The rod and spring must be transfered from the old modulator to the new one. Unfortunately, I'll wager that your problem is lack of vacuum rather than bad modulator and once the new one is working properly it will work like the old one.

You might try looking at this article if you continue to have problems after the rod and spring are restored: http://www.continentalimports.com/ser_ic20242.html

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician