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Dave
Dave, Jeep Master Tech
Category: Jeep
Satisfied Customers: 1158
Experience:  Chrysler Triple Master, ASE master Tech AA degree in Auto.
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Hey so i have a 1997 jeep grand Cherokee. i have just replaced

Customer Question

hey so i have a 1997 jeep grand Cherokee. i have just replaced the fuel pump as it quit on me. i have hooked everything up the same way but the fuel pump will only prime and run when your foot is on the brake. as soon as you take your foot off it shuts off. any ideas
Submitted: 1 year ago.
Category: Jeep
Expert:  Dave replied 1 year ago.
Hi, thank you for allowing me to help you. Please remember that all I have to work with is what you tell me so please excuse the questions. This is normally caused by a bad ground or voltage source to the fuel pump module.
The easiest way to diagnose and determine just what is going on is to use a test light.
With the key in the off position disconnect the electrical connector at the fuel pump module. You will be testing the wiring on the harness side connector. Hook the alligator clip of the test light to ground (any body bolt that's clean will work) now probe pin number 1 which has a dark green with a white tracer wire going to it. Now turn the key to run. The test light should light for about three seconds then go back out. This is normal and proves you are getting voltage to the pump. Now hook the alligator clip of the test light to battery voltage. You may need to use an extra length of wire to make it. Just make sure it is shielded and no bare wire touches a ground source. Now probe terminal number 2 that has the black wire to it and the test light should light up. It does not matter if the key is on or not. This wire should be grounded all the time. If the test light does not light then you have a bad ground.
If it is a bad ground I would simply splice another piece of wire onto the existing black wire and attach the other end to any ground source. This could be as simple as using a self tapping metal screw and attach the wire to it and screw it into the frame.
If you do not get power on pin 1 let me know as we will need to do further testing. You may need to have a helper turn the key to run so you can be watching the test light since as I said it will only light for a couple seconds then go back out. The reason for this is....when the key is first turned to run or start the fuel pump module gets power to prime it. If the PCM (powertrain control module) does not see engine RPM from the crank sensor it stops sending voltage to the fuel pump. Since we are not cranking the engine over you are only going to get voltage long enough to prime the system. We want to do it this way so we do not flood the cylinders since we know it will not start and keep running without the brakes being applied. Obviously we can not have the brakes applied to diagnose it correctly.
Let me know what you find out and if you have any other questions or concerns please do not hesitate to ask. I am here to help you until your Jeep is running correctly.