S500 closing assist

My '94 S500 has a new gremlin. The closing assist system will randomly stop working on different doors or the trunk at different times. For example, lets say that today the driver door will not close, but if I remove and reinsert the fuse to the system it will again work perfectly. However, tomorrow the trunk or a rear door may not close, but again it can be fixed with the fuse trick. Note that the fuse is not blown. The local MB dealer said that the pump for the system needed replacement at roughly $900, but I don't see how a bad pump would fail to close only one of the five mechanisms at a time. Help...
Thanks

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J.B.

The dealer is unfortunately correct in their diagnosis. It is possible for the pump to fail as you described because it is electronically controlled. I am thinking of offering a rebuilt unit if I can get the time to invest in the project. As for now, you will have to purchase a new one.