There is a Technical Service Bulletin to address a similar issue. NO: 21-04-95 SUBJECT: Vibration or Perceived Engine Miss DATE: Apr. 14, 1995 NOTE: THIS BULLETIN APPLIES TO VEHICLES EQUIPPED WITH THE CUMMINS DIESEL ENGINE AND AUTOMATIC TRANSMISSION.
SYMPTOM/CONDITION: A vibration or perceived engine miss at approximately 1100 RPM as the torque converter clutch engages. This condition occurs in 4th gear at speeds between 42 to 48 MPH.
DIAGNOSIS: After duplicating the above condition, verify the part number on the Powertrain Control Module (PCM). For proper repair procedure, refer to the guidelines given below. If the vehicle is a 1994 MY equipped with PCM P/N 56027303 or 56027304, replace the PCM with P/N 4798745. Proceed to the PCM replacement section of this TSB. If the vehicle is a 1995 MY, verify that it is equipped with PCM P/N 56028335 or 56028336, and proceed to the PCM reprogramming section of this TSB. If the vehicle is equipped with a PCM other than 56028335, 56028336, 04798745, or 04856080, replace the PCM with P/N 4798745 and proceed to the PCM replacement section of this TSB.
NOTE: IF THE VEHICLE IS EQUIPPED WITH PCM P/N 4798745 OR 4856080, THE SOFTWARE UPDATE NEEDED TO ADDRESS THIS TSB ISSUE HAS ALREADY BEEN COMPLETED AND NO FURTHER ACTION IS REQUIRED.