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84 Benz doorlocks

I have an 84 Mercedes 300sd. The power door locks are activated by the key or by the driver lock when the door is in the closed position. The lock activates at it’s normal speed when locking but when unlocking,the action can take up to 20 seconds. Can this be lubed or is there some cure for this partial failure?

The slow ‘unlocker’ is only in the drivers door?

There may be some dryness causing a slowdown.

I’m not sure if power door lock actuators can fail slowly or simply quit when worn out.

This is just a WAG, but there may be a dirty/loose electrical connection to the unlock side of the actuator.

Your car has a vacuum locking system with two separate circuits (one to lock and one to unlock). There is a vacuum powered actuator in each door, the fuel filler cap, and the trunk lock. The system is powered by a vacuum pump located in the trunk and controlled by the position of the driver’s door lock. It is likely that you have a vacuum leak someplace in the “unlock” circuit. Normally one of the actuators is the source of the leak. The various actuators look like this:

Sonofagun, learn something new every day (still). Thanks Craig.

Lots of differences between European and N/A vehicles.

It’s a pretty strange system for any car, troubleshooting vacuum leaks can be interesting. My door lock system has a slight leak at the moment (it loses vacuum after about 12 hours), I need to spend some time tracking it down too.

Not interesting,but systematic. Just get a copy of the vacuum line diagram and a Mity-Vac and work your way back from the vacuum pump. The vacuum tubing gets old and brittle after 25 years, along with a whole host of “T”,“L” and “F” fittings. Parts are cheap and the labor isn’t extensive, a good DIY job.
There are tubular rubber boots at the front of all four door that hold all the wires and lines going into the door. The flexing of 25 years of opening the doors works a hardship on vacuum lines at this point.


Thank you Craig I start my search to find the culprit Charles

I agree that a Mity-Vac is essential unless you are very, very patient. The good news is that the failed part is likely to be very cheap (once you find it).

It’s not THAT strange, Ford used vacuum lock actuators for at least the Thunderbird, up in to the '70s I believe.

I didn’t know that.

My old VW Jetta GLI 16V had this lame system of vacuum for “power” locks with occasional problems including fuel door. I don’t understand why an electrical solenoid actuator/door like the rest of civilized auto world was used. I never had a problem with electric locks in vehicles from the 150k-225k range when sold.