Tech Tip: GM’s A/T is Stuck in 2nd/3rd Gear – UnderhoodService

Tech Tip: GM’s A/T is Stuck in 2nd/3rd Gear

Some customers may comment on one or more of the following conditions: the MIL is on; the transmission is defaulted to second gear (4L80/85-E); the transmission is defaulted to third gear (4L60/65-E); the instrument cluster is inoperative.

Models: 2006 and Prior GM passenger cars and light duty trucks; 2005-’06 HUMMER H2 and Saab 9-7X; and 2006 HUMMER H3 — with 4L60-E, 4L65-E, 4L80-E or 4L85-E automatic transmission (RPOs M30, M32, M33, MT1, MN8).

Some customers may comment on one or more of the following conditions: the MIL is on; the transmission is defaulted to second gear (4L80/85-E); the transmission is defaulted to third gear (4L60/65-E); or the instrument cluster is inoperative.

Upon investigation, one or all of DTCs P0740, P0753, P0758, P0785, P0787, P0973, P0976, P1860, P2761, P2764 and P2769 may be set depending on the vehicle.

The most likely cause of this condition may be a loss of voltage to the transmission solenoid power supply circuit. This condition may also be the result of an incorrect installation of an aftermarket electronic device such as a remote starter or alarm system.

Correction
Important: If the vehicle is equipped with an aftermarket electronic device and the DTCs are being set when the device is being used, verify that the appropriate fuse is being supplied battery voltage during operation. If voltage is not present at this fuse, these DTCs will be set due to lack of voltage at the solenoids. This condition is most likely to occur with an incorrectly installed remote starting system. If this is the case, refer the customer to the installer of the system for corrections.

1. With the ignition switch in the RUN position, test for battery voltage at the circuit fuse in the transmission solenoid power supply circuit. Refer to the appropriate SI document for the transmission solenoid power supply circuit information.

2. If battery voltage is present at the fuse, inspect the ignition voltage circuit between the fuse and the transmission for possible opens.

3. If battery voltage is not present at the fuse, test for continuity between the fuse and the ignition switch. If no continuity is between the fuse and the ignition switch, repair the open in that circuit.

4. If you have continuity between the fuse and the ignition switch, the most likely cause is the ignition switch. Replace the ignition switch using the appropriate Ignition Switch Replacement procedure in SI. Refer to group number 2.188 of the parts catalog for part description and usage of the ignition switch.

Technical service bulletin courtesy of Mitchell 1.

For more information on Mitchell 1 products and services, automotive professionals can log onto the company’s website at www.mitchell1.com.

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