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Diagnostic Aids

A scan tool only indicates when ECM has turned on the 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.

To determine if gear switches are functioning properly, perform checks in all CHART C-8 charts. Switches will not prevent TCC from functioning but will affect TCC lock and unlock points. If 4th gear switch is always open, TCC may engage as soon as sufficient oil pressure is reached.

Fig 1: Chart C-8A, TCC Troubleshooting Chart 1 of 4 (1992 5.7L PFI Y Body)
G92E14872Courtesy of GENERAL MOTORS CORP.

A 2nd gear switch mounted inside transmission, is normally closed and opens when transmission shifts into 2nd gear. ECM uses gear switch signals to adjust ignition timing based on transmission shift points. Adjusting ignition timing after a transmission shift will allow for higher shift speeds.

NOTE: Test numbers refer to test numbers in diagnostic chart.
  1. Unless switch is stuck open or circuit No. 232 is open, scan tool should display NO, indicating transmission is not in 2nd gear. The 2nd gear switch should only be open while in 2nd gear.
  2. The ASR system must be disabled when performing this step. Whenever ignition key is cycled to off position and then cycled back on. ASR system will default on . Check operation of 2nd gear switch. When transmission shifts into 2nd gear, switch should open and scan tool should display YES.
  3. This step determines if ECM and wiring are okay. Grounding circuit No. 232 should cause scan tool to display YES, indicating transmission is in 2nd gear.
  4. Disconnecting TCC connector simulates an open switch to determine if circuit No. 232 is shorted to ground or problem is in transmission.


A faulty 2nd and 3rd gear switch circuit can cause an ECM code 72. If 2nd gear switch is changing states, check connections and wire routing.

Fig 1: Chart C-8A, TCC Troubleshooting Chart 2 of 4 (1992 5.7L PFI Y Body)
G92F14873Courtesy of GENERAL MOTORS CORP.

A 3rd gear switch mounted inside transmission, is normally closed and opens when transmission shifts into 3rd gear. ECM uses gear switch signals to adjust ignition timing based on transmission shift points. Adjusting ignition timing after a transmission shift will allow for higher shift speeds.

NOTE: Test numbers refer to test numbers in diagnostic chart.
  1. Unless switch is stuck open or circuit No. 108 is open, scan tool should display NO, indicating transmission is not in 3rd gear. The 3rd gear switch should only be open while in 2nd gear.
  2. The ASR system must be disabled when performing this step. Whenever ignition key is cycled to off position and then cycled back on, ASR system will default on. Check operation of 2nd gear switch. When transmission shifts into 3rd gear, switch should open and scan tool should display YES.
  3. This step determines if ECM and wiring are okay. Grounding circuit No. 108 should cause scan tool to display YES, indicating transmission is in 3rd gear.
  4. Disconnecting TCC connector simulates an open switch to determine if circuit No. 108 is shorted to ground or problem is in transmission.


A faulty 2nd and 3rd gear switch circuit can cause an ECM code 72. If 3rd gear switch is changing states, check connections and wire routing.

Fig 1: Chart C-8A, TCC Troubleshooting Chart 3 of 4 (1992 5.7L PFI Y Body)
G92G14874Courtesy of GENERAL MOTORS CORP.

A 4th gear switch mounted inside transmission, is normally closed and opens when transmission shifts into 4th gear. Switch is used by ECM to modify TCC lock and unlock points, when in a 4-3 downshift maneuver. Temperature switch is normally open. Transmission must be in 4th gear for temperature switch to apply TCC to cool transmission fluid. Temperature switch lock at 279°F (155°C) and unlocks at about 259°F (143°C).

NOTE: Test numbers refer to test numbers in diagnostic chart.
  1. Unless switch is stuck closed or circuit No. 446 is shorted to ground, 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. The ASR system must be disabled when performing this step. Whenever ignition key is cycled to off position and then cycled back on, ASR system will default on. Check 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 ensure it is not changing states under a steady throttle position. If switch is changing states, check connections and wire routing carefully. 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 4 of 4 (1992 5.7L PFI Y Body)
G92H14875Courtesy of GENERAL MOTORS CORP.
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