The Mercedes-Benz S500 battery drain issue is difficult to diagnose, as multiple systems have been known to fail prematurely and cause severe battery drain. These include modules related to the car’s seating and, more rarely, issues with the infotainment center.

The Mercedes-Benz S500 is a high-end sports car trapped in a full-size luxury sedan’s body. And while the S series has certainly had its fair share of ups and downs through the years, there’s no denying this is an absolute triumph of a car. A triumph with just a few weird little quirks, but a triumph nonetheless.

One of those quirks? The near-infamous Mercedes-Benz S500 battery drain, where you find your battery totally drained through the night without an explanation. Battery drain has affected multiple generations of this luxury car, but the fourth and fifth generations, from 1998 through 2013, are arguably the most susceptible to it.

A Mercedes-Benz S500 with battery drain issues can get frustrating fast.

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What causes the Mercedes-Benz S500 battery draining problem?

Two modules found in the seats in an S500—the seat control module and the heated seat module—are known to have failures leading to dead

car batteries

. The keyless entry system and the infotainment system can drain the battery too. But in a large number of cases, it’s actually those two seat modules causing all the commotion.

Replacing the seat control module can cost anywhere from $500 to $1,000, depending on the mechanic and the availability of parts. If it’s the heated seat module causing the problem, or the actual pad that’s draining the battery, repairs tend to be a bit cheaper, in the $300 to $400 range.

If you know your way around the inner workings of a car and you’re familiar with the use of a multimeter, you can diagnose this issue yourself at home:

Make sure the battery is fully charged. Consider replacing it first if it’s more than six years old.

Write down your

Mercedes Benz radio code

first. Also, be prepared to do a window and sunroof synchronization after this process.

Unplug accessories, turn off lights, remove the key from the ignition, and give the car up to 30 minutes to make sure nothing is drawing from the battery.

Locate your battery. This is in the trunk in many models or the engine compartment in others.

Disconnect the negative battery cable. Connect a test lead to the negative post and the other to the negative cable.

The current should read at less than 0.1 amps over 100 milliamps; 0.05 amps is ideal. If it’s higher than 0.1, there’s likely a problem.

Disconnect the fuses from the car one by one, checking your multimeter along the way. When pulling a particular fuse causes the ampere reading to drop, you’ve found your culprit.

Use a fuse chart to figure out which systems that fuse affects. If that fuse isn’t critical to safety, you could leave it out until you get the problematic component fixed.

If all of that is foreign to you, you should

take your car to a mechanic

. A professional has the tools to diagnose and find the exact cause of your Mercedes-Benz S500 battery drain.

MORE:

Jumpstarting your dead car battery back to life

How to fix the Mercedes-Benz S500’s battery draining problem

These two modules are somewhat difficult to repair, as they require the full removal of a car seat that has wiring running into the floor. You don’t want to damage that wiring while removing the seat, so removing the seat can be challenging.

Once you have the seat out, replacing these modules is straightforward enough. You simply locate them, disconnect their cables, remove the unit, and then install the new one. That part is simple. It’s getting the seats out and back in that causes all of the headaches.

We strongly advise you to have this job done by a professional mechanic. They’re far less likely to cause accidental damage, and they have tools that make this job a lot easier, too. The added labor costs aren’t always worth it with every car repair, but in this case, we think it is.

MORE:

How to save money insuring your Mercedes-Benz S500

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