hoodcleaner1970

2014 w212
Auxiliary battery malfunction came up on dash I checked battery and had corrosion on positive side I checked volt battery has 10.3 volts I cleaned everything put back together and started car still 10.3 bolts I took positive off while still running and tested the cable alone no volts any ideas why ???

JettaRed

iTrader: (1)

If you are trying to test your alternator, get a proper tool. I have no idea if you need to have the battery connected for the alternator to provide voltage to charge the battery.

CaliBenzDriver

The AUX charge current was exceeded. Look nearby there's a fuse, like 7Amp with a red color. Test it with DVM or visual inspection.

As always be careful not to short battery to GND when working with connections.

Buy a CTEK charger to prevent MAIN AGM battery drainage while parked. AUX is virtually unused while MAIN stays above 12.5V.

I wonder what type of AUX battery put out acid corrosion... is this normal vented AGM ??

Some of these 2014 chassis have a pronounced appetite for batteries - Check the voltage while driving with the cluster display to see voltage deep below 12.3V: "drain by driving". No code, no CEL

juanmor40

and, please do NOT disconnect a battery with the engine running unless you want to replace a bunch of electronic modules. Lucky if you just blew a fuse doing that.

Read @CaliBenzDriver advice because shorting a battery, or reverse polarity when jumping can take an ECU in a flash, and a few $$$ later you might get it back to life.