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  #1  
Old 08-12-2013, 09:05 PM
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rear head rest problem on 87 300D

I have an 87 300D

When I bought it a few years ago, the REAR head rests worked.

Recently, I noticed that they dont work. They just flop down, and wont stay up.

Since very few people sit in the back seat, I dont know how long they have been failing.

I verified that I have about 12 pounds of vacuum to the motor that is under the back seat, but I didnt have vacuum at the head rests, so I bypassed the motor, and got 12 pounds of vacuum at the head rests, but they still didnt work.

I have verified that I have 17 volts going to the electrical connection at the motor, but I cant get it to work.

Does anyone have any information on this?? I have searched this forum, and found some valuable info, but not enough to help me find the problem.

Thanks


Last edited by 1987300sdl; 08-12-2013 at 09:31 PM.
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Old 08-12-2013, 09:21 PM
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The headrests don't need vacuum to stay up. Use a heavy hand to set them upright and they should click into place. Vacuum to drop the headrests is shared with ACC, not central locking. I forget whether it splits downstream of the check valve ahead of the bulkhead or under the gauge cluster. If you confirm 17V at the central locking pump, you have a serious electrical issue.

Sixto
87 300D
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Old 08-12-2013, 09:54 PM
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Sixto,
The motor I am talking about with 17 volts is not the vacuum pump for the Central locking system. It is a motor on the drivers side under the rear seat. (it may not be a motor, it may be called something else.) It has a white vacuum line going to it from the engine compartment, and a white\blue stripe going out to the head rest.
The Central locking system is a yellow vacuum line and goes to a pump on the passenger side under the rear seat.
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Old 08-12-2013, 10:40 PM
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The "motor" is a simple valve with a delay function. When you press the headrest switch the valve opens, pulling vacuum on the headrest latches and allowing them to lay back. When the delay times out the vacuum stops.

As Sixto writes, the headrests should lock if you pull them up. The 17 volts cannot be correct. It should be 12-14 volts max.

Jerem
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Old 08-12-2013, 10:58 PM
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It is possible that the meter I am using is not accurate(it is a cheap one) I will borrow one tomorrow and re-test.
Thanks to both of you for your assistance.
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Old 08-13-2013, 01:50 PM
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OK-I got a good MultiMeter, and it reads 12.30 Volts with the ignition swith on, and the engine shut off. That sounds like Battery voltage, which should be acceptable.

So-I am back to my same problem. When I manually pull rear headrests all the way up, they wont stay up, they just flop back down. I have pulled very hard to be sure I am pulling all the way up, but they both just flop back down.
Based on what I have learned on this forum, the vacuum pulls the headrests down when the button on the center console is pressed, but what holds them up until I press the button to let them down??
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Old 08-13-2013, 02:11 PM
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the headrest function is pretty simple, the vacuum is to "unlock" the headrests and they flop down on their own weight, The vacuum solenoid is under the rear seat on the left (driver) it has a delay function built in, meaning it works for 2 seconds to release the headrests and then will only work after about 10 seconds.

The main vacuum feed is a white vacuum line, it is besides the yellow Y shaped piece behind the brake booster and it goes inside the car in the cable tray in the floor all the way to the back

If you say your headrests do not lock then you might need to get into the trunk and see if something is limiting movement, they only latch at the very end of movement.
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Old 08-13-2013, 02:12 PM
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There's a picture in the following thread of the hinge mechanism but it's very small. You might find a better picture on eBay.

Rear headrests - how to fold down? 87 300D

Disconnect the line at the 'motor' and see if that helps. Otherwise, pull the liner aft of the fuel tank and see if the vacuum pods are stuck. Maybe a quick puff into the pods and some non petroleum lube on the retracting pin will help.

Check the gasser forum since MBUSA sold far more 300Es than 300Ds.

Sixto
87 300D
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  #9  
Old 08-14-2013, 10:00 AM
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Problem solved!
It was Operator Error.
I was pulling the headrest all the way forward, but wasnt putting enough pressure on it to lock it in place.
Thanks for all the assistance!

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