dual vacuum system C337

cessnadriver · #1 12-22-21, 07:28 AM

HELLO ALL. Got a question for the group. Recently installed a G5 AI but needed the old vacuum AI to run the 400 navomatic. While behind the panel. I decided to remove as much old 1/4" rubber hosing as possible. So I installed 1/4" plastic tubing/ Now I have a vacuum DG, pilot's side and a Vacuum AI co-pilot's side. Hooked up the Garmin G5 and re-plumped the vacuum system. Went out and test flew my skymaster after the update. Wasn't too sure of my plumbing job, but had the G5 to give me airspeed , altitude and heading, just in case. Vacuum instruments spooled up just fine, BUT the vacuum gage ( the PSI side was not working), although the red buttons where IN. I cannot find a GOOD illustration of the skymaster's vacuum/instrument system. Some show the twin engine configuration BUT show four lines coming from the vacuum gage, mine has three, two to the vacuum manifold and one main to the AI/DG. Any suggestions?
Many thanks as usual.
Regards, BILLS

wslade2 · #2 12-22-21, 11:59 PM

I have an E model and this from my service manual for "1968 and on" (open attached, I couldn't get it to show in the body of this message).

It shows 3 lines from the vacuum gauge, 1 each to the supply side of the manifold for each of the 2 vacuum pumps and one connected to the horizon.

I suspect those supply side lines are for the "buttons"/indicators of the gauge to tell which pump is operating and the 3rd line off the gauge is for the the actual suction measurement of the system. Be sure the "3rd line" is hooked up to the horizon and does not have a leak. The Horizon has 3 ports: central supply off of the manifold, ambient (filter) and the suction gauge. The central suction off of the manifold is what spins up the instruments. The 337 suction gauge is a dead end and should be hooked up like that to the horizon (unlike some systems, like a 182, where the suction gauge may be a parallel pass through).

cessnadriver · #3 12-23-21, 10:03 AM

WSLADE2.
You know the old saying "Can't see the forest for the trees" well that is the case here. I didn't even think to check my service manual for that picture! It shows how to properly connect the lines, even though my newer AI and DG have extra ports for the gauge, I'll keep those ports capped.
Many thanks for the advise.
Regards, BILLS