That option(?) being that when you shift into reverse...the trunk lid unlatches!! I have a '94 S500 and she is a GREAT car, but this problem has bugged me for months and I was hoping someone here may have a solution. It doesn't do it all the time, maybe 6 out of 10 times, and it doesn't pop open, but rather just unlatches, but not totally unlocks. Is there a way to just disconnect the vacum line from the trunk lock without affecting those 'back-up bars' that popup when you shift into reverse? Better still, is there a way to fix it without ending up in the 'poor house?' Any suggestions would sure be appreicated....THANKS!!

Sounds to me like there's a problem at the PSE pump that's mounted under the back seat.

The seat bottom comes out real easy, two red levers at the outside bottom corners - push (or pull) and lift the front edge of the seat cushion.

As I understand the process, when you push the button to disarm the alarm and unlock the doors, the pump runs and sends pressure to the actuators in the 4 doors, and the gas flap, unlocking each. The doors are part of one circuit, the gas flap is on its own circuit. Pressing the remote button shortly thereafter (like within 1/2 second), causes a signal to open an electric solenoid valve that isolates the trunk release from the gas cap circuit, and the air pressure flows to another actuator that releases the trunk lock.

I'm pretty sure the plumbing for the trunk release "tees" into the line that actuates the gas flap - I don't think there's a separate line running back to the pump, but I could be wrong. (On a '92 300SE, the diagrams show the line teed into the gas cap circuit). If there's a separate line to the release, then the pump is the problem (actually the manifolds inside the pump are the problem).

When you put the car in reverse, the only pressure that should be generated by the pump should be in the circuit for the reversing guides.

If I'm wrong about the "tee" mentioned above, and there is a separate line for the trunk release, possibly something's wrong with the pump manifold and it's sending pressure there at the wrong time.

To test this theory, you'll need an assistant. Start the car and lock the doors. Have the assistant check to see if the gas flap is locked. It should be. Put the car in reverse. If the trunk unlocks, the gas flap should also be unlocked (but the doors should stay locked).

You can disable this feature by removing the wires from the valve that controls the flow to the remote trunk release actuator. The valve should be in the trunk - my diagrams show it to be near the passenger side lamp assembly. You'll have to remove the trunk lining on that side to get at the equipment.

If it still unlocks, that means the valve is stuck open - replacing the valve will get rid of your problem - but that still doesn't explain why pressure is in that circuit when you put the car into reverse.

Pressure in the wrong circuit at the wrong time is usually indicative of a screwed up PSE pump.

Hope this helps