CLK W208 boot not locking

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Stray Cat

New Member
Oct 19, 2015
CLK 320 W208
Hi Guys

My first posting so I hope its in the right place!

I've recently acquired a lovely 1998 CLK 320 but it was only after a couple of weeks I realised that after centrally locking the car I could still open the boot by the boot push button or the remote fob, which obviously triggered the alarm.
I've just had the boot lid lining off and the lock half out but nothing seems out of place, especially two air pipe connections, photo hopefully attached.
boot lock.jpg
I also find the emergency key only goes in about 12mm but it is in the vertical position.

Any help most appreciated. :wallbash:
First, regularly squirt some penetrating oil into the lock, keep the boot lid open to let the oil drip down into the lock.


Try unplugging any electrical connections on the vacuum pump, for 20 minutes, check them for wet/corrosion. Have the ignition off when you are doing this.

DO NOT SLAM THE BOOT SHUT WHILE THE VACUUM PUMP IS DISCONECTED from the electrical supply or you wont be able to open the boot. Drape a thick towel where the lock latch is to prevent accidental locking while the vacuum pump is disconected.

Thanks Dec,
I've read about oiling the lock but that was in a 'locked shut' situation. As the key is only going in 12mm I wondered if someone had changed the boot lock and not provided the new key....surely not.
I assume one of those two airlines actually locks the boot mechanism, i.e. prevent it from moving, while the other unlatches it? I'll try to trace the air lines back to the pump...which I believe is filler cap side?

It’s very important to have a working boot & door lock in the event that the vacuum pump or the remote fob or the car battery fails completely. Keep at the oil and work the key blade up/down/in/out in the key hole, it’s just seized up though lack of use.
Unlikely the boot lock was changed and no new key provided, test your key blade in the glove box and door lock so as to ascertain exactly how fare it is supposed to enter a lock that is not seized.

That boot is locked mechanically when it is slammed shut and doesn’t need any air to lock it.
When you push the boot lock key hole inwards, with your finger, a switch within the lock causes the vacuum pump to run for 2 seconds this blows air into the actuator on the lock and “trips” the lock open and the boot lid should rise up powered by its springs.
This switch, within the lock, could be faulty and causing the vacuum pump to run, even though its not being activated or there could be a fault within the vacuum pump.

Do the door lock pins rise, instantly, when you unlock and fall when you unlock?
When locking & unlocking, ideally, the pump should not run for more than 2 seconds, is that the case?
See similar fault here…


Thanks again Dec.
I think I'll start with the WD40 to free up the lock. It must surely have something to do with this.
Yes all buttons react immediately when locking/unlocking and the rear headrests collapse ok too and I read somewhere that the headrests were on the same air feed as the boot lock.
The head rests may be fed by the vacuum pump alright but the would not have anything to do with the boot lock.

Do you have the metal lever at the Red arrow in picture below on your lock?
You might have to take off the 2 bolts on the lock to see properly.


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From experience, unless you are using them regularly, leave one of the back seats unclipped until the lock is sorted. If the boot does eventually seize closed, it is a real pain trying to get in through the first aid kit housing.
Hi Dec
I was a bit wary of pulling the lock all the way out of the boot mount as there seems to be a plastic connecting rod going to each of the two secondary latches. Can these be easily disconnected AND reconnected?
The lock seems to be working in every other way...the latch 'arms' itself when you open the boot by button or fob. And it mechanically springs shut when a bar strikes the latch. Then the latch springs open again when you press the button or fob. It just doesn't lock itself with the central locking. There must be an air-triggered mechanism within the lock to prevent the button or the fob from releasing the latch until the central locking releases it.
If I can remove the lock from the boot I would like to test if air is coming through the two air pipes connected to the lock. I reckon one of those lines is the latch release and one the locking mechanism.
My boot lock, 1997, has the metal lever at the Red arrow… in picture above… and only one air tube, yours seem to be different , you probably don’t have the metal lever as you have 2 tubs therefore as you say, one blows to unlock/releasethe lock and the other tube sucks, to lock.
It does sound like either the tube or the actuator is leaking OR the vacuum pump is failing to suck air when locking.

If you can remove the lock from the boot lid and remove both tubes you can stick the White actuator in your mouth and either blow or suck to operate a lever connected within the lock, this will help to establish if the actuator is leaking air or not.

Not sure what you mean by the plastic connection rods going to each of the two secondary latches… got a picture?

Below link might be of some assistance, it’s a 1994 bur similar I think to yours.
W202 Boot Lock Fix. - Mercedes-Benz Owners' Forums

W208 Boot Lock

Here's some photos of my set up Dec...

They were too large for upload so this link should get to them.

Pics 1 and 2 show the left side connecting rod. (right side rod ovscured)

Pics 3 and 4 show the secondary locks on the lid and corresponding receptacles on the body.

Pics 6 and 7 showing the 'plunger' in and out after me pushing the barrel in.
Thanks for the pictures, unfortunately I’m not familiar with that set up so not going to be of any help.

This thread here… clk 230 (w208) central locking boot fault - Mercedes-Benz Owners' Forums … may help to explain how the plunger operate, (driven by air being blown into the actuator) your set up is a lot different with the presence of the connection rods so I think you should remove the Grey boot lid liner to get a better view of the mechanism behind it.

Any luck with the seized boot lock, freeing this up might yield some improvement, if only to enable the locking of the boot with the key blade.

Thanks Dec

Still soaking with WD40 but not yet free!

Yes removing the boot lining gives far better access. The connecting rods must just clip onto / into the main body of the lock as they're within the boot panel and must have been fitted that way.
I'll have to spend some time on it with some decent lighting. You can't get your fingers through the boot frame to get enough purchase on the air lines to free them or attach them so the lock has got to come out.

Stay tuned!

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