Difficulty Troubleshooting Deice Boot System

[howardk]

Boots work in the shop.  Boots do not work when the engines are running/providing pneumatic pressure.  Pneumatic boots inflate/deflate normally when connected to shop air/test kit.  Pressure regulating to 18 psi and timer works, on both GPU and battery voltage.  System works with either side, and/or both sides connected to shop air/test kit(s) simultaneously.  Deice light on left CB panel lights.  Run the engines and the boots barely inflate, for about a second.  No light.  Replaced right-side pump.  No change.  Left-side pump is quite new.  System shows 18 psi on the left pump and 15 psi on the right pump.  Only difference between working and not working appears to be running the pumps/engines!  Check-valves appear to be working.  Two bad pumps?  They make pressure but insufficient volume?  One bad pump that is putting out some air, such that the good pump can't quite close the check-valve against the weak pump?  Not sure that would matter if no leaks.  Thank you for any suggestions.  Also, the deice control valve costs $8700 new?

[klausnw]

Check the intake system to the vacuum pumps. If there is something preventing a good volume of air entering the pumps then the outflow pressure will read high because of the regulators but the volume is restricted. One possibility.

[howardk]

Gentlemen,
We were able to isolate the deicer problem to the timer. In the shop with external power, the system operates OK. When engines are running, it appears engine vibration causes the timer to come on for an instant and then shut off almost immediately. Removed the timer from the bulkhead and set it on a sponge. System operated fine. Thank you for your time and suggestions.