96 fuel pump issue

Ok. I'll try to keep this simple and deal with one problem at a time.

Long story short my girl friends 99 xj was all rusted up. So i got a great deal on a 96 with no powertrain.

I have the engine, and trans in the 96 and am working on getting it to fire just for a few seconds before i replace the water pump, front clip and tcase and stuff as i hate spending time putting stuff together only to have to remove it when something isn't working correctly.

Anyway, I swapped the black and white part of the harness from the 99 into the 96 so i can keep the 99's intake and injectors and TB sensors, as a direct plug in fit. I do not have the starter hooked up as i just want to flush the fuel line out before i connect it to the fuel rail i hooked up the battery and made sure no connections are shorted. When i turned on the key the fuel pump never primed (along with fuel gauge on peg passed fuel mark, and the oil pressure is maxed out also and it stays there with no battery) Well while playing around checking fuses (non are blown) i figured i would check the brake lights. When i pushed the brake pedal (key on) the fuel pump runs.:doh:
It will only run when the key is on and the pedal pushed, it will not run when the key is off and the pedal is pushed.

the bad thing about this is I never pulled the drivetrain, it was pulled when i picked it up. But the reason it was parked was it had a bad trans (verified by tearing it down its full of sludge)

Any ideas? I'm going to pull the brake fuses and see what happens then with the key on but i'm lost lol

It never pushed any gas, but i'm guessing they emptied the tank when they pulled the drivetrain.

Update. I unooked the black and white ECU harness from the 99 and hooked up the 96's without the injectors/sensors hooked up and it does not run the pump with the pedal, however it also does not prime the pump when the key is turned on either.

I still have no idea. I'm leaning towards the ECM.

I checked the wires and plug at the fuel tank and it looks fine. I'm unsure where the wires run with the brake wires. I'm guessing along the rocker on the drivers side? But i doubt that its a rubbed wire or it would be blowing fuses.

I don't have the starter hooked up. but i used jumper cables and grounded the body of the starter and wired it up and when i try to start it using the key the starter does nothing. So i verified that the starter works using the jumpers and it does.

I Ohm'd the crank sensor and its fine.

I was told when i got the jeep the only thing wrong with it was that the trans had been fried.

So that leaves me to think its a ECM issue?

The voltmeter, and temp gauges do move when i key on, but the fuel level and oil pressure are way beyond maxed out on the dash.

Since there are no replys i guess this is a new issue. So i'll keep updating it as a go so it can be documented.

The plan tomorrow is to check Pin A2 on the PCM to see if its getting any 12v with the key on engine off. Then test it with the brake pedal pushed. If its receiving 12v then i will trace it back to the ignition switch.

Well. No go. It wasn't getting 12v. I swapped the 96 intake back in place so i could use the 96 sensors/injectors and rewired the cam sensor plug to work with the one on the 96 harness. Still nothing, no prime, fuel and oil pressure gauges are maxed out, and with the starter hooked up (not mounted) and grounded with jumpers to the block it won't engauge. Basically its dead all around. however when pushing the brake pedal the fuel pump no longer runs like it used to. Going back to the wiring diagram and going to try to figure this thing out.

Its very close to taking a trip to the scrap yard.

You have about 15 different issues going on at the same time. You need to start with one and work your way one at a time through all of them. Why are you trying to crank the engine if you know the fuel pump doesn't work? Have you checked for power at the fuel pump connector?

The '96 fuel pump is special. It's the only one for that year. Start there. The pump gets power from the same wiring harness that the brake lights do.

You've disassembled so much stuff that it's going to be really hard to figure out what the original problem is.

I think the problem is bigger than just fuel pump related. The fuel pump ran when the brake pedal was pushed (with the 99 black and white part of the ecu harness), that should leave out the wiring to the pump as being a problem. I also did a visual inspection from the tank to where the wires enter the floor and they all look fine as does the plug.

The biggest thing i can't figure out is the lack of dash lights. No check engine, oil pressure light, or any warning light comes on when the key is turned on EXCEPT the brake light.


I'm not trying to crank the engine, i don't even have the starter bolted into the flywheel. I was just simulating to see if it was bolted in place to see if it would be a crank/no start. That test allows me to know that it is a no crank, no fuel, no dashlights problem.

I'm leaning towards just removing the flex covering from the harnesses, and tracing all of the wires for shorts. Starting at the ignition switch and working my way to the front then to the rear.

why not just swap everything form the 99 into the 96, rather than just splicing random wires together and hoping for the best?

Sounds like you've already decided what's wrong and what you're going to do about it, so no need to ask for help.

You might want to buy a multimeter though, it does a good job of making your assumptions look dumb.

mustang_gt_350 said:

I think the problem is bigger than just fuel pump related.

...

I'm leaning towards just removing the flex covering from the harnesses, and tracing all of the wires for shorts. Starting at the ignition switch and working my way to the front then to the rear.

Click to expand...

I would agree that the problem is more than the fuel pump, but I doubt opening up the wiring harness will reveal much, unless you have an area in which the harness has been substantially damaged. For the most part wires behave themselves well. They don't tend to move around much.

On the other hand, if you substantially compressed or stretched a portion of the wiring harness in the process of removing/reinstalling engines that could have an interesting effect. That sort of damage should be fairly localized and would probably have visible manifestations on the exposed portion of the wiring loom.

I would first be looking for a completely ineffective ground connection, something like the back of the block or the firewall ground. After that I would go over the harness with a fine tooth comb looking for signs it got squashed or stretched. Then it would be time for a wiring diagram and voltage/continuity from the battery forward, one circuit at a time.

Please do share what you find. You have an interesting one for sure.

souske said:

why not just swap everything form the 99 into the 96, rather than just splicing random wires together and hoping for the best?

Click to expand...

What do you mean when you say swap everything?

I have the 99's engine, trans, in the 96 now. I haven't put the tcase in as i want to get the other issues figured out before i waste time assembling the whole thing and need to remove it just to get to something else to fix the problems.

RedHeep said:

Sounds like you've already decided what's wrong and what you're going to do about it, so no need to ask for help.

You might want to buy a multimeter though, it does a good job of making your assumptions look dumb.

Click to expand...

I'm asking because i thought maybe someone would have had an issue similar to mine and could lead me in a general area to check. But i forgot the reason for a forum is so everyone searches, and never starts new posts to ask questions, so it can die... I don't know why your having an attitude about this. I never had one with you and if it sounded like i did on my last post i apologize, as that was not my intention.


I do have a multimeter. When i jump the socket for the fuel pump relay the pump will run. So that shows me that there is power at the socket since the pump runs when the relay is bypassed by jumping the relay socket.

I discovered this earlier today, and just haven't had time to post or look it up yet. I took some readings on the relay sockets for both the fuel pump relay, and the ASD relay.

The fuel pump socket reads as follows.

Key off.
86=0v
87a= 0v
85= .24v
87=0v (with continuity)
30= 12.59

Key on
86=0v
87a=0v
85=12.2v
87= 0v (with continuity)
30= 12.48v

ASD socket

Key off.
86=0v
87a=0v
85=.24v
87=.84 (no continuity)
30= 12.63v

Key on.
86=0v
87a=0v
85=12.06v
87=7.71v (no continuity)
30=12.50

http://static.cargurus.com/images/site/2013/04/08/12/39/pic-5585062978081855657.gif

I checked the ground from the firewall to the valve cover stud, and its fine. I have continuity from the neg terminal of the battery to one of the fender bolts and from a valve cover bolt to the neg battery terminal. So it should be grounded fine.

Reverse the readings for all 85 and 86. Had a dyslexic moment.

Chased the grouning circuit for the fuel pump relay around last evening/night. Ended up following it back to C19 and grounding it there and it would trip the relay, so i OHM'ed the wire from the socket to the plug and it was fine. Looked at the PCM pintout and saw it had 2 grounding pins (A31, A32), Checked them and they had nothing.. So i quit for the night since i was sure that was the problem and it was 2am.

Looked around and found both grouning wires for the PCM cut off close to the harness and i never noticed them. I never pulled the drivetrain from this jeep, a transmission "chop" did. I hooked them up, along with the starter, and changed the connector for the cam sensor(distributor) and flushed the fuel line a few times with fresh gas before i hooked up the fuel rail. It fired

I noticed there is no tach function, and the fuel gauge and oil pressure still don't work but i can keep tinkering with them. I don't expect the oil to work since i broke the sending unit

I'm just happy it fired, and i can start to put everything else around it together.

That is a unique one all right.

Good job.