84 (1993-1994 Vehicles) PCM - Internal Communication Failure

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The PCM/EC communicates to the PCM/TC by an internal serial peripheral interface (SPI) line. When SPI communications fail the information flag 84 is set and the PCM/EC will continue communicating with the PCM/TC by way of the serial communication interface (SCI) line with no performance impacts.

Information Flag Parameters

Information flag 84 will set if:

  • A certain number of incorrect or missing messages have been sent between the engine controller and the transaxle controller within the PCM.

Diagnostic Aids

If information flag 84 is active and you plug in the PDT or Scan tool a DTC 82 will set. This is due to the PDT or Scan tool also communicating over the SCI line which interrupts the PCM/EC to TC communication. This condition will cause the PCM/TC to go into back-up.

  • Clear all diagnostic trouble codes and information flags.
  • Test drive vehicle. If information flag 84 resets replace PCM.
  • If information flag 84 does not reset do not replace the PCM.

When a DTC is active it means the conditions for the DTC are currently being met, and if it is cleared, it will reset immediately.

An information flag 84 may set after a transaxle calibration download or after transaxle adaptives have been reset. This is a normal condition due to the interruption of communication between the PCM/EC and PCM/TC. Clear information flag 84.