The wiring diagrams I have seen show the gear switch grounding pin 4 of the pressure head for planes without vacuum, and pin 13 if you have vacuum.  I think this is backwards.  Pin 4 opens a solenoid in the pressure head that connects the vacuum port directly to the bellows.  This allows the vacuum to pull the bellows fully open (dump mode).  So, if you have vacuum, you want pin 4 grounded when on the ground and taking off (to prevent the pop you get when going to full power).  I added a small electric vacuum pump to mine to do this that goes on whenever pin 4 is grounded.

Pin 13 goes to a second solenoid in the pressure head, but I have yet to figure out what it does.

Pete

-----Original Message-----
From: billhogarty [mailto:billhogarty@hughes.net]
Sent: Sunday, March 20, 2011 6:29 AM
To:
Subject: Pressurization

For Pete Sinclair:

Pete:  Looks like you might be the expert on this topic.  I was looking for a preflight check  but thought that the system was grounded thru the
gear down switch.   Any help here??

Thanks, Bill Hogarty