My '96 S500 Coupe sat for 4 days in cold weather in an airport garage in January. When I returned, central locking functions were TU. I pulled the following codes, all PSE:

- 002 Central locking system air demand too high
- 004 Backup assist air demand too high
- 006 Intake manif assist retr head restraints vacuum too low
- 003 Retractable trunk lid grip air demand too high
- 005 Orthopedic backrest air demand too high

I confirmed that the pump was DOA by unplugging it, re-connecting it and trying to lock the doors from inside. All that was heard from the pump was a soft click. I also applied a Mityvac and verified that the reverse assist and door locks function, with the exception of driver door unlock.

I then removed the pump disconnected the pump motor and appiled 12V directly to it. Nothing. I then scrounged a working pump, soldered it in and reassembled and reconnected the pump unit.

1. Immediately, the pump ran continuously. I disconnected and plugged the lumbar (gray line) port. The continuous running stopped.

2. The door locks function (inlcuding trunk and gass door) except for driver unlock from inside and outside

3. The reverse assist functions normally.

4. When the ignition is turned on, the pump runs continuously, seeming to cycle between pressure and vacuum approximately 50 times, then stays quiet with an occasional burst of cycling. I conclude is that this is what fried the original pump motor.

5. While it is cycling, the reverse assist and locks work OK. It interrupts the cycling, does its thing, and then goes back to cycling.

So I (probably) have either a fault in the circuit board or valving in the pump unit, or bad input from a micro switch or a control unit in the system somewhere.

Has anyone experienced this particular failure and tracked it down? Failing that, any suggestions? I would hate to buy a new pump unit, only to find out it's an input to the unit causing the problem.

Thanks,

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe