Diagnostic Aids

A scan tool only indicates when ECM has turned on the TCC driver and does not confirm TCC has engage. To determine if TCC is functioning properly, test drive vehicle. Engine RPM should decrease when Scan tool indicates TCC driver has turned on.

To determine if 4th gear switch is functioning properly, perform checks is CHART C-8 (2 OF 2). Switches will not prevent TCC from functioning but will affect TCC lock and unlock points.

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

Courtesy of GENERAL MOTORS CORP.

A 4th gear switch mounted inside transmission closes when transmission shifts into 4th gear and this switch is used by ECM to modify TCC lock and unlock points when in a 4-3 downshift maneuver.

1. Unless switch of circuit No. 446 is closed, scan tool should display NO, indicating transmission is not in 4th gear. The 4th gear switch should only be closed while in 4th gear.
2. This test checks operation of 4th gear switch. When transmission shifts into 4th gear, switch should close and scan tool should display YES.
3. Disconnecting TCC connector simulates an open switch to determine if circuit No. 446 is shorted to ground or problem is in transmission.
4. This step determines if ECM and wiring are okay. Grounding circuit No. 446 should cause scan tool to display YES indicating transmission is in 4th gear.

A road test may be necessary to verify customer complaint. If scan tool indicates TCC is turning on and off erratically, check state of 4th gear switch to be sure it is not changing states under a steady throttle position. If switch is changing states, check connections and wire routing carefully. Also, if 4th gear switch is always closed, TCC may engage as soon as sufficient oil pressure is reached.

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

Courtesy of GENERAL MOTORS CORP.