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1990 420 SEL
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14 Posts
Discussion Starter · #1 · (Edited)
Howdy everyone,

I've been troubleshooting the driver side power seats on my 1990 420SEL. After using all of the great resources here, I'm stuck.

Here is what I know:
  • Someone ( Who I've been calling Uncle Eddie) at one point tried to make the thing work, but did a very bad job.
  • I have replaced all of the fuses.
  • The power seat diode, that fits into position G is totally missing.
  • I have pulled out the seat and tested all of the motors and they all work.
  • I have disassembled the door switch and It looked brand new inside.
  • I have tested that the plug that powers the control memory module(under the seat) gets voltage.
  • The telescoping steering wheel does not work.
  • The passenger seat works fine, and I even hear the memory try to engage on it - although it doesn't seem to do much.
Other electronic things that don't work, but might be related
  • The driver side door will only lock from the inside with the door closed.
  • There are lots of things that don't seem to get power, AC, Radio, various lights(could be bulbs but haven't checked).
  • All of the windows work, except the passenger which works occasionally.
  • I found a plug just behind the fuse box I has a green and brown wire going to it and has a label F See image below, It doesn't have much slack and doesn't seem to have a mating connector anywhere near by
  • The odometer, gas gauge, and clock do not function, but all of the other gauges do.
  • The AC was retrofitted to 134A
  • I have not idea how the distributor and rotor look as one of the screws is stripped so I haven't gotten in their to look to see if it has been replaced recently.
  • The car idles (to my untrained ear ) kind of high - Like it goes from a low to high. Kind of like a
  • The Idle Air Hose - Valve Cover to Idle Air Distributor - has a crack in it ( which I am currently attributing the weird idling to.)

Any ideas on how to proceed? Is there a way to test the memory control unit to see if it is the problem? Could one of these other electrical issues lead to the root cause? I would appreciate any help in troubleshooting further. Thanks in advance!

2705203
 

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Registered
1989 300SE 240k miles
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2,884 Posts
Ok… With the seat memory, my first question is: Does the power seat, itself, work? My second question is: Is the power steering wheel a part of the seat memory? (I never use my seat memory) Because I’d expect all the functions need to work individually to work together.

The driver’s door lock should be mechanical. A key should lock/unlock the door from the outside. You need to get that working. And while you’re working on that, you can use a hand vacuum and hand pump to test the vacuum actuator and hose going to the lock. ..The door should lock/unlock with the key in the other door and/or trunk. This is thru the pneumatic/vacuum system.

The things not getting power, have you pulled them to make sure they’re in their harnesses?

The intermittent window, I’d start with the switches. Both switches need to be fully connected for the window to work. If one switch is loose, that’ll cause your problem.

Does your ambient temperature display work? That wire harness could be for the ambient temperature sensor. That sensor has those color wires, and it runs underneath the fuse box. The display and sensor connect under the fuse box.

Odometer is probably a broken cog. Fuel gauge is probably on the sensor end, in the tank. Clock has a resistor or diode that is known to burn out. But they could all be bad/loose connections.

That cracked hose is a very strong candidate for your idle issue. Replace that and see what happens to the idle.
 

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1990 420 SEL
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14 Posts
Discussion Starter · #3 ·
Thanks for the reply!

The power seat motors work, but do not function when connected and the door switch is triggered. The telescoping steering wheel is part of the memory module. Could I take the one from the passenger seat and plug in the power and just the seat motors to test if the control module is bad?

Thanks for the tips on the other issues. I just got the vehicle and started at the power seat as it was stuck so far back I could barely reach the gas. I haven't really done any troubleshooting on them - but wanted to include them just incase it might be helpful.

I think that unplugged harness might be where the power seat diode plugs in as it doesn't look like the spot in the fuse box has any connections in the bottom.

I'll have to check the ambient temperature gauge - it didn't stand out to me. Also the little economy mode thing doesn't
 

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Registered
1989 300SE 240k miles
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2,884 Posts
Ask @MBL87560SEC for the power seat schematics for the motors. The plugs are easily accessible and the pins are labelled. And with the schematics, you should be able to jumper the wires to put the seat where you need it. And then you can work out the rest from there.

If there’s nothing in the fuse box, I’d expect there’s a problem and they simply disconnected everything
 

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1990 420 SEL
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14 Posts
Discussion Starter · #5 ·
An update. I think the reason the diode was removed because the previous owner had spliced together some wires so you could move the driver's seat back and forth, but it was easy to undo. I removed the passenger seat(the back and forth motor isn't working - so it was a pain but I got it out – car is in need of a deep clean so this had to come out sooner than later) My plan is to test the driver seat with the passenger seat control module to determine if it is the problem.

While getting the seat out I found what looks like the diode for the fuse box under the seat, but after googling the part number I found it is an r16 resistor? P/n 000-540-2481( see image below) then there is another number underneath it that says 09-8659-24 not sure what it is? Is there a resistor in the power seat assembly? could this be why the passenger seat motor isn't working?

2705424
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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5,254 Posts
I've only seen this part plugged in to a harness near the fusebox. It retards the timing of the engine. Might want to check that yours is still in place.
 

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1990 420 SEL
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14 Posts
Discussion Starter · #7 ·
A further update. While I was removing the seats I noticed a wetspot on the carpet. This led to me pulling out all of the carpet for cleaning. I found that the cowl area hadn't been cleaned out and had rusted through allowing water to get into the car. This led me to fixing some rust spots that were just starting to form in the floor and then repairing the cowl with some fiberglass cloth and POR-15. Then coated the hardened fiberglass with flex seal just in case.

With the water that had come in( some on both sides, but I think the drivers side was worse) - I imagine there is a very good chance this may have damaged some of the electronics/wiring. Any advice on how to troubleshoot possible impacts from the water damage.

Also I found my central locking system was not connected to a pump. I don't know if this would break a circuit that is connected to other things - but I'm planning to fix that first to see if it helps at all.

Thanks!
 

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1989 300SE 240k miles
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2,884 Posts
Good hunting! On the interior water-front, did you look at the drain hoses for the evaporator core? They dry rot with age. And will allow water into the front foot wells.

I wonder why someone would disconnect the pump? If you connect it and the pump runs for more than a few seconds, you have a leak/problem. Quite often it’s a stuck actuator for the fuel fill door. Took me weeks to track that down…
 

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SuperModerator
1986/1990 W126
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17,658 Posts
It's all very durable, I've even seen the seat heater relay units having been under water and working still. But it's not ideal, lol.

I think the seat power gubbins should be pretty much all you have to worry about.

The front floor pans are double skinned, so the floor you see has a fairly large cavity underneath it. Unfortunately this can fill up with water too and usually does. You can find the drain plugs under the car, on what look like the chassis rails.

Good move taking the carpets out, it's the only way.
 
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