3L80/3L80HD (400/475) Automatic Transmission

Oil pressure can be checked with the transmission in the vehicle, using a 0 to 300 psi pressure gauge.

1. Clean dirt from line pressure plug, located on left side of transmission case, and remove plug.

2. Connect pressure gauge hose fitting at pressure plug bore in case and place gauge in vehicle so it can be seen from the driver's seat.

3. Drive vehicle until transmission fluid has reached operating temperature.

4. Check and correct fluid level.

5. Road test car and check pressure as follows:

Engine Idle Pressure Check
With selector lever in Direct Drive position, operate vehicle at approximately 30 mph with throttle closed. Line pressure should be 70 psi. When a selector lever has been moved to Intermediate range with vehicle operating at a stead road load (25mph), line pressure should e 150 psi (plus or minus 5 psi).


Pull Throttle Pressure Check
With vehicle on jackstands, disconnect vacuum line from carburetor and place selector lever in Neutral and in Reverse. Operate engine at fast idle (700-1000 RPM) in each position. Neutral pressure should be 145 psi, and Reverse pressure 230 psi. Connect vacuum line.

If either pressure check was low, check vacuum modulator for loose attaching bolt, collapsed bellows, or stuck modulator valve. Also check pressure regulator valve and spring for sticking, plugged orifice, or collapsed spring. Check regulator boost valve for sticking.

If either pressure check was too high, check vacuum line and vacuum modulator for leaks or sticking valve. Also check pressure regulator valve and boost valve for sticking.