The signal is generated at the front balancer and converted to DC pulse signal by a transducer mounted into the diagnostic socket. The signal is then used by the tach. If the tach is bad it is usually the capacitors.

With gas cars a bad tach is usually the tach, with your model diesel it is usually the transducer. Unscrew it and move it up an down in the contacts. It is such a low voltage signal that sometimes the contact is poor.

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