Central locking problems in the R170 series of Mercedes Benz SLK are quite common. The issues are usually centred around the failure of the PSE (Pneumatic System Equipment) pump unit or its connecting air pipework. The pneumatic door lock actuators are simple by design and as a result are found reliable, even in older cars.
If your central locking is playing up, maybe no doors lock or unlock, perhaps one door does not function, there are a few simple checks to determine what and where the issue may lie. The first thing to try is when you lock the car from the remote key fob – does the vehicle respond and if the ‘alarm arms and disarms’ this is an indication that the key fob is sending the correct signals to the car. However if the car only either locks or unlocks from the fob, there is a chance the fob has a problem with one of the miniature tactile switches inside. These often fall off or break away from the internal PCB, these can usually be repaired (re-soldered or replaced) by carefully breaking open the sealed halves of the fob and resealing when done.
Actuating a fully functioning key fob will cause the PSE pump to run for a period, as will unlocking the drivers door, opening the drivers or passenger door, unlocking the boot or operating the dash ‘door lock’ switch. Listen out for the pump motor running in the area of the boot floor just below the rear of the fuel filler. To inspect the PSE pump unit, open the boot and remove the spare wheel, then remove the right hand plastic trim lower edge fasteners to enable the ‘foam box’ containing the PSE pump unit to be extracted sideways. There should be enough free-length in the wiring loom and yellow connecting pipes to allow its partial removal into the spare wheel well. If the foam box is ‘wet’ and you do not hear the pump run on lock actuation, then prepare yourself for a potential financial blow.(Circa £200 – £380 for a good used part) It is common for the PSE pump unit to be waterlogged from either water entering the boot from the inner arch area (corrosion) or through faulty boot lid seals and block channel drains (drains behind rear quarter glass gutter, under boot lid) – either way the water is mopped-up by the pump unit foam and damage results. Housed in the base of the PSE unit is a complex electronic PCB that controls its cycle and air distribution sequence. Even if not completely waterlogged a damp environment can do its worst, not only to the PCB but also to the small DC motor that runs the pump itself. It is worth checking the supply fuse, but often no response means replacement required. There are a few companies offering a service exchange service for PSE pumps but as yet I have not found one that does R170 types!
However if the motor ‘runs’ and seems to run at a healthy speed (buzz) you may be in luck. Mark the yellow hoses and their associated clip in connection points before removing them from the pump block. Activate the pump and see what degree of suction or pressure appears at the air manifold output spigot connections. Suck locks, blow unlocks. The three yellow pipes are connected to: The drivers door only, the passenger door and boot lock and finally the fuel filler flap and cubby box lid lock. You can manually test the function of the actuators on these components by sucking and blowing on the individual pipe ends. This takes a bit of ‘puff’ but it can be done. If when blowing on the respective pipes, you can blow constantly, without building any pressure in the pipe, there is most likely a leak. The pipes are made from a semi rigid wall Nylon and very tough, it is uncommon for them to split or cut. What is more common and is actually a design flaw in this model, is the grey right angle pipe end connectors that clip to the door lock actuators, break. This is due to the over-tight stretch of the air pipe across the door inner. Over time this work-hardens the connector (door slamming etc) and the connecting pipe breaks away, taking with it the plastic spigot on the grey push-on connector. Any air leak will affect the function of the locking. Often if you listen carefully at the latch section on the door edge while the pump is running, any hissing is a sure indication that the pipe connector has broken inside the door and will need attention.
To gain access to the door lock actuator and its yellow pipework you will have to remove the door card. Start by prizing out the SRS badge and removing the philips screw behind, then remove the chrome trim plate and plastic latch aperture cover from the door edge. Beneath the door handle locate the slot in the underside seam and insert a broad screwdriver, lever off the top half of the handle cover to reveal two more large philips screws. This will take some force to unclip the handle and you may think you are about to break it – don’t worry as long as your screwdriver is as broad as the slot will allow it will pop off – eventually!
Remove the top triangular trim piece behind the mirror area. Using a trim clip tool or spatula, unclip the trim at its lower edge and speaker section. You can see the position of the mushroom clips in the photograph of the rear of the door card.
With the bottom edge of the door free lift the handle of the door card upward with some force. Ensure that when the large handle clip disengages from the steel panel of the door, that you don’t stress the tweeter speaker wire, this could if yanked, break the fine wires to the speaker.
Once the door card is free, slip your hand behind and disconnect the tweeter connector (covered in grey foam) and then unclip the door release lever mechanism hook from its operating lever, you will have to pull back the black outer sleeve of the bowden cable and slide it from its retaining slot in the plastic block on the rear of the door card. Now you can put the door card safely out of the way.
Carefully peel back the plastic membrane from the lock area of the door to reveal the actuator and yellow pipe. Operate the lock from the dash or key and identify the leaks in the area where the pipe joins to the grey push on connector. Usually once you touch this area the pipe breaks off completely! The connector can be levered from the actuator with a screwdriver, it is a snap fit.
While you can no doubt get replacement grey push connectors for the air pipe, if you damage the pipe end in replacing it and have to cut it back even a few millimetres, it will be too short to reach the actuator. The pipe appears to be heated and sweated onto the connector and I felt it too much of a risk to try to replace the connector in this instance, so I repaired it in such a way as it would be stronger than the original part!
It is important to release the yellow pipe from the cable tie that also holds the loom section a little further across the door, this gives a few valuable millimetres of slack to take the stress off the pipe and its connector.
Now to repair the connector. Using a jewellers screwdriver or suitable size small drill, open out the plastic connector ‘break-point’ and pipe to take a short piece of nozzle tube cut from a WD-40 spray can. This tube can be scraped with a sharp knife to reduce its external diameter slightly to obtain a tight push fit inside the two broken pieces of the connector. A piece of about 8mm in length is perfect, using a light smear of bonding glue on assembly if you wished.
You will now have to mix some epoxy putty (Milliput) to add strength to the join area and complete the repair. If you shape the putty as I have done in the photographs, it will add support for the pipe and prevent the two pieces coming apart in the future, while additionally of course sealing the repaired joint. Once you have fashioned your repair, push the connector back home onto the lock actuator.
Test your locking and build up the door. I suppose if you were a purist you could try and replace the connector with a new part, but due to the restricted pipe length (the real problem in the first place) you could run the risk of damage. As the repair cannot be seen, in my view it is a simple and quick solution to the problem. This repair technique could be easily used on connectors utilised on the pneumatic headlamp levellers on vehicles of the same era, such as the R129 SL etc.
See here for more useful general background info on the Mercedes pneumatic central locking system.