Strange fuel pump/sender issue

[Ibby]

I've recently had trouble with my MJ (1991 4.0 5 spd) where it will stutter, come close to stalling and backfire a bit through the intake. I've experienced this before as I've owned several other Jeeps. I did the regular stuff, checked the ignition, dist, wires, checked the throttle, TPS, etc and all checked out fine. I checked pressure at the rail and found not quite 20 psi so I dropped the tank. I found the small rubber hose from the pump to the tube had come loose so I replaced it and tightened it down. I buttoned everything up and found the fuel pump would not run at all. I took it apart again and tested the pump at various points from the pump leads back. With 12 volts on the pump it runs like a champ. If I try to put 12 volts at the 3 pin electrical connector, nothing happens, unless the float arm is raised all the way up, and then the pump runs fine. When the arm is lowered, the pump starts to run slower, and after about a half inch of travel from the top the pump stops completely. I've never run across this before, and I'm not sure how to go about fixing it. Has anyone seen this happen before?

[Ibby]

I took apart my Renix fuel system from the other truck to test the fuel pump/sender from that truck, and it does the exact same thing but in reverse (which I kind of expected as it's Renix). I don't understand why the pumps won't pump unless the potentiometer is at max. Am I bench testing these things incorrectly?

[HOrnbrod]

If I try to put 12 volts at the 3 pin electrical connector, nothing happens, unless the float arm is raised all the way up, and then the pump runs fine. When the arm is lowered, the pump starts to run slower, and after about a half inch of travel from the top the pump stops completely. I've never run across this before, and I'm not sure how to go about fixing it. Has anyone seen this happen before?

 

IIRC, pin 3 is the signal to the fuel gauge. If you apply 12VDC on the float arm variable output the fuel pump motor will act exactly like what you are describing. On the 91, when the arm is up, there is no resistance so the pump will run. Lower the arm, operating voltage is dropped through the potentiometer, so the pump stops. Haven't looked in the manual yet, it's late, but pretty sure this is what's happening.

[Ibby]

D'oh, just figured it out in my head. I should have been using the ground pin for well, ground, not the potentiometer pin. Kind of a horrible mistake for an electronics tech. I'll blame it on a garage full of fuel fumes.

Thanks for the head's up hornbrod. You fixed my brain fart.