jabsco macerator pump wacko

[razzda]

I have a new macerator pump that will not prime on board. I can take the basic setup, pickup hose and pump place all above a five gallon bucket mimicking boat install and it will pump almost instantly, put this untouched system on board and it will not prime or pump the poop, just spins.
Non physics expert could use some input here as to what Ive suddenly done wrong.
This very same arrangement has worked for 6 years. All I did was replace the pump after malfunctions.
Tank is below the waterline but really shouldnt matter?
Why does this create a vacuum in the pickup hose ( 1 1/2 " dia, )above the waterline out of the boat in 2 seconds and not work at all below the waterline in the boat??
Perplexed and annoyed.

[HopCar]

My first thought is you’ve got an air leak in the pickup somewhere when it’s installed.

Does the pickup tube go to the bottom of the tank?
Could the pickup tube be blocked?

Just making wild guesses. Good luck.

[HopCar]

Blocked tankvent?

[razzda]

Yes thanks for the replies. No blockage, pick up does go to tank bottom, there is no leakage.
The whole set up works out of the boat, same pump, same pick up.
I dont get it.

[desodave]

On our previous boat, the one time the macerator wouldn't pump out was when the holding tank was pumped (over)full at anchor. Had to allow some of the contents out through the direct overboard before the macerator pump would work. No vent blockage, but a relatively full vent line.

[razzda]

interestingly Dave my tank is really really full, Going up to the boat today to put in a foot valve so I can maintain a prime and get it empty. Let you know how it goes.

[qwert]

you haven't accidentally changed the connections so its running in reverse ?

[razzda]

Problem solved, it was physics of course, pick up was sitting in just enough accumulated sludge to prevent a vacumm in the hose.
Changed the end point and ran like a charm.
Will partially fill and scrub to alleviate future nightmares.
Thanks for everyones replies.