GM transmission performance

This TSB applies to 2009 and prior GM passenger cars and light trucks, Hummer H2/H3 and Saab 9-7X equipped with a 4L60-E, 4L65-E or 4L70-E automatic transmission.

It may be difficult to diagnose DTC P0756 (shift valve performance on 4L60-E, 4L65-E or 4L70-E automatic transmissions). As detailed in the service manual, when the PCM detects a 4-3-3-4 shift pattern, DTC P0756 will set. Some customers may also describe a condition of a second, third or fourth gear start that may have the same causes but has not set this DTC yet. Below are some tips when diagnosing this DTC:

-          This is a performance code. This means that a mechanical malfunction exists.

-          This code is not set by electrical issues such as a damaged wiring harness or poor electrical connections. Electrical problems would cause a DTC P0758, P0787 or P0788 to set.

-          The most likely cause is chips/debris plugging the filtered AFL oil at orifice #29 on the top of the spacer plate (48). This is a very small hole and is easily plugged by a small amount of debris. It is important to remove the spacer plate and inspect orifice #29 and the immediate area for the presence of chips/debris. Also, the transmission case passage directly above this orifice and the valve body passage directly below should be inspected and cleaned of any debris. For 2003 and newer vehicles, the spacer plate should be replaced. The service replacement spacer plate is a bonded style with gaskets and solenoid filter screens bonded to the spacer plate. These screens can help to prevent plugging of orifice #29.

-          This code could be set if the 2-3 shift valve (368) was stuck or hung up in its bore. Inspect the 2-3 shift valve and the 2-3 shuttle valve (369) for free movement or damage and clean the valves, bore and valve body passages.

-          This code could also be set by a 2-3- shift solenoid (367b) if it were cracked, broken or leaking. Refer to Shift Solenoid Leak Test in the service manual for the leak test procedure. Based on parts return findings, a damaged or leaking shift solenoid is the least likely cause of this condition. Simply replacing a shift solenoid will not correct this condition unless the solenoid has been found to be cracked, broken or leaking.

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