Hey everyone, I’m hoping to get some input

I have a 2016 GL450 with 158k miles and it’s been having some scary electrical issues. It started with the AC working on and off randomly for about 1 month, then I began getting a red battery warning once in a while.

A couple days later while driving, the car threw “STOP VEHICLE – see operator’s manual,” lit up a bunch of warning lights, went into limp mode, lost power steering, and then eventually shut itself off in middle of road. I jumped the car and drove it home and left it overnight at home before I worked on it and this morning it started right up. However, when I plugged in my scanner after 5 minutes the car died. Right before it dies, the center screen turns off, the engine starts stumbling/misfiring like it’s trying to stay alive, and it won’t even let me turn the car off with the button — it just dies on its own.

I checked voltage and found the main battery down around 4.5V. I jumped it, and while running the voltage only climbed to about 6.5V, then the car died again. Another day it showed ~14V briefly at idle, but it’s clearly not consistent.

I pulled a code:
P065B – Alternator control / performance malfunction

Jumping the car will let it run temporarily, but it doesn’t last. Sometimes after sitting overnight it’ll start once without a jump, but the problem comes back.

Main Battery is only a few months old. Aux battery hasn’t been replaced yet. But would a bad aux battery cause all this?
Do you guys think I need a new alternator? Or can it be something else?

 

Dead AUX batteries can cause all sorts of inexplicable problems. I’m not saying it’s the smoking gun here, but IF it’s the original it’s time for replacement regardless so it's the cheapest, easiest first step of troubleshooting before throwing more expensive parts at it.

 

Jumping is always only a temporary solution - once batteries experience this kind of depletion they need to be recharged on a battery charging to go through the whole spectrum.

The alternator code could be a failure with the alternator or the inability to charge the batteries. What I would do is use a dedicated battery tester like this
AFTER you have fully charged both batteries on the lowest (3A) setting on the battery charger. If it's AGM which it probably is, you'll need to charge it in parallel with a known good solid state battery - unless there is some fancy new AGM charger I don't know about, which is entirely likely.

On some alternators, you can take out the voltage regulator and inspect/replace it cheaply, but I don't think you can do that with this alternator if it is 0009068706

EDIT: Also - you wouldn't be the first person in history to get a failing battery that is "new" - I've had more than one. So if the main battery won't hold a charge or keeps depleting you should ask wherever you bought it to replace it.

 

Yes, many newer chargers include an AGM setting.

 

Yes my charger has that too but it still won't charge actually fully before the charge-off gets thrown from the battery. Recommended practice is to have a fully charged lead acid battery and connect that in parallel to the AGM battery that needs to be charged. That way it bypasses the shut off from the AGM that kicks in prematurely. Ken/Mercedessource has a video explaining this better than I am doing.

I should have said a charger with an AGM setting that actually works correctly.

 

Kind of lack of common sense. No charge and you blame battery?

 

I’ve got an e320 4matic, same thing, red dash light. Regular battery under warranty so I replaced it and was fine. Light kept coming back intermittently until it stayed. Replaced alternator and problem solved. I bought a lifetime warranty one from autozone and good thing I did because it went out 1 1/2 years later. Got an agm and replaced alternator and hopefully it holds up. Btw, I kept my original alternator and plan on replacing the voltage regulator next time it goes out and get a new replacement in case it dies on me as a spare. No matter what, the brushes will wear and go bad.