So Im getting my first experience with the dreaded red battery light. I know it seems like beating a dead horse BUT I have a slightly different question about it. I removed the passenger toe board and got to the fuses. I am getting voltage on both sides of the starter battery fuse. I am at home now so I dont remember what the voltage was but my question is, can you get voltage on both sides and still have a blown fuse? I am well versed in 12v so though that may sound silly, what I mean is, can both sides of the fuse posts still have 12v because of a bleed through in circuits but not getting proper charge because the fuse is blown? trying to get my ducks in a row to get back on the car on Monday?

 

The red battery warning can just mean a bad starter battery. But, if you're measuring voltage across the fuse terminals then the fuse is bad.

What year is your SL500 and what fuse number are you checking?

 

Hi psrllc,
You are right to question the conclusion usually drawn when 12v is measured at the starter fuse. One can detect 12v on both sides of a blown starter fuse, due to the circuitry that allows the SL to start, even when there is weak or nonexistant starter battery. It took me days of tracing the schematics to prove this to myself five years ago (and I'm a degreed engineer). I'll say this before someone else does: Others have stated that 12v potential from circuits on both sides of the fuse is not possible. I stand by my own conclusion that it is possible.

That said, after going to all the trouble of getting down the the fuse in question, why not remove the fuse and bench test it? Sure, you actually need to remove both fuses to get to it, but that is a relatively minor effort at this point. The replacement fuse, if you need it, is probably the least-expensive part you will ever buy for your SL.

Also note that the SL will start quite a few times and drive fine without the starter battery receiving any charging voltage. The Red Battery Icon will remind you that there is a problem that need fixing.

Let us know what happens,

Regards, CPDay

 

Thanks guys. I would of course have removed the fuse to bench test had you not responded, I simply ran out of time yesterday. Appreciate the replies, greatly appreciated.

 

There may be some confusion about how the measurement is being done. Are you measuring the voltage to ground on each side of the fuse? I was referring to placing the voltmeter probes across the fuse and not to ground.

It's basic Ohm's Law - if you place the voltmeter probes across the fuse while in circuit and you see a non-zero voltage then the fuse is bad. A fuse is a short circuit and the voltage across a short circuit will be zero. On the other hand, if the voltage reads zero that doesn't mean the fuse is good. It's possible there's no voltage to the circuit you're testing because it's switched off. But, at that point you can switch your meter to a low ohms range and safely test the fuse without removing it. Once again, probes across the fuse and not to ground. An ohmmeter should show a value close to 0 if the fuse is good.

Hope that helps.

 

so, finally back at the shop today. pulled the fuses out of the passenger toe board today and both are perfect, so both batteries are new, both fuses are good in the passenger toe board, still have the red light. Possible battery module in the trunk bad?

 

Didn’t I read in this thread that the red battery light will remain lit until being reset by a competent diag system?

 

And it was cleared and instantly came back on.

 

The "Dual Batery System.pdf" I posted earlier in this thread says that the DTC will be set in the vehicle power supply control module (N82/1) and it can only be reset using SDS / DAS.

 

Try running the engine for at least 15 minutes and then try to clear it again if it hasn't cleared.

 

ok so I have read this a few more threads about the red battery light and the “electrical consumers offline” I have to fix this issue it’s bothering me that much ... I’m going to buy the voltage regulator for the alternator, buying both 100 and 200 amp fuses for pass footwell.. changing both relays near BCM k57 k75 ... I’m going to check the BCM for any visual electrical burns (read of one where the terminals fried) I’m going to look at the rear SAM (need location) and clean out any corrosion... consumer battery was replaced three months ago ... might buy new starter battery since I read that the “red” battery light in dash means that specific battery is low/bad... am I missing something short of replacing the alternator too?? I figured Ill throw 100$ in the easy parts for peace of mind. I have owed this car for only 6 months

 

I suggest replacing the starter battery first before buying those other parts. On the R230, per the docs, the red battery warning refers to the starter battery.

 

CPDay - Thanks for the excellent write up! Apparently I accidentally shorted the starter battery and blew its F52F1 100A fuse on my SL500 while working under the hood, and thus had the red battery message. I have battery disconnect switches ($5 at Walmart) on the ground side of the consumer batteries on my SL55 and SL500 to prevent battery drain during the winter when the cars are not driven. Two preliminary quick tests confirmed the likelihood of the blown F52F1 fuse before I followed your instructions on how to get to the fuse. First, a voltmeter showed 14+ volts present at the consumer battery with the vehicle running, but only 12.4 volts at the starter battery. The car started and seemed to run fine off the consumer battery, but had the red warning message. Second. opening the consumer battery disconnect switch, thus eliminating its ground connection, kept the car from starting. So in under 5 minutes I confirmed the likelihood that the fuse was blown, which it was.

 

Is every thing working now? Getting down to those two fuses is no simple task, is it? I'm glad my notes helped.

 

Yes, all is good now, no red battery icon. Attempted to obtain the Blue Sea Systems 5256 Midi/Ami Fuse locally without luck, so purchased it online. In the meantime, Advance Auto Parts carries a VW/Audi fusible link kit from Littelfuse for $5 that has 7 fuses, of which one is an 80 Amp. Fit is identical to the original 100 Amp M-B fuse so I installed it temporarily until the correct fuse arrives. Thanks again for your excellent diagnosis and repair procedure.

 

First post here. Just purchased a 2003 SL500. Beautiful car, just a few issues.
None of the pneumatic features, including door locks were working. I cleared out a clog in the pump and good to go (I love free repairs).
Headlights often have a difficult time firing up, so I'll look into that. About to replace the seal on the front hydraulic cylinder to get the top functional again. The trunk actuator is leaking and not pulling down far enough.
This battery light always gives me a welcome when I start the car, so it's one that is always on my mind. I'm guessing the fuse is blown as I can measure 14+ V at the rear consumer battery, but I have yet to see anything at the starter battery.
Been working on cars for 20 years, but this is my first MB. So far, this forum has already saved me close to a probably thousand dollars!

 

Hi, WebbJAC here with my first post and I found why the system battery was not charging on the 230R 350 SL model (owned by a relative);
This is a fix that took around 30 minutes of DIY input and no new parts:
The systems battery was not charging at all and a smart charger rated it as 18% charged, so the red icon was hardly a surprise. After following existing posts and useful diagrams of the dual battery system, I concluded that the starter battery was getting 14+ volts from the alternator but the systems battery was getting nothing. The battery control module and related relays were believed to be OK and the lack of 12+ volts on the system battery took me first to the the rear prefuse 60A fuse which I expected to find blown. It was not and all was good in the rear prefuse.
This then took me to the front prefuse box in the passenger footwell and with both batteries isolated, I dismantled the front prefuse where I found the problem. Damp had got into this box and all fuses had some rust showing. The 200A fuse connecting to the rear prefuse was no longer making good contact. A resistance meter quickly confirmed the poor terminal contact and associated high resistances, where there should have been close to zero ohms between each set of fuse contacts/nuts.. As a quick fix and to get the car up and running quickly, each nut on each fuse was slackened and re-torqued. This was enough to restore good contact. At a later date, I intend to dismantle all and clean all contacts properly and then coat all with dielectric grease, to prevent further corrosion. I hope this reply post will prove helpful in addition to things already reported. Summary: Diagnose with voltmeter with car running. If the systems battery has no charging value (e.g. no 13-14+ V at terminals but less than 12V), then check starter battery with car running. If this has 14+ volts at the terminals, ask 'why is this charging value not reaching the systems battery?'; this journey should take you to the two prefuse boxes to find out where the alternator feed has gone. In my case, rusty fuse terminals had resulted in a lost 200A pathway at the front prefuse which meant that the systems battery in the rear was not seeing the alternator output and so it was not able to charge at all.

 

I have the opposite issue-the front battery is not charging but the rear is.

 

When the front battery is not charging in the engine compartment, I would suggest the following:
1. Alternator is good if the Systems battery is charging
2. Starter Battery is not seeing the alternator output so the problem is on that connection pathway
3. The hot favorite is the F52 round fuse box in the passenger footwell. The bottom blue 100A fuse (f1) is either blown or making poor contact due to corrosion/rust on the blades of the blue megafuse.
4. If it's corrosion, slackening and re-torque of each nut should resolve. Best to isolate both batteries before working in there. remember to reconnect the Starter Battery first when finishing.

A new blue mega fuse is not expensive, if needed; it may not be if the problem has been caused by corrosion at the terminals. A dielectric grease can be applied to help keep moisture away from the metal.
John

 

Hi,
One thing I meant to add to my earlier reply:
Sometimes, the fuse F52f1 (blue mega fuse) will blow during a jump start procedure. This might happen if the jump leads were momentarily connected the wrong way around (or possibly if the jump start was attempted at the Systems Battery).
The problem is that the fault might not show up for some time because the starter battery might be re-charged externally by a battery charger and then the blown fuse would be masked until the starter battery began to run low. This could be several days after the fuse was blown.

I hope that proves to be helpful; F52f1 100A is the most likely cause for the starter battery not charging from the alternator.

 

Thank you I will investigate down the line. I've just been charging up the front battery once a month with the CTEK

 

Before you do the job (It might only take 30 mins if all goes well), it's a good idea to buy a new mega fuse 100A Blue (they come in other colors which would not really matter if the 100A rating was correct. With the new part ready before you start, all should go like a dream.

These are not expensive on ebay
You will not have to use the CTEK again, once this is fixed and the alternator is talking to both batteries again!
Let me know how it goes; earlier posts in this thread show diagrams of the round fuse box (F52) with the two mega fuses so these are really useful to refer to

 

Thanks, I will let you know the outcome. I just assumed my BCM fried or something.

 

Hi,
I suggested it would be worth having fuses ready before starting the work, as it appeared from the symptoms that the !00 A fuse was gone.
I have just acquired some mega sized bolt-in fuses and I strongly suspect the the fuse size needed is the Midi-sized bolt-in rather than the Mega-sized bolt in.

The midi-sized bolt-in fuses appear to be more commonly used in automotive applications while the larger mega-sized bolt-ins are more common in large trucks and boats.
The required size can also be obtained on ebay at fairly low cost and the midi link fuses seem to generally have the following color code:

midi fuse typical value versus color of fuse body
30 A (Orange)
40 A (Green)
50 A (Red)
!00 A (Blue)
200 A (Violet)
Other color variants are also out there such as a black fuse body with the value printed on the body or stamped into the metal tag at one side of the body of the fuse.
Apologies for possible misleading info referring to 'mega' when what is almost certainly needed is 'midi'

I hope this further detail will prove to be helpful

 

Hi rodeo I have done the same thing changing the aux battery and sparked it everything works but the console radio cluster will not turn on. How did you fix it?