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Attack of the MJ fuel pump


NAXJA Forum User
The Mudwest
Went to drain my fuel tank, figured that even with low pressure it would be enough to drain the tank.

So I dropped the fuel line, had the wife flip the switch. The pump came on like it was pressuring, then after the normal amount of time it cut-off. The supply line was in a gas can so it couldn't reach pressure. I only got about a cup and a half out of the tank.

I thought it could be a relay, so I switched them into my 88 XJ, it ran like normal.

Let me see if I understood your correctly.

You put the key to the on position with the fuel line going to a gas can as opposed to the fuel rail? The pump came on for about 2-3 sec then shut off?

This is normal. The system doesn't actually measure the fuel pressure then shut it off when it hits a certain value. It just turns the pump on for a couple seconds to help restore any fuel pressure losses while the vehicle wasn't on.

To keep the pump on you'll have to hardwire it directly.
Well, sort of. My fuel pump is wired to a toggle under the dash. EE's idea of a security device.

But, thanks for the answer!
But your toggle doesn't turn it ON, does it? A security toggle is an interrupt that prevents the pump from running when the vehicle system tells it to go. That's just the opposite of jumping it to run with no back pressure in the rail.
The power for the pump runs through the toggle. If you turn the key, without the toggle on, the fuel pump won't come on.

The juice to the ignition system also runs through another toggle. no toogle on, no ignition juice. Truck just cranks over!
I believe what your saying is correct, but I think Eagle is also correct. Yes, the power for the pump runs through the toggle, but is the hot side of the switch a constant hot? When you crank the engine the pump comes on, release the switch to the "run" position and the pump stays on IF the engine starts. If the engine does not start, the pump will shut off due to no oil pressure(I believe). Run a hot wire directly from your battery to the pump, that'll tell ya if it's right. It doesn't need to build pressure to work. After re-reading your post, I'm not exactly clear on what you did or what you're looking for!!??
Okay, here is the situation.

Normal XJ (is there really one), you turn key to run, fuel pump comes on, just for a few seconds, builds pressure and shuts off until Jeep is running.

My MJ, turn the key to run, without the toggle on, no fuel pump whatsoever.

I am draining the tank to replace a pump that will only hold about 12-14 psi, while the truck runs. I am just going to drag an extra battery out there and wire up direct to the pump.