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Old 01-03-2012, 02:08 PM   #1
aragu36's Avatar
Join Date: May 2008
Location: san bruno ,ca
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Fuel Pump Alwaya on

I changed my fuel filters and lost prime >
Thats another story
The problem i'm having is the fuel prime is always on or stops for 10-20 seconds and comes on again
its a black box about 6 in long and connects to a 6in round ball via air hose ? I don't know if the pump is broke it just stays on all the time
Any help

PS I think the little black thing is the fuel prime?

CUMMINS ISB 5.9 275 hp
Allison MT 643
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Old 01-03-2012, 03:01 PM   #2
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I'm thinking that black box you are looking at is a Vacuum pump.
It has nothing to do with the fuel system. It creates vacuum for the heater/air conditioning control and cruise control.
The priming pump that you have is a carter electric pump, and to fill your filters properly, is to turn on your ignition key on and off several times, leaving it on for 30 seconds at a time for the pump to fill up the filters.
Next time try filling the fuel filters with clean fuel before installing them.

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Old 01-03-2012, 03:25 PM   #3
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Originally Posted by sjholt View Post
I'm thinking that black box you are looking at is a Vacuum pump.
2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft
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Old 01-04-2012, 11:41 AM   #4
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Is it normal for the pump to run constant ?
CUMMINS ISB 5.9 275 hp
Allison MT 643
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Old 01-04-2012, 12:07 PM   #5
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No it is not. On my previous MH I had that problem and found a hose leaking and a leak in a diaphram on a vacuum control unit. You can follow the vacuum lines and pinch them off one at a time and see which ones make it hold vacuum and stops the pump. I would first pinch off the main line coming out of the pump to see if it shuts off or not. Could be the pump is bad. You can also go to a car parts store or Sears and get a vacuum tester which is a small hand pump with a gauge on it. Then you can go around to each vacuum control and hook the below pump up and test it.

Amazon.com: Actron CP7830 Hand Vacuum Pump: Automotive
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