W140 Coupe Locking Pump Near Death - PeachParts Mercedes-Benz Forum

ctaylor738 · #1 03-12-2007, 10:44 PM

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,

syljua · #2 03-13-2007, 05:33 AM

Quote:

Originally Posted by ctaylor738

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.

Have you found out which nozzel/s the cycling pressure/vacuum goes thru? To see better, you should be able to operate the PSE without
the cover on.

br,
syljua

ctaylor738 · #3 03-13-2007, 08:29 AM

... but let's go further.

So I run it with the cover off and will probably observe that the plungers on one valve are moving up and down and causing the pump to cycle.

Then what do I do with the information? Disconnect? Replace? Adjust?

Thanks,

wbrian63 · #4 03-13-2007, 08:29 AM

After being disconnected, the constant running is the pump attempting to top off the two reservoirs in the system. There's one for vacuum reserve (in a sedan, it's inside the right-front fender area just in front of the front door). The other is a pressure tank for the lumbar supports (again in a sedan, it's in the trunk atop the right rear wheel well). Since you have orthopedic backrests, that's a huge demand for air pressure. Over time (especially when sitting for days like you describe), the tanks leak down (or fill up for the vacuum side), and the pump has a full job of topping them off when you restart the car.

The constant switching between pressure and vacuum is the pump's attempt to "top-off" these reservoirs. The programming for the system states that those circuits are low priority, that's why locking/unlocking and putting the car in reverse interrupts the cycling.

I'd cap off the clear line at the pump that runs forward - that's the line for the manifold vacuum assist. See if the pump quits the constant vacuum-cycle. There's a manifold under the passenger carpet, behind a cover plate at the base of the firewall where it meets the floor board. Over time, these manifolds can leak and cause a huge demand on the pump. The part is relatively cheap (under $100) and cured my system of constant cycling on the vacuum side. I also had ruptured lumbar bladders, but I could stop the constant pressure cycling by turning the dials on the seats to 0.

The codes you pulled are there because the pump has (as you say) gone TU. It's taking too long to reach designed pressure/vacuum levels on each of the systems. It also sounds like you've got a blown diaphragm in the door lock actuator for the driver's door.

The circuits in the PSE system fall into 3 categories -
vacuum only (Manifold Vacuum Assist, Rear Headrest release, headlamp level on euro models)
pressure only (lumbar / orthopedic backrest)
vacuum/pressure (locks, backup assist, trunk handle)

I'm pretty sure the design of the system involves pressure/vacuum thresholds which must be attained in a specified time period to prevent codes from being thrown. When you unlock the doors, the system sends air pressure through the lock lines to actuate the (in your case) 2 doors, the trunk lock and the fuel door. As each of the actuators move from locked to unlocked and reach full travel, the pressure builds in the system. If it reaches the programmed cutoff point, the pump cuts off and all is well. If it doesn't reach the programmed cutoff point within a specific timeframe, then a code is thrown and the pump cuts off.

Any of the systems that has a leak will place a huge load on the pump and will cause premature failure of that pricey gadget. I'd check all your circuits for leaks and repair what's required in addition to getting a new or rebuilt (if available) PSE unit.

Circuits that are vacuum/pressure are more likely to leak on the pressure side than the vacuum. On my car, the door locks held vacuum just fine, but when I applied pressure, they leaked.

NOTE that I've constructed a special regulated air supply to test the pressure sides of the system - using your home air compressor risks popping all of those tiny little actuators like balloons. The device allows me to inject air at very low pressures (1-6psi). The system consists of a low-pressure regulator, a gauge, a valve and another gauge. I set the inlet pressure down to about 2psi, attach the device to be tested and open the valve (it's electrical, so I just press a button). If the system has a bad enough leak, you'll hear the hiss. If the leak is slow, when I release the switch, the valve closes and the 2nd gauge allows me to see how the circuit is holding the pressure. Cost about $100 to build, but has saved me many times that in shop fees alone.

The testing spec for the lock system is as follows:
System leakage allowance - at 600mbar pressure or 300mbar vacuum - no more than 30mbar/minute.
Lines and actuators - at 600mbar pressure or 300mbar vacuum - no more than 25mbar/minute.
1 bar is approx 14.5psi, so 600mbar = 8.7psi. For vacuum, 14.24mbar = 1" mercury, so 300mbar vacuum is 21.06 in mercury.

Hope this long post helps

ctaylor738 · #5 03-13-2007, 11:43 AM

Thanks for the long explanation. That was very helpful in filling in the gaps.

I plugged both P- (vacuum only) and P+ (pressure only) and still have the cycling problem, which indicates to me that there is a problem with the control unit's internals. Maybe there is an adjustment in the little round "paddles" that close the contacts that will help. My next step, as syljua suggested is to watch it run with the cover off.

But what I also did was to disconnect the lead to Pin 4 on the wiring harness. This is the power from Fuse 11 in the trunk (hot with key on), which seems to enable the P- and P+ trimming. Now I have locking, trunk open, and reverse assist and the pump seems to function normally (runs briefly when each function is exercised). So this resolves the major headache of not being able to lock the car from the driver's door and will allow the car to be driven while I sort out the trimming problem.

What I suspect is a ruptured diaphragm as was discussed in the close assist article on Pete's website:

http://v12uberalles.com/Closing_Assist_Pump.htm?

I will replace the element in the driver's door ASAP, but it still locks/unlocks OK with the key and the pump does not seem to mind.

Stay tuned.

JimF · #6 03-13-2007, 11:51 AM

My pages describe the 'leaking' Closing Assist Actuator in MENU#28. Pete's describe the pump-fix and rebuild.