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2000 Mercedes-Benz R129 SL500 AMG SL1 + SL2
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Discussion Starter #1
Hi!

I recently bought an R129 SL500 from 2000. Soon I discovered that the central lock buttons of the remote control couldn't lock the trunk. I could open the trunk manually with the rest of the car locked, and the alarm went off. So yesterday I decided to investigate this further, and maybe it's a common fault, I don't know. However, I'd like to share since there might be other people out there with similar issues of either trunk or any other lock.

So, the central lock pump is as we know situated in the right of the trunk near the battery. It appears to be working well, but it kept running for 10-20 seconds extra when it couldn't lock the trunk. I figured something must be wrong with the actuator at the lock itself then. There are three hoses running from the pump in the back plate. One of them just continues on the left side to the front, and appears to have nothing to do with the trunk. It's probably for the left door.

Now, there are two hoses left. One is white and operates the pump to the left that operates the trunk door elevator. The other is yellow and goes directly to the actuator. To visualize all the hoses you need to remove the carpet and round plastic cover at the back plate. I checked for leaks whilst operating the remote control, unlock-lock-unlock-lock and so on. No leaks in the hoses.

It appeared that the actuator was unable to lock the trunk, but it managed to unlock it when I had helped the plastic lever manually over to the locked position. If I listened carefully, there was an air leak inside the actuator. So I dismantled it. Inside there is a rubber membrane which functions as a piston in a cylinder, driven by pneumatics. Close inspection by naked eye revealed a tiny tear in the periphery of the membrane. I put some sticky glue on both sides of membrane, let it dry and reassembled everything back in place. No leak! Problem solved, central locking system now works as it should!

Hope this is helpful to someone!

Best regards,
Robin
 

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94 SL600, 89 Jag XJS conv, 85 Daimler Princess, 97 993 63 Avanti 16 Lincoln MKZ hybrid 16 Silverado
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720 Posts
Thank you, well written DIY for a common problem with older vehicles.;) When you mentioned sticky glue, did you just "weld" together the small hole sides?
I hope your 2K SL slowly lets you use your mechanical and analytical skills!
 

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2000 Mercedes-Benz R129 SL500 AMG SL1 + SL2
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29 Posts
Discussion Starter #3
UPDATE:

So it appeared today that the problem had re-occurred. Once again I opened it all up, and what happened is probably that when the glue had stiffened, the repair broke when the rubber membrane moved back and forth.

I went to a random repair shop and bought a rubber repair solution for 29,90 Swedish crowns (around 4 USD). Application was easy, the rubber welded together nicely.

Hopefully it will work and stay put this time.

/Robin
 

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'85 380SL, '99 SL500, '18 C300, '78 911SC, '91 Alfa Spider
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43 Posts
Sorry to revive such an old thread, but just want to thank you so much Robin for helping me pinpoint such an annoying issue (with such an easy fix).

I'm currently waiting for the rubber epoxy to cure so that I can test that the fix worked, but it did turn out to be a tear in the rubber plunger inside the vacuum actuator.

I've taken some pictures of what this looks like disassembled so that if anybody else encounters this, they have a clearer idea of what's going on.

The actuator itself is very easy to remove (don't start pulling out the entire locking mechanism like I did...) just with a plastic clip pulled back and then sliding the assembly downward. Disconnect the vacuum line and the power cable and you're set.

Once you have the actuator removed, there are four clips stopping the lid from coming off that need to be pulled back (one is hidden under the lip) and voila.
 

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