Installing a PCM

By Brian A. Marks

Tools Required:
  • Sockets

Parts Required
  • PCM

The Upgrade

  1. Disconnect the negative battery terminal.
  2. Remove the driver side cross brace.
  3. Remove the airbox lid, filter, and ducting.
  4. Disconnect the existing PCM and note the blue and clear connectors and their positions.
  5. (For 1997 vehicles only: open the small cover on the PCM using a torx bit and remove the knock sensor module and install into the new PCM)
  6. Install new PCM including clear and blue connectors.
  7. Install PCM back into airbox (or wherever you had it placed) and replace the ducting, filter, and lid.
  8. Replace the cross brace.
  9. Reconnect the negative battery terminal.
  10. Start car and let idle for a few minutes to ensure smooth operation.
  11. IMPORTANT: If you are installing a reprogrammed PCM, make sure to take the car to the dealer to get the crank sensor position (or CASE) learned. This is not required for 1997 GP PCMs because the data is already stored in the knock sensor you switched above. You may get a "Service Engine Soon" lamp after the installation - this is due to the CASE learn not having been performed. Whether you get the SES light or not, you should still get the CASE learn procedure performed. Any GM dealer or official service shop can do the procedure if they have a Tech2; they may charge you a nominal fee for the service. In brief, here is a description from the GM Service Manual:
    The crankshaft position system variation learning feature is used to calculate reference period errors caused by slight tolerance variations in the crankshaft, the crankshaft balancer interupter rings, and the crankshaft sensor hall effect switches. The calculated error allows the PCM to accurately compensate for reference period variations. The crankshaft position system variation learning feature enhances the ability of the PCM to detect misfire events over a wide range of engine speed and load.

    The crankshaft position system variation values are stored in the PCM non-volatile memory after the crankshaft position system variationlearn procedure has been performed. DTC 1336 set indicates that the crankshaft position system variation values have not been stored in the PCM and the crankshaft position system variationlearn procedure must be performed. The learn procedure is required after the following service procedures have been performed, regardless of whether or not DTC P1336 is set:
    • PCM replacement
    • Engine replacement
    • Crankshaft replacement
    • Crankshaft balancer replacement
    • Crankshaft position sensor replacement
    • Any engine repair(s) which disturbs the crankshaft/harmonic balancer to crankshaft position sensor relationship.
    © General Motors Corporation
    Please note that this is not a defect or problem - it is simply to help the PCM know exactly what is going on at any particular time.