GM Says Don’t Fry the TCM

This bulletin applies to a host of GM vehicles from 2007 to present (just one example is 2014-2019 Corvette). After hooking up the Active Fuel Injector Tester (AFIT) to the vehicle’s ECM connectors, the engine may begin to turn over, then exhibit a no-crank condition afterwards. 

On vehicles equipped with an automatic transmission, it may also be found that there is no communication with the transmission control module. This issue can occur when the AFIT SIDI testing cables (cables for testing direct-injected fuel injectors) are plugged directly into the AFIT MCU (main control unit) with the MCU powered up. 

On vehicles that are equipped with an automatic transmission, this can cause internal damage to the transmission control module, preventing the engine from cranking and the TCM from communicating.

The fix: Replace the TCM to remedy the no-crank/no-start condition (on auto trans applications). The AFIT SIDI testing cables were intended to plug into the DMU (drive & measurement unit), not to the MCU. 

Note: GM dealers were sent an update kit to prevent the MCU’s power supply from being connected if SIDI testing cables were connected directly to the MCU.

Example of a testing cable with guard installed (right).

With a guard installed, it will cover the power plug port on the MCU if the SIDI cable is mistakenly plugged into the MCU.

About the Author

Information courtesy of Mitchell 1

Information for Technical Service Bulletins comes from ProDemand, Mitchell 1's auto repair information software for domestic and import vehicles. Headquartered in San Diego, Mitchell 1 has provided quality repair information solutions to the automotive industry since 1918.

About the Author

Mike Mavrigian | Editor

Mike received a BA degree from Youngstown State University in English Literature with a minor in Journalism in 1975.