volume fade only while engine runs?

Car: 2000 Lexus SC400 System: Stock Nakamichi Headunit and System Xtant X603 Amplifier for front + sub channel only Main Issue: Extreme Volume Fade that returns to normal volume and fades back intermittently while standing. Other symptoms: Momentary loss of balance control on HU, problem occurs on all source inputs. Note: (1)Setup fully functional for 2 months w/o any issues or additional modifying. (2) Alternator tests good at 15.10 while running with the amp on. (3) Isolated Xtant amp by removing power and problem persists through the HU direct powered rear channels. Backstory: The volume on the stock NAK HU disappears w/o warning. It will go from full volume to no-sound over various bumps (even my aftermarket amplifier running half the system is following along with the act). Figured it was a bad fuse or two. Replaced all radio fuses thanks to the OEM manual, but problem persists. Everything will play clear and loud as long as the car is in the ACC or ON position. As soon as the engine is running, the volume fades back to the "intermittent" mode (somewhere between no volume and way too loud). As soon as the engine is killed, the clarity and volume returns immediately. Tested this while standing still and it is the same results. What gives? Help...

How are you providing the signal to the amp? Is it your stock alternator that is pushing over 15 volts to a 12 volt system? I have had customer vehicles with bad voltage regulators that have had similar effects on the audio system. A guy with a 70's bronco had a decrease in output power with the motor running, and if he reved the motor everything would shut off. metered the truck and found a 15.5 volt idle, and a 24 volt peak when he reved past 4 grand. He had polk momo amps.

The signal to the amp is a built-in line-in connection for the Xtant X603. No line converters here. The stock alternator before my amp pushed about 15.4 continuously. The power regulator is built into the alternator. Replacing the alternator is over $200 in just parts. I would like to be sure that this will solved the audio problem before making such a large investment. Is there anyway to be more sure, or any other possibilities first? Note: Several 12V sources inside the car still read 12V with the car on so the regulator still has to be functioning somewhat.

Mark Levinson system?

i am an idiot wrote: Mark Levinson system?

No. The Nakamichi was the premium system before Lexus inked a deal with Mark Levinson. Still looking for answers though.. Who else can chime in? Mods?

Is there any way to isolate the stock NAK headunit or stock amp as the problem? The headunit uses a balanced 5 wire input into the amp, so I am not sure how to test the signal. Or furthermore, how to test the amp's integrity either. ideas?

that 15.4 volts from the alternator sounds a little too high. we had the same problem with an 04 malibu that ran 15 volts, 12 volts at idle everything was fine, start the car then nothing intermittent on/off from the deck. had the car checked out by gm and sure enough the regulator was bad in the alternator and they replaced the alternator and everything works fine now. also the guages read 13 volts even though it was  more than that at the battery.git yer swerve on!

Yep, sounds like the alternator/voltage regulator to me.  Take the car to your local auto parts store and have them test the alternator.Support the12volt.com

DYohn wrote: Yep, sounds like the alternator/voltage regulator to me. Take the car to your local auto parts store and have them test the alternator.

I didn't know that they could test the regulator. I thought they only checked if the output was above 13v or so. I will look into this further. Thanks for the support guys.

Just to eliminate the alternator from the puzzle, without having to go get it checked.  Why not disconnect the charge wire.  If the problem goes away then the alternator may be the problem.