Greetings,

I am a huge fan of the 97 LS400. I put 335k on my last one and have 200k on this one.

Recently I began to experience sudden voltage drops when applying the brakes while there is a heavy electrical load (headlights and A/C on). The drops are getting so bad that the car will stall when applying the brakes. I rebuilt the battery ground and the problem seemed to go away for a couple weeks but it is back. The problem is intermittent and inconsistent seeming to come and go for no reason but it is consistently deteriorating. The battery seems good for running and cranking. The alternator is putting out 14 volts. The vehicle runs fine at idle showing between 11 and 13 volts consistently. However when I apply the brakes it will drop to 10 or less volts and the system will dim for a split second. In heavy stop and go traffic I am having to drive with two feet to keep the voltage up to prevent stalling.

I opened the ECU to check for leaky capicitors and there is zero sign of leakage. There are no irregular codes from the OBD.

Any suggestions on where to look next? The car is becoming dangerous to drive.

 

I don't have a solution for you, but I happened to be reading an auto electric book that had something similar. Depressing the brake pedal caused several other electrical glitches to occur.

The Short To Voltage Story.
From the text: "Because most of the trouble occurred whenever the brake pedal was depressed, the technician decided to trace all the wires in the brake light circuit. The problem was found near the exhaust system. A small hole in the tail pipe (after the muffler) directed hot exhaust gases to the wiring harness containing all of the wires for circuits at the rear of the truck. The heat had melted the insulation and caused most of the wires to touch. Whenever one circuit was activated (such as when the brake pedal was applied), the current had a complete path to several other circuits. A fuse did not blow because there was enough resistance in the circuits being energized so that the current was too low to blow any fuses."

It doesn't concern a Lexus, obviously, but maybe something similar is going on.

 

Not heard of any such problems with the series III LS400,the earlier models (90-94) had a problem with the left side trunk hinge wiring loom causing starnge electrical problems with brake lights and other things.
Does the fault still happen if the car is stationary and the brakes applied?
Could be a fault on the brake lights wiring loom.

 

11 V is too low, even at idle unless you have a very unusual load (e.g. hi-wattage audio system).
It may not be your main problem, but should be investigated.

 

Yes the problem will still happen at idle.

 

Will you inform us the idle rpm when at P and D?

 

If the voltage drops to 10v with brakes applied, it sure does sound like the alternator is going(or maybe hi-z wiring to the alternator). You could get a current probe and see what kind of current you are drawing with and without the brakes applied to see if it exceeds the output of the alternator.

I know my LS will NOT charge the battery when I have every load possible on it and at idle(600 RPM). That's just the way it is. But that is not the normal mode of operation and is just an occasional exception.

My bet is on your alternator going bad. It could put out 14v under lighter load and then fall flat on its face with heavier loads. Perhaps a phase of the alternator is gone? Have you tried setting your voltmeter on the AC setting and seeing if you get any significant reading?

And another question: Are you seeing any 'Battery' light on the dash occasionally?