A couple months ago the central locking system stopped working on my '87 300D. I checked the vacuum pump under the seat and had 12v at the two wire plug. I also had 6 volts between common and either of the other two wires on the three wire plug at the vacuum pump after pressing the lock or unlock button on the remote key fob. I can hear a relay clicking under the dash on the drivers side when I press the lock or unlock buttons on the key fob. There is a Becker security system under the passenger floor mat that has been disconnected since I bought the car. I connected this and was rewarded with a beeping horn at a red light several miles into my trip.

Back to the vacuum pump. I've got voltage at the pump, and the switch seems to be sending a 6v signal at the three wire plug depending on which button was pressed. Every once in a while the vacuum pump would "chatter", but the pump would not function. I figured the vacuum pump must be bad. I purchased a used pump on Ebay that was garanteed to work and came with a 100 day warranty. It looks much newer and cleaner than the pump that was in the car. This morning I installed the "new" vacuum pump but the door locks still don't work. I no longer hear any noise at coming from the vacuum pump, but the relay under the dash is still clicking when I press the unlock button.

I've downloaded the wiring diagram, but I don't see as it's much help as I've got voltage at the vacuum pump.

What do I need to look at next? Oh yeah, I've checked all the fuses.

Thanks in advance,

Lukejt

Bump...

I've checked the wiring diagram, everything looks OK. The repair manual doesn't really tell me very much. Should I be looking for that relay under the dash? What is the correct signal that will turn the pump on? Is it possible I've gotten a bad pump?

Lukejt