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Old 04-06-2015, 06:44 PM
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w124 Seat Switch Repair Questions

Hi All:
Been doing some searching and cannot find any info regarding repairing a passenger (non-memory) seat switch, specifically a post 1992 switch which has less internals than the earlier style switches with springs and balls etc.

The later W124 switches use a series of micro-switches to soldered to a circuit board. The micro switches perform all of the same functions as the older style, but are each individually cased, and use spring connectors much like a relay that are switched with the movement of the respective switch pole. Each switch is encased in its own plastic box.

So my seat bottom fore and aft movement has recently stopped working reliably. I know that the motors work, because after messing with the switch I occasionally can get the seat to operate fore and aft, but it wont repeat after letting go of the switch.

I opened up the switch as the fore and aft is the only non-working function, i thought I could clean and restore function. Upon opening the switch case and confirming that the contacts were not positively connecting to the other poles when tripped. I cleaned the contacts, slightly bent the contact arms to ensure positive connection when activated, yet after hooking up to test, nada, nothing, as if nothing was done.

I also checked power to the switch contacts with the cover removed and was getting 12V to the static poles, but no motor engagement when the switch contact was tripped, even manually. I'm stuck, the contacts work as they should mechanically, but not electrically

Does anyone know what is on the underside of the PCB? The board seem attached to the plastic case via the solder joints attaching the backside of the male plug pins. I see three plastic guides two of which are melted but after cutting them flush, the board remains rigidly affixed, and seeing no other connection method I assume that the pins are preventing lifting it from the plastic casing. Not sure if there is circuitry that might be preventing the current to flow all the way through. Any ideas? switch wiring diagrams?

TIA

Picture: Red circles are contacts for fore/aft switch, yellow oval is the three pole connection with the wiring harness, presumably for the fore/aft function

[IMG][/IMG]
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Old 04-06-2015, 07:41 PM
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Ive seen the solder joint where the pins connect to the board be a problem. Break out the iron and reflow everything you can get to.
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Old 04-06-2015, 07:53 PM
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Quote:
Originally Posted by duxthe1 View Post
Ive seen the solder joint where the pins connect to the board be a problem. Break out the iron and reflow everything you can get to.
Thanks duxthe1
will give that a shot, have you seen the board on the backside? Any idea of how the micro switches are attached and if there are any other parts back there?

Thanks again
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Old 04-07-2015, 11:55 AM
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Well I resoldered the post connections, basically anything that was accessible and still no go. All other seat functions work except fore/aft and the switch mechanically is working and making contact, letting current thru just no seat movement.

Any ideas?
TIA
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Old 04-07-2015, 12:16 PM
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I wonder if you can just put in the old style seat switches.They are a lot easier to open up and restore. I guess all you would need to do is change the wires out so the connectors could fit. I did a similar mod to my w201, but for the O2 sensor. Since the Ford mustang 5.0 (1990) shares the same Bosch O2 sensor as the w201 just with a different connector and at a fraction of the cost, instead of always cutting the O2 sensor connector after purchase, I just changed out the connector on the car. Now I can purchase the sensor w/o having to cut the end.
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Old 04-07-2015, 09:30 PM
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It's alive....

Well after cleaning up the pin contacts a bit and swearing how the car will end up in the crusher, outloud so that the car could hear it
In any event the switch seems to work fine now, repeatedly, let's see how long that holds out.

Thanks to duxthe1 and the others for some great suggestions. I think this came down to two causes. The fore/aft being the most often used seat eventually bends the contact arms to the point where a weak contact is achieved. After correcting that problem, and still having erratic operational issues, weak/cracked/cold solder joints as duxthe1 suggested were rectified and now all seems well.

The later switch design is far simpler mechanically to service and rectify over the earlier design with all of the springs and ball bearings. It has no pieces that fall out of place or need to be tracked when the cover comes off and it is a very robust design, but like anything in use for 20+ years, some corrective adjustment is acceptable and expected.

I also picked-up a spare from the junk yard and unfortunately that also had a few non-functioning switches, so I'll tear into that one and post the findings here. My main curiosity is how to separate/remove the board fro the plastic housing, if one wanted to replace an individual switch rather than repair it, how it could be done. Also of interest is the shared electrical pathways on the circuit board that go between switches. I'll try to take some detailed pictures of the board layout front and rear.

Thanks to all
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