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I've had my '88 300E for almost 2 years now and for the first year i didn't replace the seat adjustment switches because I didn't need to. I got fed up with everyone in the family complaining about not being able to adjust the seat so I got to it and in doing so discovered that you CAN adjust the seat as long as the door was OPEN. Weird. Is this just a simple crossing of wires somewhere along the way? Any advice would be much appreciated.
 

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1995 E320
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I think that the opening and closing of the door over the years has frayed the wires connected to the switch to such an extent that they are only in contact when the door is open.
 

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About a dozen 1988, 1989, 1990, and 1991 sedans, wagons, 4Matics and 1 coupe
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Only *part* of your car's seat adjustment circuitry is working.
1) With the engine/ignition switched off, the seats can only be adjusted when either front door is open. This allows you to adjust the seats manually or activate either memory setting (driver's side) before you take a seat. This so-called "comfort" feature is handy when two driver's have different body proportions (like me and my wife). I don't have to cram myself into the seat after she has driven the car.
2) With the engine/ignition switched on, you can adjust the seats with the front doors open or closed.
If the latter situation is not true, check fuse #2 first. If fuse #2 is good, the problem is almost certainly the troublesome little diode that plugs into the underhood fuse box where the relays are. To access it you have to remove the 6? screws that hold the lower cover plate in place. The diode location is shown in the diagram below. Easy to fix if you have a soldering iron and a Radio Shack nearby. Diode is a 1N4003 IIRC. Get a pair for $1 or so.
It sounds like your car's diode failed "open." When it fails "shorted" you get other weird symptoms. I've seen both cases.
 

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