My daughter has a 1990 LS400 that had a charging problem. Her car also had strange intermittent engine quirks. Not sure if this will help or not, but here goes. We solved the charging problem, but not until we cleaned up one of the ground to frame connections. It is the ground strap near and a little behind the alternator. It goes from the engine to the frame. Here is what happens. The strap is connected to the painted frame by a self tapping screw. Here in Wisconsin where we use salt on the roads for a few months each year, this bolt gets rusty and you lose the ground. If you remove the screw on the frame side, then remove the paint on the frame down to the metal, clean up the screw and apply some dielectric grease before reinstalling. Putting in a stainless screw would even be better.

Hope this helps.

 

What did this end up being? I'm having the same problem.

 

I have sold that car since then. The problem was the ground strap. Cleaned the connection and it was fine. This was an amazing car. Fast, comfortable and built like a tank. Too much maintenance for me. Too expensive to maintain. My Buick has served me much better for much less cost.

 

Thanks for the response! Mine turned out to be a bad battery cable. I don't know where the ground strap is, so I hope it doesn't go bad!
I had an old '84 Century for a while. Durable cars. I actually had my eye open for a mid-90s Regal when I bought my LS. Gotta dig a Buick.

 

In case this ever comes up as a problem in the future. If you have had an aftermarket anti theft system installed, double check the wiring. These anti theft systems could have not been installed properly and could be the cause of your issues like was the case on the car I have pulled into the shop right now. Make sure to always take into account any previous modifications made to the car in the past and double check those first because more than likely, someone has taken shortcuts.