2-35. TRANSMISSION SYSTEM TESTS (4L80-E)
The 4L80-E Transmission system is equipped with a computer called the Transmission
Control Module (TCM) (located left rear seat compartment; for M996A2 and M997A2
vehicles, located in the left exterior stowage compartment) which interprets, analyzes and
records electronic signals form sensors and switches located on the engine, brake pedal, and
transmission. The recorded codes stored in the TCM are known as TROUBLE CODES which
are retrieved using the Diagnostic Switch Cable on the J2 connector of the TCM. The TCM
can protect the transmission from damage by locking it in Second Gear, with maximum fluid
pressure, until the problem has been corrected. The following procedures will detail
diagnostic testing, troubleshooting and corrective action for any existing faults. These
Transmission System tests may be run any time you think there is a transmission problem or
if you were sent here by another test chain. This paragraph has a different kind of flow chart
to guide you through testing because of the may problems that can occur. The most common
problems are loose or corroded wiring connections. To troubleshoot the transmission, you
will need a diagnostic switch cable, digital multimeter and a throttle position (TP) sensor test
Do not disconnect battery without recording
Trouble Codes. Failure to do so will result in
loss of diagnostic test codes.
The diagnostic test codes (DTC) are transmitted from the TCM to transmission lamp located
next to the shift lever. When Diagnostic Switch Cable is connected to the J2 connector, the
system is placed in the diagnostic mode, which causes the transmission lamp to flash. The
transmission lamp will flash once, pause, flash twice (meaning code 12) pause, flash once,
pause, flash twice (code 12 again), and do this one more time for a total of three times, which
means the system is operational.
Diagnostic test code 63 will be set at all times.
For example if the TCM is sending the trouble code 74, the lamp will flash 12 three times,
flash 63 three times, which is set all the time and is to be disregarded, flash code 7 and 4,7
and 4, 7 and 4 for code 74, and then 12, 12, 12, which ends the diagnostic check. These
codes will repeat again, if not taken out of diagnostic mode.
The TCM J1 connector will be used to diagnose and troubleshoot wiring, sensor connectors,
pins, solenoids, and brake switch, to insure all external problems are checked and corrected
prior to TCM and transmission faults.
On the following pages you will find diagrams and charts of the major portions of the
transmission circuits. These are designed to help you isolate a problem and correct it.