Join Date: May 2009
Vehicle: SLK230 2002 Silver/Marlin
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I have had a similar problem and solved by cleaning the switches. I should have taken pictures and if this ever happens again I will. Here's what I did.
Pull the seat, unlug all the wiring. Uncscrew and pull off the seat side panel to get access to the switches. Remove the screws holding the switch block to the side panel and separate. I then unscrewed the main connection bracket so I could remove the harness from the seat area and work on it on my bench.
On the front side of the switch block the 4 swiches are covered by grey covers, 1 for the recliner and 1 for the other three. My problem was with the fron seat riser so I pulled off the cover covering the three switches. This exposes the switch mechanisms. I then removed the toggle for the non working switch by prying aside the clips securing the toggle and lifting it out. Careful here as yours might have loose springs though in mine they were all captive. You can now access the switch contacts. Note how they are fitted. Lift out the moving part of the contacts so you can see the contact area. In mine they were black and oily and also rather pitted. I cleaned them and added just a fraction more bend in the contact arms, not too much or they will not move correctly and you could end up with a short. Reassembled everything works just fine. You need to be careful putting it all back together to make sure the big switch levers mate up correctly with the switches, try them to make sure you can feel the switch action before putting it all back together.
You can check the switch action with a multimeter. If I recall correctly the two switch commons go to the motor then when operated in one direction it switches + volts to one common whilst leaving the other common connected to - Switching in the other direction puts + on the other switch common leaving the first common at - When at rest both commons are connected to -.
Provided you are careful the switches can be cleaned and rebuilt Mine have worked for months now and seem fine. It would seem that perhaps the mottor current is a little more than the switches were designed for and this is or the motor back emf is what is causing the pitting of the contacts.
YMMV, I rpovide this info as is and without warranty of any kind. Hope you find it useful.