GM Says Don’t Fry the TCM
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.
Mike Mavrigian
Motor Age Editor
Mike Mavrigian has written thousands of automotive technical magazine articles involving a variety of specialties, from engine building to wheel alignment, and has authored more than a dozen books that crisscross the automotive spectrum. Mike operates Birchwood Automotive, an Ohio shop that builds custom engines and performs vintage vehicle restorations. The shop also features a professional photo studio to document projects and to create images for articles and books.