Why does the lighter / power socket fuse keep blowing on my Mercedes, BMW, Audi, or Mini?

Why doesn’t my lighter work on my Mercedes Benz or BMW?

It’s a question we hear often, “Why does my lighter/power outlet fuse blow when I plug in my cell phone adapter?”To answer that, let’s spend a few minutes reviewing the history of the ubiquitous lighter socket, now known as the “power outlet” in newer cars.

In the United States cigar lighters started appearing as standard equipment in automobiles in 1925. In newer cars, the socket often ships with a plastic dummy plug, without the lighter heating element due to declining popularity of smoking. However, the socket continues to exist to power consumer electronics in cars as the primary purpose.


Why The Fuse Blows
Now that you know more than you ever wanted about lighter sockets, let’s talk about the fuse problem. The simple answer is that most power adaptors are designed for US size lighter sockets. The European standard is close, but slightly different, both in diameter, 20.93 mm compared to 21.51mm, and in electrical layout.

While the physical dimensions are different, it is the electrical layout that is really the root of the problem.

Domestic/American Cars
The only battery positive contact is at the bottom of the socket well – see photo, that’s why your lighter adapter has a small spring loaded “button” on the end, to make contact with the positive post in the socket.

European Autos
Use a different layout – see photo, they also have the positive contact at the bottom, but in addition they have two small “U shaped arms” which are also positive contacts, that run the side of the lighter socket.

The Secret
Here’s where the problem happens, if you look at your average power adaptor – see photo, you’ll note two small spring contacts on the side, these are for the negative power connection. Normally this works fine, it holds the adapter in place, and makes a nice electrical contact. The problem arises when these two spring contacts touch the positive “arms” of the European lighter socket, pop goes the fuse.

Puzzled, the owner replaces the fuse, plugs the adapter back, in, and all is good in the world until the contacts touch again, which may be days, weeks or months. Hence the mystery why the lighter fuse seems to randomly fail.

The Fix
A quick tip I suggest is to make a small “index” mark on the opening of the lighter socket with a Sharpie marker, this helps you align the plug, to miss the contacts, when inserting.

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*Special thanks to Atlantic Motor Car for the information in this article!


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