fuel tank humming

[Vlad ink]

The most annoying sound I've heard from my jeep in a while
is a low bowel shaking hum coming from gas tank. If the tank is full
it's not as loud but close to empty it resonates thru the entire vehichle
like a continuous low E note.

Also runs fine but sometimes will not start no sound of the starter clicking nothing. But if I go shake all the wires It will start. Just bought a
new battery.

[Saudade]

The hum may be your fuel pump. Check the ballast resistor in the engine compartment. Then check the pump.

What wires do you shake to get it to start?

[scuba4321]

It is your fuelpump! It not as loud full because the more fuel insolates the sound, make sure your battery cables are clean, and on tight!!

[Vlad ink]

Thanks,
I shake different ones sometimes the wires on the starter work.
sometimes the harness near th air cleaner. My jeep doesn't seem to
care as long as I get out in the cold and open the hood.

BTW what does the ballist resistor do?
Am I checking for continuity or something else ?
connections there look clean resistor itself looks old.

[scuba4321]

the ballest drops the voltage, get your volt meter, check the voltage on the from, than on the rear connection, not sure the difference I belive when I checked mine it was 2-3v drop!

[NJPagan]

I've seen the ballast resistor too, and I know they drop voltage...But across what? I used to see them in point ignition vehicles, they dropped the voltage across the points after startup to keep from burning out the points too quickly...
What's the function on say my 88 Cherokee?

[Saudade]

A ballast resistor is used in the fuel pump control circuit. It's by-passed during start mode. During running mode, it reduces the speed of the pump by lowering pump voltage. This ensures normal speed in running mode. It's 1-ohm (according to my FSM).

It's also possible that the pump mount may be loose or some of the shock isolation/gasket material is no longer good (hardened, worn thorugh, etc.).