RA81722

replaced the alternator on my e55 and now when I get on the power the voltage drops to 11v and I get the red battery warning! Just cruising the volts stay at 14v and everything runs fine! Is it a blown fuse or something else? Anyone else?

Blade Canyon

How old are your two batteries? When I started having voltage problems, I had to replace the rear battery as well as the alternator. The front battery had been changed a year before.

mrmotoguzzi00

I honestly don't know anything about the E55 specifics but could that be a loose belt ? when you get on the gas the belt could be slipping ?

at steady cruising belt is not slipping ?

Just a thought...

Mike

BoostedAero

Could you possibly you have a faulty alternator?

"This will be the second alternator being parts warrantied and the fourth time I've pulled one out of the car. Alternator measures ~14v at the back on start-up, but as soon as the a/c kicks on, it drops down to 11.5-12v and doesn't recover."

But I would think about doing a load check on the battery and verify it is healthy.

I am thinking it is the alternator though


https://mbworld.org/forums/w211-amg/...r-removal.html

RA81722

the rear is maybe 1.5 years old! Idk about the front

RA81722

but even with ac on and all that it still stays at 14v just cruising! Idk if maby it's a fuse or the front battery is too old!

austingtir

As others have said it was actually the BATTERY that destroyed my alternator due to a dropped cell. So be very careful and get both batteries checked out as you could kill another alternator here.

mightar

Have your voltage regulator checked out as well...

RA81722

The alternator is brand-new and so is the voltage regulator

RA81722

just hooked up a voltmeter and the alternator is charging find under load. I went ahead and replaced the small auxiliary battery and will drive it around for a bit to see if the issue comes back. What's also weird is that my oil level sensor is bad now too. Is the sensor just a plug and play or will oil come out if I replace it?

austingtir

A voltmeter wont really tell you much at all about the problem I am talking about. You need to either buy a battery testing machine (not that expensive for a cheap one) or goto a battery retailer and they should have the right test equipment. I use a solar branded one similar to the one in the video below. These testers can also test alternator with car running and do a cranking test they are very good for quick diagnosis on issues like this.

I think you might be very surprised if you saw those batteries on a proper tester. A dropped cell will do damage to your alternator and wont show with normal testing on a volt meter.



In my experience its just not worth the hassle to mess about with these things and try to guess it. There is to much electronic **** to wreck in this car to dick about with it.

RA81722

i had the ac, driver and passenger heated and cooled seats on, defrost, headlights, fog lights and high beams and it was still at 13.5-14v the whole time! The alternator isn't the issue! The small aux battery was only showing 12.5v so I went ahead and replaced it! The main battery is less than a year old and is oem Mercedes

Ders

I had the exact same issue and after replacing the aux battery it fixed the issue. None of my creature comforts stopped working and voltage was fine the whole time.

bbirdwell

You have to understand the car's sub-systems. You just replaced a perfectly good battery.

The auxiliary battery is not tied into the main electronics system all the time. The K57/2 relay must be energized to connect the auxiliary battery to the main electrical system. So, you were measuring the battery voltage while it was disconnected from the circuit.

This K57/2 relay will activate in event of main battery or alternator malfunction. With a properly functioning system, the relay will only activate once per ignition cycle to allow the auxiliary battery to charge through the relay.

Auxiliary battery voltage is monitored by a polyfuse and the BCM (Battery Control Module). The above mentioned activation once per ignition cycle (for "a short period of time" or <15 minutes IIRC but I need to verify) will occur after you key on and start the car.

So, connect your voltmeter to the auxiliary battery, measure voltage, then start car and monitor voltage. If the K57/2 relay is good, you should see the voltage increase when the BCM activates the relay thus tying the auxiliary battery into the charging circuit. If no increase, good possibility you have a bad relay and that is what is triggering the dash alert.

Star diagnostic system should show the source of the fault. You're going to want to look under Control Units/Body/Battery Control Module/Maintenance operation, adaptations, or exchange/Fault Codes.

The relay is located under the plastic trim of the windshield wiper cowl next to the cockpit air intake next to the auxiliary battery.

PM me your email address and I'll send you some info to assist your trouble shooting if you would like.

Good luck on it and please post your findings.

FWIW, I use a Solar BA-5 battery tester (looks like a Solar battery tester in the video) to monitor the health of my batteries. Hasn't steered me wrong yet. When my batteries measure CCA at half the new rating, out comes the old battery and in goes a new one.

Tommyboy928

I blown 1 alternator, charged batteri in back to the top , and drive with a used replacement alternator from 2002 ,all checked, worked fine for 7 miles, then red alert batteri sign, Batter in back changed 5 years ago.

Tommyboy928

K57/2 is where in SL cars ? just blow one alternator, and the regulator on a alternator taken from E55 ,2002, alternator was fine, but after 3 drags and 5 miles battri sign came on on the 2007 SL55 AMG

bbirdwell

Try searching here:
https://workshop-manuals.com/mercedesbenz/

Tommyboy928

hi, do you know where K57 /2 sit on the sl 230, AMG ?

changed K57 by the passenger side , trunk in wall , i understand it is 2 relays?