W126 Power Seat Tutorial and Test Tool - PeachParts Mercedes-Benz Forum

micalk · #1 06-12-2010, 02:04 PM

The main reason for this post is to detail a little tool I made to help me with the seats, and it has other uses too.

First an overview. This is the tutorial portion.

In normal use, when a window switch is at rest (neutral position) +12VDC is connected to both sides of the window motor through the switch contacts, i.e., both motor connections are at the same electrical potential, no current flows, no motor movement. When the switch is engaged in one direction, +12VDC is removed from one pin and ground is applied in its place - difference in potential, current flow, motor movement. I don't know why MB decided to have +12VDC running through the wires except perhaps it is probably easier to chase +12VDC around when troubleshooting the window circuits.

The seat motors operate in the same fashion, though the seat switches make no connection to power or ground when in the neutral position. Prior to 1984 both +12VDC and GND are applied when the switch is engaged, and there is a single connector under the seat that connects the switches directly to the seat motors. 1984 introduced memory seats and a solid state seat controller under the seat which only applies power to the motors when needed, and the switches only apply GND to the controller; each motor has a separate connector to the controller. For all model years, the seat motors operate the same way as the window motors - polarity to the motor is switched to change direction.

The tool I made has two pins that plug into the motor connector, two connections for power, and a window switch to control polarity to the motor. I took a rear window switch and a two pin male connector and re-jiggered them into forward/reverse seat switch (for a 1984 300SD). Can also be used to troubleshoot windows.

There is a slight safety concern with this tool that I didn't consider when I built mine. I mimicked the design of the window switch and motors, that is to say that my wire colors indicated the expected connections: red is +12VDC, brown is -12VDC or ground. As described above, the plus connection goes through the switch to the motor when the switch is in the neutral position. This means that if the battery from the car is used and the pins are left hanging, then the pins could short to practically any bare metal that is grounded. So the colors on my power wires are bass-ackwards from the safer design described below.

What you need:

35-40 watt soldering iron
Heat gun for shrink tubing
Solder
Isopropyl Alcohol
Tooth brush
Flat blade screw driver to disassemble connectors
1 ea, 12 ft brown 16 ga wire
1 ea, 12 ft red 16 ga wire
2 ea, 1 ft 16 ga wire, any color (white in my case)
3 ea, 4 ft 3/8" shrink tubing
8" of 1/4" shrink tubing
Rear window switch and switch connector (from a rear door, not the center console)
2 pin male connector (1984+) or female connector (1983-) from somewhere under the hood of that junker your wife always complains about.

Soldering tip #1: The pins are rather sizeable and will sink alot of heat. A 25W soldering will work but will take much more time to do the job. A 40W iron works perfectly with these pins.

Soldering tip #2: The connector shell is bakelite, and will take the heat from a soldering iron. You can use the shell as an assembly jig while you are soldering to arrange the wires with appropriate lengths and proper locations and wire dressing.

Soldering tip #3: After soldering a connection, it's always good practice to clean the connections thoroughly with isopropyl alcohol using the tooth brush. It's not truly essential with modern rosin core solder, but it's still good practice.

I'll use the term 'socket' to describe female ends, 'pin' to describe male ends, and 'cups' to describe the solder cups on both the sockets and pins.

1.) Disassemble the switch connector and unsolder the wires from the socket cups.
2.) Sockets 3 and 7 both need ground (brown wire), so I soldered them internally within the switch. The wire has to be tucked down between the two sockets to do this because another wire will cross its path. The alternative is to bring the two wires out and solder them together outside the connector. If you make it internal, use a small piece of 1/4" shrink tubing to protect it.
3.) The next wire is socket 4 (red wire) and is paired with the brown wire on sockets 3 and 7 to go to the battery.
4.) Assemble all three sockets to the connector and put 11 feet of 3/8" shrink tubing over the red and brown wires leaving about a foot of exposed wires at the battery end.
5.) Solder each white wire to a socket and install in connector locations 1 and 2.
6.) Install about 7 inches of 3/8" shrink tubing over the two white wires and shrink it down
7.) Now install 1/4" shrink tubing on each white wire before soldering the pins/sockets on the other end.
8.) Disassemble the male two pin connector (1984+) or female two socket connector (1983-) and unsolder the wires from the cups. Solder a one foot white wire into each cup.
9.) Push the 1/4" shrink tubing over the pins/sockets leaving enough of the pin/socket uncovered to fully seat into a socket/pin.
10.) Re-assemble the top to the switch connector and install it onto the rear window switch.

Using the tool

The motor connectors at the seat controller (1984+) under the seat come off easily and the two pins on the tool will only connect to the motor wires in the connector, so there's no real way to screw up the connections. For earlier models, the motors are all on one connector, but are wired separately (no common connections) so even if you manage to put the sockets onto different motors nothing gets hurt when you apply power.

The switch passes ground to both connectors when in the neutral position, so there should be no danger with the pins/sockets flopping around when the power wires are connected to the car's battery. When the switch is engaged one side goes to 12V and the other remains ground, and the pins switch polarity when the switch is engaged in the opposite direction.

When assembled with pins, the tool can be used to troubleshoot windows. When the pins are connected to sockets 1 and 2 in the window switch connector, you should have a direct connection to the window motor. I don't have a way to check right now, but you can probably also put the switch in question onto the test tool and test the switch as well. (Are the rear door-mounted window switch connectors the same as the console switch connectors?)

Hope this helps someone out there. I know when I removed the seat from my car to change the seat springs, this tool helped me disassemble the seat much quicker.