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How to repair the K40 relay on a Mercedes.

Model: Mercedes Slk R170, MK 1 - Years 1997-2004
Part: Fuse box

The K40 relay fitted on the 2001 Mercedes SLK in the video is a multifunction module that manages several essential circuits. When it fails, it causes starting issues, horn and cruise control malfunction, and non-functioning air suspension. Do not confuse it with the immobiliser: if the dashboard lights up, it is working; the problem is the K40. The fault is due to solder points separating from heat or vibrations, as shown in the video. To repair it, remove the relay from the car, open the plastic casing, identify the broken solder points, heat the existing solder with a fine-tipped soldering iron, and apply a bit of new solder, ensuring adjacent terminals do not touch. Check other points that may seem oxidised, then close the casing and reinsert the relay in its place. It is worth repairing, as it is an easy job, while purchasing this multifunction relay, for example on Amazon, costs over £160.

Posted on 8 May 2026 by ScegliAuto

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