Retrofit early to late model seat switch W126 - PeachParts Mercedes-Benz Forum

eatont9999 · #1 02-22-2013, 02:08 PM

Is it possible to get a later model seat switch for a W126 to work in an earlier model car? The ones on my 83 300 SD are beat up and need to be replaced. I have some later model switches and pigtails that I want to use. Any suggestions about how I can go about this without turning it into a hack job?

jay_bob · #2 02-22-2013, 07:36 PM

There were three variants of the electric driver seat in the 124 which is the electrical basis of a gen 2 126.

First there was the non memory seat. All power for the motors goes straight through the switch. All passenger seats are like this across the entire 124 range.

Second there was the early 124 memory seat. The switch only operated control wires to the logic box under the seat. All power switching done in the box. Individual wires from each switch contact to the logic.

Third was the later 124 memory seat. The switch had logic in it and a data bus connection to the under seat controls. Only a few wires between switch and box.

I would suspect an early 126 would have a seat of the first or second kind.

I would obtain ETMs for your car and the donor and see what you have side by side.

LandYaghtLover · #3 02-22-2013, 07:42 PM

My 91 has many wires going to the brain box. So I suspect no bus system. My memory seats are dead on both sides, oddly. But controls for the seats works fine. The memory module is just a passive middle system.

jay_bob · #4 02-22-2013, 08:33 PM

The memory box is not passive. The switches ground the respective control lines which in turn switch relays in the box to move the seat motors. The seat will not do anything without the box.

Not trying to talk down but are you sure it's broken? I thought mine was until I realized I was working it wrong. You put the seat in the position you want, then press and release the circle button, then press and release one of the number buttons (the 2 rectangles if the paint is worn off like mine are).

Then someone moves the seat to some other position. When you want to recall your seat position, you have to push and hold in the number button until everything quits moving. It moves one motor at a time.

Each motor has a potentiometer on it that is used for feedback to the controller. If the box loses one of the sensors, it cannot remember the position of that element, and so when recalling that motor will not move.

Currently my steering column sensor is out because the column adjuster works but it does not remember the position. That's pretty low on my list since my son and I are both 6'3" tall so we keep the seat and column in the same position.

LandYaghtLover · #5 02-22-2013, 08:43 PM

Quote:

Originally Posted by jay_bob

The memory box is not passive. The switches ground the respective control lines which in turn switch relays in the box to move the seat motors. The seat will not do anything without the box.

Not trying to talk down but are you sure it's broken? I thought mine was until I realized I was working it wrong. You put the seat in the position you want, then press and release the circle button, then press and release one of the number buttons (the 2 rectangles if the paint is worn off like mine are).

Then someone moves the seat to some other position. When you want to recall your seat position, you have to push and hold in the number button until everything quits moving. It moves one motor at a time.

Each motor has a potentiometer on it that is used for feedback to the controller. If the box loses one of the sensors, it cannot remember the position of that element, and so when recalling that motor will not move.

Currently my steering column sensor is out because the column adjuster works but it does not remember the position. That's pretty low on my list since my son and I are both 6'3" tall so we keep the seat and column in the same position.

I did not say the box was, I said the memory module is.

And yes, I know how to use the system. Been over it many times and a thread or two here. From what I can tell there is no power to the memory portion. Power is cut when the car is in drive, for example, to prevent accidental movement. Yes in drive one can still manually control the seat. I first suspected the seat relay, but thats not the issue. Somewhere a wire is cut or ground is bad. This is also why I said the memory module is passive, in that without power things still work.

jay_bob · #6 02-23-2013, 09:23 PM

There is a separate power source for the memory electronics that comes into the front of the under seat box. On 124s it comes from X30 the miscellaneous power distribution under the driver foot rest. Maybe that is the problem with the memory.

turbobenz · #7 02-23-2013, 10:25 PM

The memory switches will not cross over to non memory. I would check the box first, jumping the wires for the switch (to eliminate it as a possibility). Ive seen a few of the boxes fail, one the seat back didnt return and the other was constantly powering the seat forward.

This is just the bottom half not the memory board, Its possible to clean or replace the relay but I just wanted to take it apart.

eatont9999 · #8 02-25-2013, 10:54 AM

I apologize for not being more specific but what I am aiming for is finding a good solution to retrofit a switch with the removable plugs in place of what I have, which is soldered connections to the switch back. I don't have memory seats and I'm not planning on it right now. I want to use the existing wiring that is in the door and under the seat but the problem is the wires are soldered to the switches. Perhaps some electrical diagrams/color codes will help. I want to avoid having a bundle of butt connectors in the door, so maybe soldering pins on the ends of the wires will work? I'm open to suggestions.

The replacement switches do have the memory buttons but I believe that I do not need to connect them to anything to have the rest of the switch work.

The front seats are cloth and from a 500SEL Euro.

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