Electrical Issue

1990 560 SEC Euro Spec model originally from Japan.
I have a strange issue with my power windows. At any given time they all fail to work, along with the power seats. Then by
some miracle the cure themselves and work for a while.
I have changed the fuses involved to the copper electrode ones from the aluminium ones, and have even swapped relays around to no avail.
Sometimes when they are not working, if I open the drivers door the windows will suddenly start working again!
Could this be an ignition switch issue? is there some other relay involved that i am not considering?
Any insight would be much appreciated.
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It could be, but I would check for loose grounds and wires first.
Where are the PW/ PS switches located?

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GM wrote:

Well, I've always thought it weird that the power windows on the 3 Benzes I've owned will work when the ignition is on OR when the door is open. So I figure the circuit must be slightly more complicated than you would expect. There's probably a relay involved.
X.
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On my W123 coupe there is a switch that engerizes the power windows when the door is open. It would be nice if the powered the sunroof too.

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I would have to say it is ground issue. Which power seat control are you talking about?
I would take apart the window switches and clean the contacts inside it before ripping anything else to check the ground straps.
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Perhaps I didn't make myself clear.
When the windows fail to work, they all fail at the same time, so my thinking is that its probably not the switches since they wouldn't all fail at once and then all work again at once. The window switches are located in the centre consul, but the power seat controls are in the doors.
I think I can rule out grounding issues, since I don't think that all 4 windows plus the 2 seats would share a common ground, other than the one common ground that all the circuits in the car share.
Is there some other point of commonality that these circuits share besides the obvious fuses, and the one indicated relay?

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GM wrote:

Have you checked if the power seats have the same (normal) behavior as the windows? I mean if they only move if either the ignition is on or the door is open. If so they could share that mechanism.
X.
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I wouldn't make that assumption.
One test you can do is attach a test light to the pin that provides power to the switch and grounding it to a known good ground. If the light is on during a malfunction, you know it is not a power problem.

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With this info, first thing I would change is the ground cable at the battery...it is simple and easy.
Another possibility is the ignition switch, but I see this when AC fan fail to come on.
Last assumption is the OverVoltage Protection Relay... but I don't think it affects the windows... just the starter.
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