clean it up, replace the VR, reassemble it and finally reinstall it . . . . . . . . and then have an old bearing or old rectifier diode bite the dust in a few months.