Gentlemen,
Asking for a friend. 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(s.) Pressure regulating to 18 psi and timer works, on both GPU and battery voltage. System works with either side, and both sides connected to shop air/test kit(s) simultaneously. Deice light on left CB panel lights. Run the engines on the ground or in flight and the boots barely inflate, for about a second. No light. Replaced right-side pump on time. No change. Left-side pump is quite new(OH.) 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 (or three!) 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??!

Where are they hooking up the shop air? Is the vacuum side still hooked up to the pump while testing?
The reason I ask those questions, is that the deice valve applies vacuum to the boots after the pressure cycle completes. And if the pumps are still hooked up, and they are applying the air to the pressure side of the pump, IF the deice valve is not isolating the vacuum side from the pressure side, you could see your symptoms. The vacuum side should be disconnected, and checked for airflow during the inflation cycle, there should be no airflow till after the inflation cycle completes.

Rick,
Thank you for the suggestion. I believe the vacuum side is still connected to the pump when the kit is connected to the pressure side, right at the pump. I'll check that.

No vacuum is supplied for anything directly from the inlet side of the pumps. The inlet hose attaches to the firewall. On the back of the firewall there is a wristband filter for the inlet air.

I stand corrected, the vacuum is supplied by a venturi. I had my aircraft types mixed up.

Find out if the shop is removing the outlet hose from the pumps and attaching the test kits to them, or if they are removing the outlet hose from the firewall and attaching the test kits to the airframe using a shop supplied hose. The outlet hoses are routed along the engine mount and if poorly secured, get holes chafed in them. The holes are often on the bottom and hidden by the wire bundle, which is also secured to the mount.

Rick,
I believe they are connecting the test kits right at the pumps. I'll confirm though. Thank you.

Dear interested viewers,
Everyone hates an unsolved mystery. It seems we were able to isolate the deicer problem to the timer. As noted, 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! Another freak problem. Hopefully a new timer will solve it. Thank you for your time and suggestions.