July 27, 2022

Troubleshooting GM 8L45, 8L90 No Pressure/No Movement Problems

Jim Mobley

​​​​​​​The GM 8L90 and 8L45 transmissions have been plagued with symptoms of no pressure and/or no movement out in the field, even though they have a state-of-the-art, off-axis pump assembly setting in a pan of ATF. This pump assembly also has great self-priming capabilities even though, in some cases, it may not seem so.

Improper assembly of the pressure regulator assembly can be a cause of no pressure and/or no movement. Shops have had reports of all four pieces installed incorrectly in some cases, such as the spring on the wrong side of the valve or the main pressure regulator valve backwards in the bore. Other times we have seen the inner shuttle boost valve and bushing installed facing outboard. Anything other than the image shown here is incorrect. Correct 8L90-8L45 PR Valve Assembly

To verify pump priming capabilities and pressure regulator function on the bench, assemble valve body and place face down in a medium-sized clean drain pan. Then place manual valve in Park, insert 1-quart funnel into suction of pump and fill with ATF. Then rotate pump shaft clockwise with 1/2" chuck drill to prime pump.

Sonnax offers drop-in pressure regulator and shuttle valve kit 154740-02K for the OE lineup in instances of worn valves/sleeve/casting.

Jim Mobley is a Sonnax technical communications specialist. He is a member of the Sonnax TASC Force (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax Transmission Company technicians.

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