Circuit/System Testing

WARNING: This page is about a different car, the 2018 GMC Yukon XL, 2018 GMC Yukon, 2018 Chevrolet Tahoe, 2018 Chevrolet Suburban 3500 HD, and 2018 Chevrolet Suburban. However, it is still accessible from the selected car via links, so may be relevant.

NOTE:

You must perform the Circuit/System Verification first.

Verify the transmission line pressures are within the specified ranges. Refer to Line Pressure Check .
- If pressures are not within range
  Refer to Fluid Pressure High or Low .
- Go to next step: If pressures are within range
Ignition OFF, remove the Q8 Control Solenoid Valve Assembly.
Verify a pressure change occurs for each solenoid when performing the solenoid test. Refer to Control Solenoid Valve and Transmission Control Module Assembly Solenoid Performance Test .
- If a pressure change does not occur
  Replace the Q8 Control Solenoid Valve Assembly.
- Go to next step: If a pressure change does occur
Verify the conditions listed below do not exist. Carefully inspect each component for that condition. Repair or replace as necessary.

P0776 - Control Valve Upper Body Assembly (304)

3-5-R Regulator Valve (336) stuck/sticking - debris, binding, damaged valve, or scored bore.

P0776 or P0777 - Q8 Control Solenoid Valve Assembly (314)

O-ring seals leaking.
Valve body filter plate assembly cracked, blocked or damaged gasket seal.

P0776 or P0777 - Control Valve Upper Body Assembly (304)

Actuator feed limit circuit supply fluid to solenoids restricted or not available, debris or sediment blockage.
B-R-1/4-5-6 clutch regulator valve (327) not in the applied position - debris, binding, damaged valve, or scored bore.
4-5-6 clutch boost valve (329) not exhausting - debris, binding, damaged valve, or scored bore.

P0776 or P0777 - Torque Converter (w/Fluid Pump) Housing Assembly (2)

Fluid pump housing vanes/rotor/slide damaged/scored.

P0776 or P0777 - A/Trans Fluid Pump Cover Assembly (219)

Isolator valve (236) stuck/sticking - debris, binding, damaged valve, or scored bore.

P0776 - 1-2-3-4 and 3-5-R Clutch Housing Assembly (51)

3-5-R clutch backing plate retaining ring (400) not seated.

P0776 or P0777 - 1-2-3-4 and 3-5-R Clutch Housing Assembly (51)

1-2-3-4 clutch backing plate retaining ring (429) not seated, causing clutch pack/piston over travel.

P0776 - 4-5-6 Hub/ Shaft Assembly (58)

4-5-6 Hub/Shaft - broken hub weld.