Code 27, 28 Or 29 Gear Switch Circuits - 3.8L: Notes

Fig 1: Code 27, 28 Or 29 Gear Switch Circuit Diagram (3.8L H Body)

Courtesy of GENERAL MOTORS CORP.

Fig 2: Code 27, 28 Or 29 Gear Switch Circuit Diagram (3.8L W Body)

Courtesy of GENERAL MOTORS CORP.

The gear switches are located inside the transaxle. Switches are normally closed. As road speed increases, hydraulic pressure applies the specific gear clutches and gear switch opens. The ECM uses the gear switches to help determine control of fuel delivery and TCC operation.

Code 27 will set if no Code 29 is present, circuit No. 581 indicates ground or closed switch for 10 seconds when vehicle is in 4th gear, or circuits No. 581 or No. 446 indicate an open when the engine is first started.

Code 28 will set if circuit No. 108 indicates ground or closed switch for indicate an open when engine is first started.

Code 29 will set if circuit No. 446 indicates ground or closed switch for 10 seconds when the vehicle is in 4th gear (TCC locked and brake not applied) or if circuit No. 446 indicates an open when engine is first started.

1. A digital volt-ohmmeter must be used in this test. A test light will not work due to the low voltage supplied by the ECM.
2. Checks if the circuit is grounded through the switch.
3. Checks for a good, properly operating switch and checks circuit within transaxle for an improper ground.