Diagnostic Aids

A scan tool only indicates when ECM has turned on TCC driver and does not confirm TCC has engaged. To determine if TCC is functioning properly, test drive vehicle. Engine RPM should decrease when scan tool indicates TCC driver has turned on. An engine coolant thermostat stuck open or opens at too low a temperature may result in an inoperative TCC.

Fig 1: Chart C-8A, TCC Troubleshooting Chart 1 Of 2 (3.1L PFI W Body)

Courtesy of GENERAL MOTORS CORP.

The 2nd gear switch will be open in 2nd and 3rd gear. ECM uses this signal to disengage TCC when going into a downshift. The 3rd gear switch will open in 3rd gear.

1. Different makes of scan tools will read differently. Be familiar with type of tool being used. All switches should be in closed state during this test and tool should read the same for 2nd and 3rd gear switches.
2. This test determines whether switch of signal circuit is open. circuit can be checked for an open by measuring voltage with a DVOM at TCC connector. Voltage should be about 12 volts.
3. Because switches should be grounded in this step, disconnecting TCC connector should cause displayed switch states to change.
4. Switch state should change when vehicle shifts into 2nd gear.

If vehicle is road tested because of a TCC related problem, ensure switch states do not change while in 3rd gear. TCC will engage. If switches change state, carefully check wire routing and connections. If ECM, TCC solenoid and control circuitry are functioning correctly and TCC still does not apply properly, problem may be hydraulic or mechanical.

Fig 1: Chart C-8A, TCC Troubleshooting Chart 2 Of 2 (3.1L PFI W Body)

Courtesy of GENERAL MOTORS CORP.