How to drop rear head restraints on W124 300D - PeachParts Mercedes-Benz Forum

theref · #1 09-08-2005, 12:45 AM

I needed to remove the rear seat and had to stop when I could not get the rear seat head restraints to drop using the switch on the dashboard. No bad fuses apparrent.

Hit Man X · #2 09-08-2005, 12:58 AM

Just because a fuse looks okay doesn't mean it works... if you've never replaced them I'd highly recommend doing just that. Also, the switch could be bad... the sunroof switch on my SD went bad.

lietuviai · #3 09-08-2005, 10:48 AM

I've got the same problem with my '87. I've checked my switch and it's OK and there's power getting to it. I haven't checked any further than the switch. I've seen somewhere that the headrests are vacuum powered. Could it be be the actuators?

SW · #4 09-08-2005, 12:56 PM

What works for me is to have someone in the backseat force the headrests back while I hold down the switch. It works for me 99% of the time.

tolive31 · #5 09-08-2005, 03:10 PM

They are vacuum actuated. Use your mighty-vac to apply vaccum and they'll drop, or you can just manually push them down.

lietuviai · #6 09-08-2005, 03:12 PM

I've tried to push mine down manually w/o pushing the button and they won't budge.

tolive31 · #7 09-08-2005, 03:23 PM

Push harder.

I wouldn't do it very often, however.

lrg · #8 09-08-2005, 10:21 PM

Pushing without releasing the lock is NOT a good idea. These are supposed to be able to stay up when they are smacked hard by your head in an accident so they won't (and shouldn't) go down by just pushing them. The switch is electrical but the actuators are vacuum powered. Ifind I sometimes have to hold the switch for as much as 10 seconds to get my headrests to retract. My guess is I have a slight vacuum leak but I've never followed through to fix it. If the electrical circuit is good you need to troubleshoot the vacuum system. DON't just push them down, at least try holding the switch for a few seconds to see if enough vacuum can build to release them. If you break the locking mechanism they'll be useless from a safety standpoint.

AlexTheSeal · #9 09-09-2005, 05:02 AM

There are vacuum solenoids under the rear deck, accessible from the trunk. These rarely fail in my experience, and neither does the switch. But there's a switchover valve or "vacuum relay" under the rear seat cushion, on the driver's side, that often fails. To get the cushion out, wiggle the orange release levers and pull it up and forward. It'll be obvious how the system works when you see the vacuum lines and wires under there, and it's easy to figure out what's broken with your voltmeter and MityVac. If it is the switchover valve, a new one is about $25, or was the last time I bought one (they've been bad on every old 124 I've seen).

#10 09-09-2005, 09:19 AM

You can hear the solenoids click when you push the button, even if there is no vacuum. Also, mine do not work unless the engine is running which would suggest that the vacuum is supplied from the engine, not the door lock vac pump. You may have a vac leak too and need to use a Mity-Vac to determine if there are leaks.

AlexTheSeal · #11 09-09-2005, 11:14 AM

Quote:

Originally Posted by BusyBenz

You can hear the solenoids click when you push the button, even if there is no vacuum.

That's the switchover valve you're hearing, see my previous post. Just because it clicks doesn't mean it's working correctly, though.

Quote:

Also, mine do not work unless the engine is running which would suggest that the vacuum is supplied from the engine, not the door lock vac pump.

Correct. The only thing the lock pump on 124s (under the rear seat on the passenger side) provides vacuum for is the locks (doors, trunk, and fuel filler door).

theref · #12 09-12-2005, 03:07 PM

Thanks to everyone for the response. i will go digging under the rear seat for the oft-broken vacuum device.

lietuviai · #13 09-12-2005, 05:56 PM

If the vacuum units were broken, wouldn't it affect the entire vacuum system?

AlexTheSeal · #14 09-12-2005, 06:13 PM

Quote:

Originally Posted by lietuviai

If the vacuum units were broken, wouldn't it affect the entire vacuum system?

Good question, but, no, not usually. The switchover valve under the seat is like a relay. It only supplies vacuum to the solenoids under the rear deck when actuated by the switch on the dash. So when the switch is not pressed, they are isolated from the rest of the system. This is very different from having a leak in your climate control system, for instance, which affects everything else since one or another part of the ACC always has vacuum in it at some point; or the common problem with 123s, where a leak in the door lock diaphragms affects the ACC since those systems both get their vacuum off the engine.

fgreynolds · #15 01-11-2006, 05:39 PM

I hope you've already fixed this by now; in case you have not, I had a similar problem on my 87 300D Turbo. Dropping the head rests requires vacuum from the vacuum pump, & I suspect you have a leak somewhere in the vacuum system. You can test this, but I was never able to find a schematic that told you what the vacuum should be throughout the system. But you can use a Mityvac to provide vacuum to the system to see if you have a leak and/or if the rests will drop under vacuum.

In my case the leak was in the EGR system--I had a bad vacuum valve transducer that leaked, so the vacuum going to the head rests was not sufficient to drop them. On my 300D, there are 4 small plastic vaccum lines coming out of a rubber "X" fitting on the front of the vaccum pump. When you face the front of the engine, 2 lines go left (to the EGR/vaccum transducers), and 2 go to the right. One on the right feeds the transmission vacuum amplifier, and the other feeds the "accessories" line that powers/drops the rear head rests.

I fixed my problem by pulling the EGR vacuum lines out of the rubber "x" fitting on the vacuum pump, joining two short pieces of plastic vacuum hose joined together with a piece of rubber hose to make a "U" and inserting the ends of the vacuum tubing into the "x" fitting to bypass the leaking EGR transducers.

Good luck. Frank

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