bigjontpa

Recently, My 2008 550SL will present a warning when entering the car- which says "BRAKE" (in red) on the left side display, and "Reduced Brake Effect" (in red) on the right side of the display. When I start the engine, the warning message disappears. Drive the car, park it and return to the car, it doesn't repeat the warning- until the next day, or two or three days- when I use the car again- after it has sat idle for more than a day.

Any ideas on what can cause this error to show up when the car sits for a while? Have noticed zero reduction in brake performance when driving.

Your feedback is welcomed and appreciated.

Big Jon
Tampa

kbob999

Jon - The SBC brake pump is electric, so whenever the rear battery voltage gets too low, the car wants to warn you (first) that you may not have braking.
After you start the car, the brake warning will go away (voltage back up), and it will then say (second) you don't have convenience features until the voltage gets back to normal.
Your brakes will work fine until the SBC pump refuses to run on low voltage, and you exhaust the brake pressure reservoir of pressure. Then you have Flintstone brakes and about 10% of what you normally have. Not good.

This symptom happened to me when my alternator was dying as I was driving, and it would warn me about brakes as the voltage slowly dropped.
If I charged the rear battery, all would be well, then it would slowly drop voltage during driving and throw the brake warning again.

Bottom line, you may need a new rear battery (very common), and you should plan to keep the car on a maintenance charger (Battery Tender Junior is my favorite) any time you don't plan to drive for a couple days.

As a test - after a couple days, check the rear battery voltage before starting the car. It will likely read 11 volts or something lower than the normal 12+.
Lower voltage is the normal car discharging the rear battery...all R230's do this.
You could also have an abnormal discharge caused by a bad alarm, or many other amp sucking devices. There are a lot.
The abnormal drain would need to be investigated and fixed, which requires an amp meter test.

Best of luck with your car and drive it like you stole it!

bigjontpa

Thanks for the information. Spot on. I did not mention the "convenience features" issue- but you're exactly right... that was there too.

I will look into the new battery and find a battery tender.

Big Jon
Tampa

bigjontpa

Before I go digging into the rear of the car- are the front and rear batteries identical? When I do a search on some websites they show "auxiliary" battery, but don't list a starter battery. Are they one in the same? If they're the same, I have no intention of ever swapping one for the other, just for my own edification.

Thanks again for the advice.

kbob999

The batteries are not identical.
You want the 'aux' rear battery (I believe it is Group 48) and get the 'dry' AGM (Absorbed Glass Mat) version. These are available everywhere. They only last about 4 years in this car.
The front starter battery is smaller, is the 'wet' cell type, and can typically be found only at the dealer. It has an uncommon size, which I'm sure can be cross referenced to another size or type that fits. These last about 10 years.

One last thing to know. If you disconnect batteries - always do the rear first, then the front. When connecting, do the front first, then the rear.
You will need to reset your windows (hold down for 5 seconds, then up for 5) and ESP steering (lock to lock a couple times with the car running) after disconnecting the rear battery.

ray0214

gaazmon

I think O'Reilly Auto Parts also carries the starter battery as well now

Rudeney

I've not replaced the starter battery, but I have replaced the consumer battery as well as the batteries in other MBZs. I really like the Advance Auto Platinum AGM. If you order online and pickup in store, you get 20% off. They also carry the starter battery, though it's just their "Gold" version and not an AGM. Both carry a full 3-year replacement warranty.

ray0214

Blazeracer

Got the same thing going on with mine. Red warning came on yesterday after driving all around town. Battery voltage is at 12.04 after sitting overnight and goes to 14 plus as soon as it's started. I drove it around cautiosly last night up the Walmart and back, the brakes work fine.

Blazeracer

Took it to the dealer Monday. They scanned it and it was the SBC pump. Got a new one for zero dollars. SBC is under warranty till 2027 on mine, unlimited miles.

Rudeney

Excellent! I'm glad to see dealers are honoring this warranty and not making owners jump through hoops.