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1984 300 SD
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207 Posts
Discussion Starter #1
My 84 SD driver power seat abruptly activates and moves backwards, when I make a turn or go up an incline. Any ideas? Happy Holidays,
Peter
 

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Premium Member
1989 560 SEC - V-8 Kompressor - 95 E320 Estate
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1,902 Posts
Most likely your door switch has gone wacky. With the switch still in the door take a small flat blade screwdriver or pick of some sort and gently pry off the buttons/levers off the door switch. Get a spray can of MAF cleaner with the small tube attachment and spray into every opening on the switch. You want to flood the internals of the switch with the solvent. During this process wiggle all the posts protruding out of the switch. Don't worry about using too much MAF solvent as it will not harm the plastic or door material. The MAF solvent has an extremely high flash point and will evaporate within a minute or two. I do this procedure on all old switches and 99% of the time they work like new. 5-10 minute exercise which is a lot easier than taking off the door car, removing the switch then taking it apart (not for the timid) cleaning and putting all back together. Good luck! Mike
 

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1983 300SD
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845 Posts
Did anyone ever modify or fix the seat switch? If it has memory buttons it's not original. The Gen 1 is incredibly simple. IIRC it's just point to point wiring in there. If it's after the switch I'd think something to do with the plastic harness under the seat but the wire has to be getting power somehow.

Pic below is a rear view of the wiring harness mildly dissembled. To get the seat to move backward one of them needs 12v while the opposite one is ground.

Bear in mind I have very little idea what I'm talking about with the following suggestions. The order I'm putting it in is just based on how easy it is.

Best bang for you buck on initial troubleshooting is to look under the seat and see if some PO did a hack job with it. Then I'd just spray electrical connection cleaner into the seat switch because why not? If those didn't yield anything I'd pull the door card and see if any wires to/from the switch are loose or have been spliced poorly. After that I would release the seat and tip it back to check the wires going to the motors.

Replacing the older style switch is pain. To do it correctly you have to solder it in. You could use butt connectors though. I'd only do that as a last resort. If you wanted I think I have some junkyard spares, I could send you one. I didn't test them but generally they don't fail like this.

Honestly, if it's a DD I would pull out one of the forward/back wires in that harness and live without it before replacing the switch.

2613500
 

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Registered
1984 300 SD
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207 Posts
Discussion Starter #4
Most likely your door switch has gone wacky. With the switch still in the door take a small flat blade screwdriver or pick of some sort and gently pry off the buttons/levers off the door switch. Get a spray can of MAF cleaner with the small tube attachment and spray into every opening on the switch. You want to flood the internals of the switch with the solvent. During this process wiggle all the posts protruding out of the switch. Don't worry about using too much MAF solvent as it will not harm the plastic or door material. The MAF solvent has an extremely high flash point and will evaporate within a minute or two. I do this procedure on all old switches and 99% of the time they work like new. 5-10 minute exercise which is a lot easier than taking off the door car, removing the switch then taking it apart (not for the timid) cleaning and putting all back together. Good luck! Mike
Thanks Mike
 

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Registered
1984 300 SD
Joined
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207 Posts
Discussion Starter #5
Did anyone ever modify or fix the seat switch? If it has memory buttons it's not original. The Gen 1 is incredibly simple. IIRC it's just point to point wiring in there. If it's after the switch I'd think something to do with the plastic harness under the seat but the wire has to be getting power somehow.

Pic below is a rear view of the wiring harness mildly dissembled. To get the seat to move backward one of them needs 12v while the opposite one is ground.

Bear in mind I have very little idea what I'm talking about with the following suggestions. The order I'm putting it in is just based on how easy it is.

Best bang for you buck on initial troubleshooting is to look under the seat and see if some PO did a hack job with it. Then I'd just spray electrical connection cleaner into the seat switch because why not? If those didn't yield anything I'd pull the door card and see if any wires to/from the switch are loose or have been spliced poorly. After that I would release the seat and tip it back to check the wires going to the motors.

Replacing the older style switch is pain. To do it correctly you have to solder it in. You could use butt connectors though. I'd only do that as a last resort. If you wanted I think I have some junkyard spares, I could send you one. I didn't test them but generally they don't fail like this.

Honestly, if it's a DD I would pull out one of the forward/back wires in that harness and live without it before replacing the switch.

View attachment 2613500
Thanks
 

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Registered
1983 380-SEL, '67 220D (sold), '65 200D (sold), '75 L-206D (sold)
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298 Posts
Inside the seat controller are rocker contact, ball bearings and small springs. For each switch there are 2 springs, 4 ball bearings, two rocker arms and the plastic switch pole and base units and a small metal pole to lock the switch pole units in place. The top cover locks down with plastic rim clips all around the seal. The top cover has protruded guides that push down on the two upper ball bearings per switch. So, if the cover comes loose, all these parts scatter inside the controller, because there is nothing to hold them in place anymore. Usually popping the cover means the controller is toast. I have cleaned, retentioned the rocker arms and put everything back in place and locked down the cover and successfully fixed seat controllers. It's hard to do and the parts are so small, it's easy to loose something.

There are several things that eventually can go wrong with the controller. Long use + rainy weather, window seal leaking or window down in rain, or using a lot of water to wash the car and leaking through an old window seal can cause corrosion inside the switch. The seat doesn't get the 12 volt connection and one by one the motor directions on the seat stop working. Another problem comes from slamming the door and pushing hard on the controller interior door cover trying to line up during removing and replacing the door panel can cause the top cover to pop away slightly and then all seat motors stop working because the parts are loose in the switch. I can clean and rebuild the switch in 30 minutes if you are local around Los Angeles. The whole controller can be bought, I think it's around $150. Getting it back on can be a bit of a job, if your car is older than 1984 or so. Junk yard parts sometimes need internal cleaning from use in damp weather over the years. They can fail unless it's cleaned and the rocker bars checked for correct shape. Older cars have solder joints and two connectors need to be bridged with a small wire. I think it's B4 and A3. There is a guide in the forum. I have a copy, but it's easier to buy the matching controller for your car. I just replace mine 2 weeks ago and needed to add the cross bridge connection. I still have to swap two cables because the rear of the seat goes up when I push down. Darn! Otherwise it's working fine and I do have some parts to the old style '83 controller.
 

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1984 300 SD
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207 Posts
Discussion Starter #9
Any updates?

Sent from my Pixel 3 using Tapatalk
I still haven't figured it out yet. I switched out the door switch. I had new seat skins and cushions put in late last year. That could be the problem, as he removed the seats.
 

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1983 300SD
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845 Posts
I still haven't figured it out yet. I switched out the door switch. I had new seat skins and cushions put in late last year. That could be the problem, as he removed the seats.
Have you tried blasting a bunch of electrical cleaner into the switch yet? The more I think about it that's where the problem probably is. Possible the switch is faulty but I think you can get a can of the stuff for under $5 at Walmart. It fixed my issue of the seat not moving forward.

Sent from my Pixel 3 using Tapatalk
 

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Registered
'87 560 SEC, Pearl Grey/blue; 300,000+ mi; '07 CLS 550, Barolo/stone; 90,000+ mi
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482 Posts
Have you tried blasting a bunch of electrical cleaner into the switch yet? The more I think about it that's where the problem probably is. Possible the switch is faulty but I think you can get a can of the stuff for under $5 at Walmart. It fixed my issue of the seat not moving forward.

Sent from my Pixel 3 using Tapatalk
Did you disconnect the knobs then spray it in while still connected in the door, or remove it then turn it around, etc?
 

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Registered
'87 560 SEC, Pearl Grey/blue; 300,000+ mi; '07 CLS 550, Barolo/stone; 90,000+ mi
Joined
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482 Posts
Most likely your door switch has gone wacky. With the switch still in the door take a small flat blade screwdriver or pick of some sort and gently pry off the buttons/levers off the door switch. Get a spray can of MAF cleaner with the small tube attachment and spray into every opening on the switch. You want to flood the internals of the switch with the solvent. During this process wiggle all the posts protruding out of the switch. Don't worry about using too much MAF solvent as it will not harm the plastic or door material. The MAF solvent has an extremely high flash point and will evaporate within a minute or two. I do this procedure on all old switches and 99% of the time they work like new. 5-10 minute exercise which is a lot easier than taking off the door car, removing the switch then taking it apart (not for the timid) cleaning and putting all back together. Good luck! Mike
Does the MAF cleaner work differently than electrical cleaner? I would be concerned about a fire hazard from current--I have some sort of parasitic draw on fuse 15 (seats are part of that circuit).
 

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Registered
1983 300SD
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845 Posts
Does the MAF cleaner work differently than electrical cleaner? I would be concerned about a fire hazard from current--I have some sort of parasitic draw on fuse 15 (seats are part of that circuit).
Electrical cleaner seems to evaporate extremely quickly. I can't speak to the chemical properties or general smart people aspects of it. But it does seem quite different from the other automotive aerosol cleaner sprays.

Sent from my Pixel 3 using Tapatalk
 

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SuperModerator
1986/1990 W126
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13,577 Posts
I disconnect the battery before spray cleaner. You can spray it in any old how and move the buttons around to clean the contacts.
 

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Registered
'87 560 SEC, Pearl Grey/blue; 300,000+ mi; '07 CLS 550, Barolo/stone; 90,000+ mi
Joined
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482 Posts
I disconnect the battery before spray cleaner. You can spray it in any old how and move the buttons around to clean the contacts.
Great Ian, I was thinking that may be the best way. Is it recommended to wait a few minutes/an hour for it to evaporate before reconnecting the battery and testing?
 

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SuperModerator
1986/1990 W126
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13,577 Posts
Yeah for a bit I guess. It doesn't take long though.
Once I forgot to disconnect and a little wisp of smoke came out. Lesson learned.
 
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