P0776 on Chevrolet: How to Diagnose & Fix

Silverados with the Hydra-Matic 8L90 (and 8L45) 8-speed automatic are widely reported to hesitate, then shudder, jerk, clunk or 'hard shift' when changing gears — some owners describe a shift so violent it feels like being rear-ended, plus a torque-converter-clutch shudder that feels like driving over rumble strips. The root cause is internal friction/hydraulic issues and torque-converter-clutch micro-slip, often worsened by the original transmission fluid breaking down, sending metal shavings through the unit. GM issued 13+ TSBs and a fluid change (to Mobil 1 Synthetic LV ATF HP / DEXRON-ULV) as the primary remedy, and the defect is the subject of a certified class action covering 2015-2019 Silverado and other GM models. Very high search and complaint volume.

Gen 5 Camaros with the 6L80/6L90 six-speed automatic commonly exhibit a 2-3 upshift 'flare' where engine RPM briefly rises before the gear engages, sometimes followed by a firm bump. GM issued bulletins calling a mild 2-3 slip/flare a 'normal characteristic,' but worsening flare typically indicates a real hydraulic leak, most often at the 3-5-Reverse clutch drum or clutch-regulator valve, and can throw pressure/ratio codes. Left unaddressed, slipping accelerates clutch wear.

The early (Gen 1) 6T40/6T30 automatic uses a stamped steel wave plate in the 3-5-Reverse clutch pack. On 2011-2012 builds the plate cracks and shatters without warning, instantly wiping out 3rd gear, 5th gear and reverse. The car will usually still move in 1st, 2nd, 4th and 6th, so drivers often describe it as 'it flared, then it won't back up.' Debris from the shattered plate circulates through the valve body and contaminates the whole unit. This is distinct from the well-known torque-converter shudder - it is a hard mechanical failure, not a driveability complaint. GM revised the wave plate in later 6T40 builds and the failure rate drops sharply on 2013+ cars.

First-generation Traverse models built with the 6T70/6T75 six-speed front-drive automatic use a 3-5-R clutch cushion (wave) plate that on early production was improperly heat treated and left too brittle. Repeated compression cycles during 2-3 and 4-5 shifts crack the plate; once it fractures, the vehicle loses 3rd, 5th and reverse, and the debris migrates through the transmission, plugging the filter and scoring other components so what began as one $50 plate becomes a full rebuild. GM acknowledged the condition with Special Coverage Adjustment #14404, extending coverage to 10 years/120,000 miles for affected build ranges. Owners on Traverse Forum describe the classic progression: a late or flared 2-3 shift for weeks, then an abrupt no-reverse.

Equinox models with the GM 6T70/6T75 six-speed automatic suffer shudder, harsh or delayed shifts, slipping, and loss of reverse. A common root cause is failure of the 3-5-reverse wave plate, which can crack and cause extreme shudder when reverse is engaged and eventual loss of drive. Overheated/degraded fluid that loses viscosity accelerates internal wear. Failures typically appear between roughly 60,000 and 150,000 miles. Owners report bucking when engaging gear, failure to shift out of 2nd, and inability to engage reverse.