I repaced the smog pump on my daughter's '88 420SEL; I understand that not only is it possible to cause failure of an emissions test, but also that not repairing it may lead to a premature (and expensive) catalytic convertor replacement. See:

1988 560 SEL Air Pump Seized

http://www.peachparts.com/shopfor...?threadid=16303

http://www.mbca.org/discussion/thre...rch=smog%20pump