I am trying to determine if the vacuum pump in my 84' 500SEL is defective or not. The pump only works when I disconnect then connect the electric plug on the pump itself. Then it will work for a short period (a few hours) when I use my door key, after that it stops working again until I disconnect the plug at the vacuum pump again.

Tommy

It could be the door switch itself or the connection there. The vac pump has to run long enough to 'switch over' (complete an open or close cycle and turn off) or it will sit there and play dead until its disconnected.

Try getting it working and run several cycles of lock/unlock using the key in the door and see what happens.

If these is a leak in the vacuum plumbing, the pump will not build up enough pressure to switch over to the other cycle. It will run until the relay timer shuts it off and then just sit there.

Thank you for your reply. I have tried many times with the key locking and unlocking the door, and (when the pump is working) it worked everytime. I am fairing sure that the door switch is okay. There doesn't seem to be any leak in the system as all the locks operated very quickly. I think, for some reasons, the pump is working intermitently. Anyway, I found a good deal on a new pump so I am crossing my fingers that it will solve the problem.

Thanks.