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Fuel pump keeps coming on when key left on

Old 05-06-2019, 06:23 PM
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Default Fuel pump keeps coming on when key left on

I'm having a problem with my fuel pump coming on after the initial normal time when you turn the key on. If I sit there with the key on after a while it will come on a couple times on its own, then turn off. Then a few minutes later it comes on again, then sometimes it doesn't. I've already put in a new complete fuel pump assembly, (new regulator, pump, and wiring assembly). Starts just fine, no problems there.

Can someone tell me if they have had a similar problem, and what it could be?

Thanks for any help.
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Old 05-06-2019, 06:50 PM
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The computer commands key-on prime by providing ground to the coil side of the fuel pump relay. It doesn't necessarily prime the pump on every key-on. There appears to be a certain amount of time that has to have elapsed before it will prime again (at least this is what I observed on my 96). Ignition switch provides positive to the coil side of the fuel pump relay when in the RUN or START positions. The computer is unaware of fuel pressure - so it is not triggering off anything like that. I think the most likely culprit is your ignition switch is cycling power and occasionally this is causing the computer to re-prime. Just a guess. But you could probably put a meter on your ignition switch (via the backside of the ignition switch connector) while fiddling with the key to see if you get any transient drop-outs of the positive feeds. I'll try to look up which wire(s) on the connector would be the ones to check.

START and RUN positions should energize circuit A21 which is the dark blue (DB?) wire at the ignition switch connector. The steering column is typically grounded, so you can use bare metal on the column with the black probe, while using the red probe to read the voltage on the dark blue wire. It should only be energized when in the run or start positions.

If nothing is observed here, we'll probably have to move downstream.

Last edited by jordan96xj; 05-06-2019 at 06:57 PM.
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Old 05-06-2019, 07:06 PM
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Originally Posted by jordan96xj View Post
The computer commands key-on prime by providing ground to the coil side of the fuel pump relay. It doesn't necessarily prime the pump on every key-on. There appears to be a certain amount of time that has to have elapsed before it will prime again (at least this is what I observed on my 96). Ignition switch provides positive to the coil side of the fuel pump relay when in the RUN or START positions. The computer is unaware of fuel pressure - so it is not triggering off anything like that. I think the most likely culprit is your ignition switch is cycling power and occasionally this is causing the computer to re-prime. Just a guess. But you could probably put a meter on your ignition switch (via the backside of the ignition switch connector) while fiddling with the key to see if you get any transient drop-outs of the positive feeds. I'll try to look up which wire(s) on the connector would be the ones to check.

START and RUN positions should energize circuit A21 which is the dark blue (DB?) wire at the ignition switch connector. The steering column is typically grounded, so you can use bare metal on the column with the black probe, while using the red probe to read the voltage on the dark blue wire. It should only be energized when in the run or start positions.

If nothing is observed here, we'll probably have to move downstream.
Thanks, that would be great. Now that you say that, I believe I actually seen the GEN light come on while doing this as well. Like I said its more randomly or intermittent rather than a constant thing that happens. I had just changed out the Temp Sensor, and was checking to see if the code was gone on my scanner. Sure wish the computer was able to utilize all the things this little scanner can do. I have a couple old meters, not sure if they work they were the old mans. One more thing to teach myself how to use, sure am paying for all the times he fixed my car.

Thanks again for helping me, maybe one of these days I wont be afraid to go out of town with the fear of breaking down, sure would be nice.
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Old 05-06-2019, 07:15 PM
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Quote: START and RUN positions should energize circuit A21 which is the dark blue (DB?) wire at the ignition switch connector. The steering column is typically grounded, so you can use bare metal on the column with the black probe, while using the red probe to read the voltage on the dark blue wire. It should only be energized when in the run or start positions.

If nothing is observed here, we'll probably have to move downstream.


Ok, guess we were posting at the same time. I'm gonna have to find my way around what your talking about. Gonna have to watch some videos to know what and where to look for what you`re talking about. First I gotta get a meter that works. Sorry, I'm a girl, you're gonna a to give me little slack till figure what I`m doin. I'll get back to you. Thanks
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Old 05-06-2019, 07:54 PM
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Originally Posted by Cheapy Jeepy View Post
Quote: START and RUN positions should energize circuit A21 which is the dark blue (DB?) wire at the ignition switch connector. The steering column is typically grounded, so you can use bare metal on the column with the black probe, while using the red probe to read the voltage on the dark blue wire. It should only be energized when in the run or start positions.

If nothing is observed here, we'll probably have to move downstream.


Ok, guess we were posting at the same time. I'm gonna have to find my way around what your talking about. Gonna have to watch some videos to know what and where to look for what you`re talking about. First I gotta get a meter that works. Sorry, I'm a girl, you're gonna a to give me little slack till figure what I`m doin. I'll get back to you. Thanks
The Jeep doesn't know your a girl. So should be no problem. To get at the ignition switch connector, you have to take off the plastic trim/shroud that is right behind your steering wheel. The one that the steering wheel adjustment handle, turn signal lever, and hazard lights pop out of. The adjustment lever can be unscrewed and set aside. The rest stay where they are. I believe the fasteners for the shroud are T20 torx (holes are on the bottom). It is clipped together in the rear of the shroud so don't try to rip it apart our you'll break the fragile clips that hold the rear together. Look inside with a flashlight once you have the fasteners out to see what I mean. Once you do it once, it is about a 5 minute job. Once it is off you can clearly see the ignition switch and the large connector that feeds it.

A test lamp would also work in this case. If you happen to have one, or can borrow one.
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Old 05-06-2019, 08:15 PM
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Originally Posted by jordan96xj View Post
The Jeep doesn't know your a girl. So should be no problem. To get at the ignition switch connector, you have to take off the plastic trim/shroud that is right behind your steering wheel. The one that the steering wheel adjustment handle, turn signal lever, and hazard lights pop out of. The adjustment lever can be unscrewed and set aside. The rest stay where they are. I believe the fasteners for the shroud are T20 torx (holes are on the bottom). It is clipped together in the rear of the shroud so don't try to rip it apart our you'll break the fragile clips that hold the rear together. Look inside with a flashlight once you have the fasteners out to see what I mean. Once you do it once, it is about a 5 minute job. Once it is off you can clearly see the ignition switch and the large connector that feeds it.

A test lamp would also work in this case. If you happen to have one, or can borrow one.
Good answer man too funny. Yeah, all the meters, except one, the batteries corroded in the connectors, and just tore apart when I took the battery out. I have a good Fluke 88 that I need to get a battery for. I use to have a test lamp, I'll have to look for it.

Oh I'm willing to take a shot, just hesitant cause I've never fooled with this stuff before. My old man was the smart one, he could work on anything. I'll get my act together, and try to take a shot at it this weekend. Thanks for putting up with me!

Last edited by Cheapy Jeepy; 05-06-2019 at 08:17 PM.
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