Vacuum Seat Locks – Previous Owner Issues

One of the cool features on the 450SLC and in fact any vintage Mercedes-Benz is the vacuum system that controls everything from the HVAC system, the central locking, the height adjustable headlights and of course the seat locks that prevent the seats from folding forward when the car is underway.

The seat lock system is really elegantly implemented. Instead of a mechanical system that consists for a manual lever that the driver or one of the passengers would need to operate, the Mercedes-Benz system is vacuum controlled and almost completely automatic.

When the car is started and the doors are closed the seats lock into an upright position. When one of the doors is opened the lock disengages and the seats can fold forward to allow passengers to get in or out of the back seat. There’s also a couple of discrete buttons in the back that releases the vacuum and manually overrides the lock.

This hasn’t been working in my car for as long as I’ve had it. I had peaked under the drivers seat previously and the complete lack of vacuum tubes were are cause of concern. So this was my starting point, and the first thing I did was to remove the stainless steel trim over the sills, where I was expecting the worst. To my surprise everything was in tact and actually looked to be in reasonable shape.

I immediately found the T-piece that connects the blue vacuum lines to the seat and enters into the sill. I pulled this out and found a short length of blue vacuum tubing that was about 30cm long. My first thought was that this had been cut by the previous owner, but it was actually just the right length. It must have just retreated back into the sill at some stage.

After connecting it, I started the engine and was hoping for the best, but unfortunately it still wasn’t working. What I did notice however was that if I manually engaged the seat lock and opened the door, vacuum would release and it would disengage the seat lock. This was great news to me as it indicated the there was life in the system.

I then had another look at the vacuum diagram in the service manual. What I found was that both sides were connected with no check valve between the 2 sides. Perhaps the passenger side has the same problem?

Over on the other side of the car, the vacuum tubes were all connected as they were supposed to be. So this obviously wasn’t the problem.

The next step was to look go back to the source and start testing the vacuum and check valves. I found the blue lines (Mercedes-Benz had the foresight to colour code the different vacuum systems) and started with the check valve right at the start. I removed it from the car and blew into it – Nothing! Blew into the other side – Nothing! This check valve was blocked.

Upon closer inspection I found something I never expected:

That’s definitely not genuine Mercedes-Benz or a reputable supplier of Mercedes-Benz OEM parts.

A quick online search, found that these valves are ridiculously overpriced in Australia (about $50 including shipping). A shout out to @mrfrotop for putting me on to Pelican Parts in the USA. They had the Vaico brand of valve for $2.75USD – a relative bargain even with the cost of shipping. Of course, I couldn’t make an order and just get the part I “needed”, so I’ve ordered some trim clips so that I can finally take of the duct tape that’s currently holding it in place. I’ll hopefully have all the parts by the end of the week.

In the mean time, I swapped over the check valves between the central locking and the seat locks, just to test it out.

My footage is sloppy at best, but still oh so satisfying to watch as it locks into place.

3 days later my new check valves arrived. I decided to replace all 4 check valves and it was a good thing I did. 3 of the 4 valves were blocked. After replacing them I found that my central locking (that was already working) was now working even better. Unfortunately, the passenger seat lock is still not working – so I’m going to have to look into that another time.