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K6JRF's Page formerly W6FZC My Mercedes Benz S500 Coupe |
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Headrest Repair DIY by Jon Lowe Jon found that his W140 Coupe's headrest had broken similar to my problem. My 'fix' was to replace it but Jon has actually fixed the broken gears. This is his writeup showing the steps necessary to repair the gears. Great job! If you have questions please email him at jonlowe@aol.com |
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The W140 Coupes, unlike most MB sedans, use electric motor powered rear headrest folding mechanisms rather than pneumatic mechanisms. They are relatively simple in concept. The motor output shaft has a metal worm gear, which drives a shaft mounted small plastic gear. At the other end of the shaft, is another metal worm gear, which drives a large plastic gear attached to an output shaft. All of the gears are in a metal case. The output shaft protrudes through a sleeve bearing in the case and has the output arm at the other end which drives the folding mechanism. The mechanism is spring loaded and goes over center at each end of travel, taking most force off the gears inside. I suspect that most breakages are by passengers trying to force the headrests up or down by hand. The symptoms of a broken folding mechanism are well documented here. The only thing I will add is, usually, you can get the headrest to move up or down with a little help by pulling or pushing it in the appropriate direction while pushing the retract/extend button. The headrest must be in the fully up position to remove the mechanism from the car. This repair procedure is not that difficult but takes a fair amount of time. Just keep at it and follow the directions carefully. I tried a few shortcuts that didn't work properly, so don't try to cut corners. Tools/materials needed to repair the mechanism: Torx bit to fit the gearbox cover fasteners: Hysol/Loctite 9462 epoxy and dispenser  Non-flammable brake cleaner aerosol: Denatured or isopropol alcohol: Dremel or other similar rotary tool with cutoff wheel, small sanding drums, and other misc bits: 1/8" roll pin: 1/8" cobalt drill bit: Drill press: Clamps to hold mechanism in drill press: Plastic compatible grease: Vise: Screw type hose clamp large enough to fit around large plastic gear: 12 volt power supply, or 12 volt battery with alligator clip leads: Vise grips and/or channel lock pliers: Small flat bladed pry bars: Optional but required if small plastic gear is broken: Hose clamp to fit around small plastic gear: Thin wall 1/2" brass tubing (available at well stocked hobby shops) : 1/16" roll pin: 1/16" cobalt or titanium coated drill bit: Typically, the large white plastic gear is what cracks causing the chattering when trying to move the headrest up or down. The worm gear driving the plastic gear opens up the crack and the teeth skip. By applying a little force, the gear moves past that point to solid teeth and the mechanism moves normally. Replacement gears are not available and even if they were, most of what is required to accomplish this repair would be required anyway. The plastic gear is apparently directly molded onto the output shaft and seems to shrink slightly over time, causing stress cracks at the hub of the gear which eventually extend out to the teeth. Even when cracked, the gear is extremely tight on the shaft and very difficult to press off. This is because the splines on the shaft are the same diameter or slightly smaller than the shaft itself. There is no way to repair the gear while it is still on the shaft, as clamping it in place will not close the cracks anywhere near enough. 
With the mechanism out of the car, start by removing the four torx screws holding the cover of the gearbox the gearbox/motor
combination. Three of the screws should be accessible but the fourth will be blocked by the output arm. Two of the screw holes
are shown in blue in the picture.
Remove the three screws that you can. Then power the mechanism thru the two pins in the
connector. You will have to help the mechanism to move. If it goes in the wrong direction, simply reverse the connections at the
connector. You should now be able to remove the fourth screw. Please note that the retract mechanism is spring loaded and could
snap back on you. Separate the gearbox from the cover. The output gear will remain with the cover. It will take some finnegaling and twisting to remove the gearbox/motor combination, but you can. Note that there is a flat and a wavy washer on the output shaft between the large gear and the bearing in the gearbox. Not the order in which they are on the shaft and remove them. 
Clean the grease off the large gear and inspect it. You should see one or more cracks from the hub all way out to the teeth and
two of the teeth separated by a crack. Now comes the fun part. You need to remove the gear from the output shaft. If it is like the two I've removed, it will either be hard or extremely hard to press off the shaft. You will want to use your pry bars as close to the hub of the gear as you can. 
It may take some cursing and pounding but it will eventually start to move and come off the shaft. In one case, I had the output
bearing in the cover come loose before I could get the gear off. I had to epoxy it back in. Thoroughly clean the large plastic gear now that you have it off. Use both the brake cleaner and alcohol to get all traces of grease off of it and out of the cracks. You may want to open the cracks a little with a flat bladed tool as you clean.  Rough up the surface of the plastic between the gears and the hub with a small Dremel sanding bit. You want to give the epoxy as much mechanical grip as you can. Clean the gear again. Mix up a small amount of the Hysol epoxy. You can get it from McMaster Carr in a couple of days. You will need a dispensing gun similar to what is in the photos in the first part of this DIY. This is an extremely strong industrial epoxy, that is tenacious in its adhesion properties. It also stays in place and won't run. It takes 24 hours to reach full strength, but the wait is worth it. It is much stronger that JB Weld and its ilk. Get as much of the epoxy deep in the cracks as you can. Open them up a little to help. Once you have as much in the cracks as you can, put on the hose clamp on the outside of the teeth and tighten it as much as you can. Now mix up a fairly large amount of the epoxy.  You are going to completely fill the area between the teeth and the hub of the gear with epoxy on both sides of the gear. If you don't mix up enough at first just mix more. You have plenty of working time before the epoxy starts to cure. Level the epoxy as best you can between the hub and the teeth. The clamp will keep most of the epoxy out of the teeth. The epoxy will provide two functions. One is adhesion in the cracks. The second, by filling area between the hub and the teeth, you are providing a mechanical lock holding the gear together once the epoxy hardens. Set the gear aside and let it cure for at least 24 hours. 
Now inspect the small plastic gear for cracks. The shaft it is on is easily removed by sliding the black plastic bearings out of
the gear box at either end. There will be washers and shims between the shaft and the black plastic bearings. Note their number
and orientation. Clean the gear. If the gear is not cracked, I'd just leave it alone. If you notice a crack, it will likely
soon fail, the gear will spin on the shaft and it is likely the worm gear will chew the plastic teeth up and ruin it. If that
happens, you are dead in the water and will need another mechanism. |
 If you see a crack, press the gear off the shaft. It will come off a lot easier than the large gear. Clean it thoroughly. Note that there is plastic hub on the gear that is smaller in diameter than the OD of the teeth. Cut a piece of the 1/2" brass tubing no longer than the width of that hub. I use a cut off wheel on my Dremel to do it. Now apply Hysol epoxy into the crack in the gear. Just a little, as it will squeeze out. Place the small hose clamp around the teeth and tighten it up. Clean out the excess epoxy out of the teeth and hub as best you can. While that is curing, test fit the brass ring you cut from the tubing on the OD of the plastic hub. It should be a tight press fit. Apply a small amount of the epoxy to the ID of the ring, and press it on to the hub completely; I used a small vise to do it. Clean up excess epoxy and set aside for 24 hours. This ring will add a lot of strength to the repair. Please note that the hose clamp goes on with the gear OFF the shaft, not on it, as shown in the picture. This was a shortcut that didn't work! 
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Back to the large gear. After 24 hours, remove the hose clamp and clean up any excess epoxy from the teeth. Use a small drum
sander on the Dremel tool and carefully enlarge the center hole of the gear until it can just be pressed on over the output shaft
by hand. You will note that once the gear gets to the splines on the shaft, the fit gets looser, which is normal. Once you have
the proper fit, put on the thin washer between the gear and the gearbox cover, then press on the gear. Note that the gear teeth
are not the same on each side of the gear. The side with the solid edge on the out circumference goes toward the gearbox cover.
Now take a close look at the following pics: 
Note the green arrow on the gearbox cover. That is the direction of rotation of the
gear with the output arm in the position shown when it goes to the other position. The gear only travels 180 degrees from fully
retracted to fully extended. "Clock" the gear on the shaft so that the teeth that had the crack will never come in contact with
the worm gear from fully retracted to fully extended. In the pictures, I've marked the damaged teeth with a green line, and
marked the approximate location of the worm gear with a green line on the gearbox cover. Note the green dot on the gear. That
is where you will drill a 1/8" hole through the gear, thru the output shaft, and through the gear hub on the opposite side. Clamp
the assembly on a drill press, and carefully line things up so that you are drilling at right angles to the gear, and the center
of the shaft. Use a sharp 1/8" drill bit, preferably a cobalt bit, and drill very slowly thru the gear and into the shaft. Back
the bit out often, and blow out the hole with compressed air. Fortunately, the output shaft is not hardened, so the steel is
relatively easy to drill through. Again, drill very slowly and clean the chips out of the hole often. Continue drill through the
shaft into the gear hub on the other side but not all the way out the other side. Unfortunately, I don't have a good picture of
the completed drilling but the picture shows a dotted blue line that marks where the hole will be drilled.
When you are done drilling, remove the gear from the output shaft, clean out any remaining chips and inspect the epoxy repair.
If all is in order, replace the gear on the shaft, again making sure the washer between the gear and the gearbox cover is in place.
Line up the holes you just drilled and carefully press in a 1/8" roll pin. If your roll pin is too long, cut it off with your
Dremel cut off wheel. The pin when fully inserted needs to go all the way through the shaft and be in full contact with the gear
hub on each side. Ensure that the end of the pin is below the bottom of the teeth in the gear. This concludes the repair of the
large gear.If you are not repairing the small gear, reassemble the gearbox to the mechanism, using some plastic compatible grease on the output gear. Be sure to install the flat washer and wavy washer on the output shaft between the gear and the gearbox bearing. Once reassembled, apply power to the motor and make sure the mechanism goes from full retract to full extended positions. 
If you are repairing the small gear, remove the clamp after 24 hours, clean any excess epoxy from the teeth and remove material
from the inside of the hub until the gear can just be pressed onto the shaft. Drill a 1/16" hole through the brass ring, gear hub
and shaft, all the way through to the other side. Install a 1/16" roll pin and cut off any excess with the
Dremel cut off wheel. Reinstall the gearshaft and bearings into the gearbox housing, being sure to install the washers and spacer
shims as you removed them. Grease gears and bearings with a plastic compatible grease and reassemble the gearbox to the assembly
as in the prior paragraph. 
One final word. If you cannot get the mechanism to go up or down before or after the repair while helping it, inspect the end of
the worm gear shaft with the motor running. The shaft should be turning. If it isn't, cut a groove in it with your Dremel cut
off wheel and turn it with a screwdriver. That should enable you to get the mechanism to move, but the small gear may be too
damaged to repair.Good luck! |