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Diagnostics Aids

Scan tool only indicates when ECM has turned on TCC driver but does not confirm TCC has engaged. To determine if TCC is functioning properly, engine RPM should decrease when scan tool indicates TCC driver has turned on. To determine if gear switches are functioning properly, perform checks in CHART C-8A (2 OF 2). Switches will not prevent TCC from functioning, but will effect TCC lock and unlock points. If 4th gear switch circuit is always open, TCC may engage as soon as sufficient oil pressure is reached.

Fig 1: Chart C8-A, TCC Troubleshooting Chart 1 of 2 (1991 - 5.7L PFI Y Body)
G92I14868Courtesy of GENERAL MOTORS CORP.

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

NOTE: Test numbers refer to test numbers on diagnostic chart.
  1. Unless switch or circuit No. 446 is open, "Scan" tester should display "NO" or "OFF", indicating transmission is not in 4th gear. The 4th gear switch should only be open while in 4th gear.
  2. This test determines if the ECM and wiring are okay. Grounding circuit No. 446 should cause "Scan" tester to display "NO" or "OFF", indicating the transmission is not in 4th gear.
  3. Checks operation of the 4th gear switch. When the transmission shifts into 4th gear the switch should open and "Scan" tester should display "YES" or "ON".
  4. Disconnecting the TCC connector simulates an open switch to determine if circuit No. 446 is shorted to ground or the problem is in the transmission.
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