Central Locking symptoms - PeachParts Mercedes-Benz Forum

C Holmes · #1 01-28-2024, 04:50 PM

My 87 300D's central locking is driving me crazy, literally, when I got the car it worked but not well. It had a few leaks so I fixed all by changing the drvs door, trunk and fuel flap lock actuators. Worked perfectly so I added the keyless system and again it worked perfectly for 2 or so months. Then yesterday driving along it locks on it's own for no particular reason. I tried it a few times and always the same, although the time it would take varied. I put it in the shop thinking something frozen and tried it a few times and then it stayed unlocked. This morning it doesn't work at all so I pulled the seat and checked it out. There appears to be no 12v at the connector. Fuse is good, no problem there, no leaks at all, pump will run if I connect 12v to it but it will do the same thing, the keyless will work as it should but when I unlock it it now will immediately lock. I tried it several times, the drvs key will not lock or unlock (with the pump) and it always did. So I am baffled, it makes no sense unless it is the drvs door actuator stuck in the unlock position but that still doesn't explain no 12v at the pump. I vacuum tested, no leaks.. Does anybody have any idea what is going on with it? It seems so simple but apparently not so much.

Diesel911 · #2 01-29-2024, 12:59 PM

Sounds like you need a wiring diagram so you can trace the VP circuit.

jdulle · #3 01-30-2024, 06:46 PM

You said the fuse was good, but you might want to spin it back and forth and/or replace it. I have had numerous little problems like that that pop up and then are quickly solved by spinning the fuse around.

jdulle · #4 01-30-2024, 06:51 PM

Since it seems like you are getting power to the pump, I suppose it's not the fuse like I suggested above...
Another thing I thought of is if one of the front door locks is unable to unlock all the way when unlocking, it locks everything again. So if you are unlocking with the key from the driver's door(or I suppose the keyless fob), if the passenger lock plunger and mechanism can't raise all the way up from the power of the pump, that could be your issue. I would watch closely when you unlock and see which door doesn't unlock quickly. Then lubricate, or even remove door cards to fully clean/lubricate.

ah-kay · #5 01-31-2024, 10:31 AM

The lock system on the 300D is very simple. A pump, one vacuum line and that is it. The control is underneath the passenger seat. Control line to the pump to command it to pump or suck ( 12v or 0v ). I would go back to basic first.

1) remove all the keyless electronics to go back to stock. Keyless installed by professional or self?

2) try to lock and unlock the car using key at driver, passenger door and trunk. The car should be able to lock/unlock at these 3 locations. If not resolve it FIRST.

3) Put the keyless back in.

Good luck.