Below you find 2 pictures of ACMS data from the same aircraft, in which the 390 F sensor of the precooler control valve failed. You can see how the PRSOV is closing and making the bleed duct pressure lower and lower. After replacement of the sensor, the ACMS data shows a correct duct pressure again, proving that the 390 F sensor is working correctly and opening the PCV to cool the bleed air. The 390 F sensor was replaced after the bleed air health check was performed iaw amm, and all other parts where found working ok. The 390 F and 450 F sensors are not tested with this test. They have a special shop-test which cannot be performed during normal aircraft maintenance and troubleshooting. Experience learns that this sensor is often defective. Thanks to Ferry B. for his input !
