Central Locking Vacuum Pump Repair
By Jerry Cole, Technical Advisor
I don’t mind spending $600 on a major part for a Mercedes from time to time because it’s part of the cost of ownership, right? So I did. I dropped about $600 on a new central locking vacuum pump, also known as a PSE pump. This pump, in some form or another, is found in many models and many years of Mercedes. In addition to central locking, it also controls rear seat headrests, inflatable lumbar supports and trunk handle, if so equipped. Moving ahead four years, the pump failed again, so it’s time to peel off another stack of Ben Franklins and buy a new one, right? Not this time.
The failure is almost always the same. The brushes wear off in the small electric motor that runs the pump, causing the pump to stop. No running pump, no vacuum to the door locks and anything else it controls. Just like any other failed part, go ahead and open it up and have a look. After all, it’s already broken. I left all of the vacuum lines attached to mine and carefully rolled it over. The cover can be easily removed with two flat bladed screwdrivers.
Inside, you’ll find (amongst other things) a motor and a lot of fine black powder. That powder is what remains of your pump motor brushes. You will need to desolder the two wires from the pump after taking note of their orientation. Red to +12v and black to negative. My original motor wasn’t marked, so I scratched a “+” next to the red wire on the motor casing. Once the wires are removed, you can lift the motor out of the pump enclosure and disconnect the pump hose. You now have in your hand the failed component, the motor, with a small pump attached to it. The rest of the unit left in the car is most likely in perfect working order, as is the small pump attached to the motor.
All of this is well documented on the internet. What is not well documented is what type of motor to buy as a replacement. eBay sellers are all too willing to sell you one that fits for $100, but a quick search reveals that motors of that type and size sell for between $3 and $15, depending on where you buy it. Looking at the motor, it probably has a Bosch symbol and a part number on it. Guess what? It’s proprietary and Bosch won’t give out any details. It gets better. Once you separate the pump from the motor, you will see the name Johnson stamped on the motor casing. Johnson Electric had the motor made, not Bosch, not Mercedes. The original manufacturer of the motor appears to have been in China and Johnson won’t spill the beans on the motor details either. That’s OK, because there isn’t just one right answer. There are plenty of right answers when it comes to a replacement.
To get a good look at the motor, you’ll need to remove the pump from it. I drew a line with a Sharpie through all of the pump and mounting bracket parts so I wouldn’t have to guess at their orientation during reassembly. Remove the two screws that hold the pump to the motor, but try not to allow the pump sections to come apart. There is a series of pump vanes inside that is a pain to put back if they are allowed to fall out. You can look inside; just don’t spill the contents of the pump. It will be obvious what I mean, once you see it. Using a small flat bladed screwdriver, you can gently pry the pump off the motor shaft, a little at a time, taking care not to break the pump. Once removed, set the pump assembly in a safe place. Be careful to keep the sections together.
After a few cups of coffee and a fair amount of research, I ended up buying a Mabuchi RS-545SH motor, which cost less than $10 shipped to my door. You can search for the same part number I used, or select your own. The key things you are looking for are high torque and low RPM. The RS-545SH has a speed of about 6,000 RPM at 12v (depending on which chart you use), a shaft length of approximately 17mm with one flat side (D shaped) to engage the pump. The shaft diameter is 3.17mm (1/8”). The body dimensions should be 50-67mm in length and have two threaded mounting holes for the pump that are spaced 25mm apart, which is very common. The RS-545SH does not have a built in fan inside. A fan is a nice feature, but in this application, the motor only has to run for a few seconds at a time. If it runs until it times out each time, you probably have a leak in one of the systems served by the pump that needs sorted out. In addition to the Mabuchi, I also bought a Johnson 9167AK motor that would have worked fine as well, but I would have had to create the flat spot on the shaft for the pump. I’ll be keeping that motor on the shelf for a spare. Just know it’s another option.
Reinstallation is obviously the reverse of disassembly and you’ll be glad you made those orientation marks as you reassemble the pump. Once it’s back up and running, you can take the $590 out of $600 you didn’t spend replacing the whole pump unit and spend it on something else, like a new Mercedes mat set, a few evenings out on the town, several tanks full of gasoline, a bottle of Pinot Noir and still have enough left over to renew your MBCA membership and that’s why you DIY.