Neither of the seat backs will lock on my '89 300CE. I've done some research to try and determine what might be the problem. Most of the problems I researched were particular to the W123 chassis. Is the system similar on the W124? I pulled the back left seat cushion and looked at a small valve with several baby blue vacuum lines exiting from it and travelling to the front seat. The lines all look to be in very good condition and were snug on this valve. I noticed that when I open and close the driver's door this small valve clicks when the ignition is turned off. I'm thinking that perhaps one of the vacuum elements underneath the front seats is leaking. Where should I test to see if they are defective? The central locking is functioning perfectly fine. Is the locking actuation for the seats on a different line than the other vacuum components?

My second problem is with the driver's side memory seat. Every so often the seat reconfigures itself to a different position. It usually adjusts itself just after I have turned the ignition on. The headrest and steering column will also move themselves on their own. I have pulled the seat controls off to clean the connections as well as disconnected the battery to clear the stored memories but the problem persists. I did a search and found very little information about a problem similar to this. Is it the seat switch on the door that houses the memory unit that is likely the culprit? Sometimes when I open the passenger side door the seat will adjust itself even though the ignition is off. What do I need to look for to correct this gremlin?

If it is similar to the W123, you've probably got a vacuum leak somewhere. Mine won't lock until I select defrost on the climate control due to (I think) a leak in the pod for the defroster flap, but then the check valve holds it closed.

Do a search for vacuum diagrams, get a Mity-Vac and get to troubleshooting.

The seat memory control is located under the driver's seat. The memory's control function is the A&B buttons on the seat switch located in the door. The seat position information comes from potentiometers integrated into the seat motors. The A&B switches are used to "capture" seat position info into the memory registers and to recall "captured" seat position info from memory registers. Pressing the button for a period of time will store a seat's position info and pressing the button momentarily will load the stored position info and the controller will cause the seat to move to the stored and then recalled position. Each button A&B stores one seat position.

In the case of the passenger seat it sounds as if in the seat position memory signal is being activated when the door is opening or closing. This would lead me to believe that your problem is likely an issue of worn/broken wiring causing an open or a short at the point where it flexes between the door and the door jam. Testing the wiring for continuity or lack thereof would be a good place to start.

For the drivers seat it may be the same issue, where the state of the A&B switch outputs is changing. This could be the wire from the switch to the module under the seat either opening or shorting, or it could be that these switches are only getting power to them intermittently with a similar output fluctuation be the result. Intermittent open/shorts caused by road vibrations could be the cause also.

It sounds like there is some apparent cause and effect with the doors moving, this would lead one to suspect the seat/wheel control module under the seat not to be origin of the problem. That's not to say if definitively is not, but you would have to eliminate the electrical wiring as a potential problem to determine if the electronics are at fault.

With regard to the seat back locks. The system uses vacuum solenoids to secure the seat backs in position; the button on the side of the seat back vents the vacuum momentarily releasing the solenoids and allowing the operating rods to release tension on the locking mechanism.

I’m not certain of the vacuum source for this system, whether engine vacuum or the electric pump used to operate the door lock system. The way to diagnose would be to apply your Mighty Vac to each of the two solenoids separately and find out if /or which one is leaking, test the release button the same. If there is no leak discovered under the seat you would have to check the lines between the seat and the source.

The under-seat vacuum stuff can be checked by removing the four 10 mm bolts at the seat corners after moving the seat forwards and backwards to allow access to the bolt heads, and the single 10 mm bolt securing the seat belt rail. You can then lean the seat backwards after unplugging the seat’s electrical connections. There is one vacuum solenoid on each side of the seat located near the seat front with an actuating rod running to the rear locking mechanism. Should be a four way vacuum connector under there; one line to each solenoid, one to the release button and the last would be the vacuum source.

If you don’t want to tilt the seat you might be able to release the seat covering along the seat’s front lower edge and then access the vacuum stuff from the front and sides. To do this you sit on the sit compressing the springs inducing slack in the seat covering material. Then the folded edge strip can be pulled downward releasing the seat covering from the bottom if the seat’s spring frame, you can then lift the loose covering material.

I should have made it clearer that there is no memory settings on the passenger seat, only on the driver's side. When I open up the passenger side door the driver's seat starts to adjust. How would opening the passenger door with the driver's door closed and ignition off cause the driver's seat to adjust itself? Could it be that there is problem with the module underneath the seat?

Yes it very well could be the module under the seat, I was just trying to figure out and therebye eliminate the potential apparent cause and effect I'd thought you'd described.

As far as the passenger door relating to the driver's seat's movements, the only correlation I can figure has to do with the way the seats are adjustable without the key in the ignition only when the door is open. So if there is a problem with say the open/short wiring on the driver's side door there would only be power to the seats with the key out of the ignition when a door is open. In effect the opening of the door acts as a power switch, then the seat moves because it has both a control issue but only has power when the ignition is turned or when the door is open.

It sounds like you have disassembled and cleaned the seat switch and you are certain that the memory switches (A&B) are functioning properly, they make contact only when pressed and break contact when released. One way of dianosing the potential of a switch or switch wiring problem would be to disconnect the wiring control plug under the seat, open the connector, identify the two A&B switch wires, remove them from the connector(noting their position and insulating them) then replug the connector to the module. Then you should not have any chance of errant control signals to the module, open the passenger door, the lack of the seat movement that you had previously observed would point to a control issue. If you still have the seat moving at unexpected times it would point to a problem with the module itself.

I had the same problem with my '93 300CE in that the seats wouldn't lock. The small vacuum relay under the back seat was the culprit. The vacuum comes from forward on the engine. We removed this relay and put in vacuum tees. Everything works great. I'm not sure of the function off this relay though,
Anziani
'93 300CE 180K
'95 E420 117K

How did you diagnose that the relay was bad? Do you know what it looks like?

Actually my mechanic made the call. He said it was hot and humming. He went ahead and teed it out. We still don't know what it was supposed to do but so far everything works as it should. When I get the rest of the problems solved I will probably go back and put in a new one.
Anziani
'93 300CE 180K
'95 E420 117K

This device sounds like the vacuum valve that is electrically controlled by the switch on the upper section of the center console to allow the rear head rests to return to the folded down position!

Dang, I bet you are right. Since I never have the rear headrests up because no one ever sits in the back I haven't had any need to raise and lower them. Thanks
Anziani
'93 300CE 180K
'95 E420117K

It is interesting that you noted that the relay was getting warm. I've noticed that when I open and close the driver's door several times I can hear that small relay with the vacuum hoses coming out of it start to click and get hot to the touch. I wonder if this is my problem and how I can troubleshoot it to determine if it is indeed defective.

You should disconnect the hose going to it and plug its' end, if it cures your seat lock problem then that was it! If you want to diagnose the valve, disconnect the hose coming from it that goes to the rear headrest locks, connect your MityVac to the valve's outlet and observe the gage. There should not be any vacuum at the valve's outlet, operate the dash switch which should energise the valve and you should see vacuum at its' outlet.

I've done a bit of testing and it appears that I have a leak on the passenger side seat. I can hear an air leak underneath when air pressure is applied. I was able to get the driver's seat to lock by blocking one of the vacuum lines on the relay valve under the left rear seat. I need to remove the passenger side seat to get a better look at things underneath but it is proving rather difficult. I've managed to pull a few bits of the plastic trim to get better access to the front 10mm bolts but the one on the right side won't come free. I don't want to break any of the plastic that surrounds the seat base. I sat on the seat and tried to loosen the trim but that did not work either. Am I missing something here?