How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Lou P. master tech Your Own Question
Lou P. master tech
Lou P. master tech, Mercedes Technician
Category: Mercedes
Satisfied Customers: 4954
Experience:  15 years master certified 100% fix rate and have access to factory radio codes, wiring, and instructions.
27644750
Type Your Mercedes Question Here...
Lou P. master tech is online now
A new question is answered every 9 seconds

My 2005 sl 55 omg is not charging the batteries. I had the

Customer Question

my 2005 sl 55 omg is not charging the batteries. I had the alternator replaced but it still isn't charging
JA: Have you tested the battery with a voltmeter? And can you see if it's leaking or not?
Customer: I replaces both batteries
JA: Are you fixing your SL yourself? What have you tried so far?
Customer: I had a mechanic friend do the work. Now I am trying to figure it out. I have pretty good knowledge of cars and I am an electrician by trade so I figured i could get it working
JA: Anything else you want the mechanic to know before I connect you?
Customer: no
Submitted: 2 months ago.
Category: Mercedes
Customer: replied 2 months ago.
The car will run when the batteries get low but I get a message on the screen that says to start the engine because the voltage is low. It is running when the message appears. The rear battery dies very quickly
Customer: replied 2 months ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call.
Customer: replied 2 months ago.
Let me know if you need more information, or send me the service offer(s) so we can proceed.
Expert:  Lou P. master tech replied 2 months ago.

Hello, my name is***** and i will do my best to assist you.I am going over your story now and will reply back soon with some info,thanks!

Expert:  Lou P. master tech replied 2 months ago.

assuming you are using a voltmeter to check charging what battery are you checking the voltage at and what is the voltage when the engine is running?

Expert:  Lou P. master tech replied 2 months ago.

ok? well the rear battery is the main battery and the only one that should show a charging voltage. if you can let me know the static voltage when running that would be ideal, then we can check the wire at the alternator for voltage, you may have gotten a bad alternator depending on where you purchased it. rarely do reman or aftermarket alternators work in mercedes.

Customer: replied 2 months ago.
The battery read 11.7 volts with the car off and when I started it the voltage started to drop down to 11.4 and then it kept getting lower. I charged both batteries last night and took it for a ride earlier which is why the voltage is so low. I have it back on the charger. Both batteries are interstate and the rear one is a AGM battery. It sure does pull a lot of power fast off the back battery.
One mechanic told me that there was a relay that charged the front battery when it needed it and I did some research and found that there was a recall on the voltage regulator. I think it is in the alternator on that car. It seems odd that with only 32,000 miles the alternator would go bad.
I think this problem began when I was cleaning some grime in the engine bay. I didn't hit it hard with water but I did use some simple green and water to wash it away if that helps.
Expert:  Lou P. master tech replied 2 months ago.

ok, there is a module that controls the batteries and the voltages and charging and all that jazz, so lets make sure that the alternator wire (big one,) has battery voltage with the key off. lets make sure the fuse did not blow for that wire. then we will need to check for 12 volts on the wire to the little connection when the engine is running. if we have both then the alternator is bad. if we are missing one let me know and ill direct which way we need to go. Lou P

Customer: replied 2 months ago.
I'm getting just over 11 volts on the main wire coming off the alternator. I had someone rev it up a bit and the voltage didn't go much higher. There is also a connector with 2 blue wires. I didn't mess with that. It's tight in there. I can't seem to figure out which fuse it is.
Hope this helps
Expert:  Lou P. master tech replied 2 months ago.

ok, you should be able to unplug that wire connection from underneath i think, ill check the diagram and see what wire does what.

Expert:  Lou P. master tech replied 2 months ago.

ok, with the engine running both wires off the small connection should show 12 volts. with them connected to the alternator.

Customer: replied 2 months ago.
DO YOU WANT ME TO CHECK VOLTAGE ON THOSE 2 SMALL BLUE WIRES? I CHECKED THE FUSES IN BOTH FUSE PANELS UNDER THE HOOD AND THE ONE BEHIND THE PASSENGER SEAT AND NOTHING APPEARS TO BE BAD. THERE ARE A BUNCH OF RELAYS THAT COULD ALSO BE THE PROBLEM. I'M STILL HAVING TROUBLE FINDING THE FUSE INDEX ONLINE. THE BOOK SAYS IT'S IN THE TOOL KIT BUT IT'S NOT THERE.
Expert:  Lou P. master tech replied 2 months ago.

OK, SO the fuse is not seen, there is a prefuse in a hidden fuse block under the passenger carpet. thats why i asked if there was voltage on the alternator big power wire with the engine off, if there is then the fuse is ok. when the alternator is running the small blue wires will have 12 volts on them so yes we need to check that.

Customer: replied 2 months ago.
I'm getting just under 11 volts on both blue wires with the engine running. Nothing when it's off
Customer: replied 2 months ago.
I'm thinking that the alternator is bad if it's only putting out 11 volts or so. Shouldn't it be putting out 13 to 14?
Expert:  Lou P. master tech replied 2 months ago.

It sounds like the alternator is bad , those wires should be 12 volts , the bigger wire will be 13-14.5

Expert:  Lou P. master tech replied 2 months ago.

I have taken my time to answer your question the best I could and I will continue to help if you need follow up information , all I ask is that you PLEASE take a second to rate my service so I may receive my credit for assisting you. Thank you. Lou P.