ATA 29-12, Hydralic Pump Low Pressure Switch

MD-11 with growing history of Hydraulic System Test failures after engine start. Problem centered around the #3 engine right pump. No troubleshooting done , but maintenance control requested low pressure switch S1-8120 be replaced.

The location of hydraulic manifold - under right hand T/R half. Switch S1-8120, rear and outboard.



Heat shield must be removed to access switch in case. Upper shield has two halves that can be split. Safety wire inboard and screws around perimeter. One band clamp has to come off also.





Switch exposed. Hex head recessed below lower shield so it's somewhat difficult to place a wrench on flats (a "really" deep socket would of done the trick). Got enough of a bite with a 1-1/8" open-end to get job done.




Removed switch. We had a small amount of Skydrol leaking down during the actual removal, but at least it wasn't puking out.



A leak check is somewhat tricky. You want to see the actual switch and case with pressure on it and you can't light off the engine with the T/R open. What to do?? Well.... you get pump pressure during engine start so a spin-up without putting fuel on will do the trick. A full engine start for a hydraulic test is needed after heat shield installed and T/R closed.

As a note: the left and right low pressure switches are NOT the same part number. Ref IPC 29-12-01-10

AMM 29-12-34-2-2