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There are two possible problems. Bob P. gave a good description of the pressure switch. In your case the pressure switch contacts could have 'welded' shut. The other, and IMHO more likely, possible problem is that you have a leak in the plumbing or the manual pneumatic switch. When the master is on the pump gets power via the pump breaker and pressure switch. If the pump is running then you have a good circuit. If the pump doesn't stop running then you either have leak or a bad pressure switch. The compressor keeps the switch pressurized all the time. The manual pneumatic switch simply connects the door seal to vent OR to the pressurized circuit.
Find the pump / pressure switch in your plane. Pinch off the hose that runs to the manual pneumatic switch. Turn on the master. If the pump runs and stops, you leak is downstream. If the pump loads down but doesn't stop the problem is the switch. If the pump does not load down then look for the leak around the pump.
Brent Regan
On 6/19/2014 3:00 AM, Lancair Mailing List wrote:
After turning on the Master switch on a 40F morning at S Lake Tahoe (6000' +), the door seal motor started on its own accord. Numerous attempts to cycle the Master, the motor CB, and the motor switch, provided no help. So I flew back to AZ unpressurized. After landing, the motor operated normally. Any ideas?
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