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  #1  
Old 02-18-2002, 12:48 PM
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Join Date: Apr 2001
Location: Central Oregon
Posts: 91
Question No Power Seats w/door closed

I have an 85 500SEL (USA) with 112k and experienced very few (but common) problems.

Recently the power seats and power windows all stopped working. When I have the car off, and door open, the seats work fully. Of course the windows do not operate in either case. I did my searches, and found many discussions about switches, motors, and the diodes.

Since they all went out at once, I doubt it is a switch.
Ditto with motor(s).
Not sure about diodes, relays?

Is there a common denominator to the windows and seats, that gets bypassed with the engine is off?

I have recently relocated - so my independent MB guru is no longer available. There is a MB dealership in town, but we all know what that means

Mahalo
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  #2  
Old 02-18-2002, 09:37 PM
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Join Date: Aug 2001
Location: Northern Virginia
Posts: 2,911
If your car was designed by the same people that did my 1987 300D, there is a module in the relay box that contains nothing but a single diode. On the 300D it is the furthest back, hardest to get to module. Also, on the 300D you get to the relay box by taking out a few screws - it is in the same big box as the fuses, but it has an additional cover.

The "diode" module provides a signal that causes power to be provided to the power seats with the doors closed and the ignition on. If someone has pressed down too hard on this plastic module, it can break the diode.

I know because I did just that, and got the symptoms you describe. It is possible if you are good with mechanical skills that you can take apart this module and replace the diode. It is only a two pin module. I had similar diodes on hand and soldered one in - a free fix.

If you are familiar at all with old crystal-controlled radio transmitters, the module looks a lot like an FT-243 crystal holder as shown below:
Attached Thumbnails
No Power Seats w/door closed-radiocrystal.jpg  
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  #3  
Old 02-19-2002, 12:20 PM
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Join Date: Apr 2001
Location: Central Oregon
Posts: 91
TK - yes, similar but the older generation MB's do not allow normal window operation unless the key is "On", regardless of door open/closed. My problem is that the windows/seats do not function while cruising down the road. The power seats work fine when I open the door w/out the key in the ignition, so wherevever the fault exists, it is being bypassed with the door open/ignition off signal. And I am not even sure the power seats/windows problems are related - just hopeful to make it a simple fix.

Ken - I think you are on the right track. Is there a simple way to test/inspect these diodes. I have easy access to several in the back of the fuse box, and am not sure how to determine if one is bad. My best course of action would be simple replacement - if I knew what I was looking for.

Mahalo
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  #4  
Old 02-19-2002, 01:03 PM
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Location: Northern Virginia
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It's a little hard to comment on a car I'm not familiar with. Generally, you need to have sufficient information to know what is inside the plug-in module you are looking at. My car has a lot of these modules that are relays - they tend to have six pins on the bottom. The only module I have with just two pins is the one with a diode inside it.

The module is mostly plastic, with metal pins of course. The plastic shell has fingers that can be carefully pried apart to allow the shell to come open to reveal the contents. Inside there should be a small diode soldered between the two metal pins.

In my case, the diode was physically broken. The diode component itself is encased in glass or ceramic (or something like that) that is breakable under bending stress. I suspect if the module is bad, physical stress has a good chance of being the cause.

Diodes are one-way devices. They must be properly connected in the right orientation. This is usually marked with a stripe on one end of the diode. Should you decide to have a replacement diode soldered in, this direction orientation must be followed. That is, the stripe end of the diode must always be soldered to the same pin of the module.

It is possible to electrically test the diode module without disassembly. With an ohm meter, you should get low resistance in one direction and high resistance in the other. Try using a resistance scale in the mid-range of what the meter offers. Some meters have a special diode junction testing position.

--------------
Too much detail?

Yes, I know. This is one of those tasks where the Mercedes electrical troubleshooting manual was essential.

Ken300D
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  #5  
Old 02-19-2002, 03:00 PM
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Location: Gainesville FL
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While I haven't tried it to confirm, all US model MBs will not allow the windows to work without the key on.

Unless the law has changed I know this to be a fact as it was one of the things that had to be converted to DOT spec when we were bringing in grey market cars in the early eighties.

Another interesting sidebar is that when my short wife bought her new ML in late 98 one of my biggest gripes was that the seats DID NOT move with the door open key off. The real interesting thing is that this is programmable in the AAM (all activities module), BUT it is specificaly stated in the WIS (workshop information system) that cars in the US can not be programmed that way.

Because I am 6'4" I have programmed mine anyway, but I must be certain that if kids are left around the unlocked car that they don't strangle one another with the windows.
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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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  #6  
Old 02-19-2002, 03:30 PM
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I should mention to clarify my posts that the diode inside the module only affects the operation of the power seats. The relay that provides power to the power seats can be thrown by either of two signals, the door being open or the key being in the ON position. The diode serves to keep those two signals electrically separate - so that they do not interfere with one another in other functions that they control.

When the diode is broken, the signal from the ignition key does not reach the relay - so the seats only work when the door is open. At least on my 87 300D that's how things were set up.

As for the power windows, as Steve mentioned they only work with the key in the ON position.

Ken300D
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  #7  
Old 02-20-2002, 12:23 AM
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Join Date: Oct 2001
Location: Vienna, WV
Posts: 121
My 87 190D works the same way. The power seats only adjust with a front door open. The Electrical Troubleshooting Manual shows on page 138/10 the diode. The diode isn't listed in the component location list and I'll be darned if I can find it. The diagram would lead you to believe it plugs into the fuse panel (Electrical Center) but all the devices on the panel are accounted for. Does anyone know where it is on a 201?
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Tom Savage
Vienna, WV
1984 300D Euro
1995 S320
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  #8  
Old 02-20-2002, 12:51 PM
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Join Date: Apr 2001
Location: Central Oregon
Posts: 91
I'm having trouble locating the diode as well.
I went to a "recommended" independent who did a quick courtesy check of the fuses and connectors - nothing there. When I brought up the diode question, he seemed perplexed, but was willing to investigate at $70/hr.

I am going to keep searching. Anyone out there who can help, please chime in!!

Ken - do you have the part number for the diode. I know many parts are used on different chassis, so this may help us

Danke
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  #9  
Old 03-07-2002, 08:23 PM
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Join Date: Apr 2001
Location: Central Oregon
Posts: 91
Talking Problem Solved

To bring closure to my post [for those who use the search feature]

I ended up taking the car to the MB dealer.
The problem ended up being under the fuse box, in the terminals leading to fuse #7 (for those of us with a W126). There was a small crack which they soldered back, and Whala! everything works like new. That explains why both windows and seats were affected universally.

Lesson learned - don't be forceful when working around these older electrical parts. Not certain that I caused the break, but I will exercise more caution in the future now that I know.

$75 later, and I was on the road again.
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  #10  
Old 03-08-2002, 12:04 AM
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Join Date: Oct 2001
Location: Vienna, WV
Posts: 121
On my 1987 190D the electrical manual clearly indicates that you should be able to adjust the seat with the door closed and the key in accy, run or
start. The problem was finding the diode on device xF. I never did find
it but I've deduced that the devices that start with a small x are on
the bottom of the Electrical Center (EC). I went looking for it but I couldn't lift the EC high enough with the harnesses plugged in to see or reach it. I was reluctant to pull on the wiring connectors - they seemed pretty tight and probably have some sort of latch. I've since learned that you can reach up to the back of the fuse box from under the dash. I'll do this next time if I have any problems with my current setup.

Not being one to give up - here is the solution I came up with. I purchased a 50V, 1A Micromini Silicon Diode from Radio Shack. Cat No. 276-1101A. I trimmed the leads and crimped the anode to a ring connector lug, crimped the cathode to a butt splice, heat shrank this section, ran a 14 ga (a lot bigger than needed) from the other side of the butt splice to a Tap-In Squeeze Connector. The other half of the squeeze connector was wrapped around the wire that supplies power to the power seat relay from the door switches. I bolted the ring connector to
the unused 15R terminal on the lower front of the electrical center. On a W201 the power seat relay is K5 but seems to be in different locations on the Auxiliary Relay Box with each model year. For an 1987 W201 the applicable pages in the Electrical Troubleshooting Manual are 103/2 and 138/10.

On the W201, bolting the ring lug to the terminal is easier said than done. To my great suprise there was no nut behind the terminal. I had to file down the flats on a nut to get it to slide behind the terminal. You also have to unbolt and unscrew the electrical center and lift the front to reach the terminal. There has got to be an easier spot to tap into 15R. Even 15 would work. As far as I can tell the only difference is that 15R has power when the key is in 3 positions - 15 only in run or start.

At any rate, the job is done and seems to work fine. I'll let everyone know if any problems crop up.
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Vienna, WV
1984 300D Euro
1995 S320
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