Long wheelbase models of the W126 could be equipped with an electric reclining rear seat. From the factory the W126 reclining rear seat was standard on the 560SEL and optional on other models. local subsidiaries may have ordered other models with this feature as standard in their markets too. At least here in Australia it is always seen on the 560 models, commonly seen on the first generation 380SEL and occasionally seen on the 420SEL. Privately imported cars vary, but it is more commonly seen on the higher specification models like the 500SEL.
The W126 reclining rear seat was not working on my 560SEL when I purchased it. I bought the car from the original owner and he could not remember when it ever worked. The seat was stuck in the reclined position, which was not ideal to fit a child seat.
Back in May, I did some initial troubleshooting. There was not a blown fuse, there was nothing jammed in the mechanism, and the motor did not run when the button was pressed. It is fairly common for the control cables to become dislodged, which means the motor runs but the seat does not move.
Today I looked at the system in a bit more detail, as I wanted to fit a child seat. The diagram above shows how the seat works. Its quite simple really. The two switches send voltage in the direction the motor must run to move the seat. Using my power probe, I had voltage on one pin and nothing on the other. This seemed odd, as I would only expect to see voltage when the button was pressed. When I pressed the button, the voltage was no longer present.
The next step was to test the motor directly. The power probe allows for voltage/ground to be applied easily. As soon as I applied power, the mechanism moved in the correct direction. I was able to easily move the mechanism into the non-reclined position with the power probe. Confirming that the motor, cables and mechanism worked highlighted that I likely have a switch, wiring or ground problem. With the mechanism in the right place, I bolted the seat back into place.
As I will have a child seat installed, it is actually better that the seat does not work. I don’t want one of the kids pressing the button and damaging the motor or mechanism as it tries to fight against the child seat. I would like to fix the seat in a couple of years once I remove the child seat. This will be early 2024 when my youngest child no longer needs one. The W126 only has a lap belt in the middle rear, which is incompatible with a booster seat. The booster seat requires a lap/sash seat belt.
It is not clear right now where the problem lies. If it is a switch or some wiring or ground problem. A switch seems more likely.
With the seat in the non-recline position, you can see the huge leg room available in the back of a long W126. If only the airlines could provide something comparable. I have now fitted the child seat, in preparation of a club drive I am planning to join in a couple of weeks.
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