bass_guy

ok... so the sweet baboo and i bought a 90 limited.. 4.0 power everything...
great deal... it died on the previous owner and refused to start for some time... i found that voltage for the fuel pump was coming off the ballast resistor... so i ran a wire to the pump and bypassed the possibility of a short between the 2 points...
its running like this...
but at first the gas gauge was reading when the key was on, and the engine wasn’t running... (after fuel pump had quit for a couple of seconds) ... to me this said a short... bust out the tester... disconnect plugs behind kick panel, test wires going to pump from there... tests good... ok... so it has to be somewhere else... start digging around... and not figuring out where they could cross again... the fuel gauge has gone to reading full.. happened before when the jeep was running (obvious sign of short) but... when power was shut down, it would go to zero (it has now quit doing this)
so my first thought is that there has to be somewhere that voltage is getting to this line (or lack thereof) traced it all the way up to under the dash... but still nothing that stands out...
am i missing something in all this?
still unable to find the short... or where the power is getting lost on the way to the pump and guage....

Turbo X_J

"baboo"

Ha!, that dude from Seinfeld?

bass_guy

no... referring to peanuts....

http://peanuts.wikia.com/wiki/File:Sweet_Babboo.jpg

just showing my age....

DFlintstone

A "short" is connecting positive to ground. It will make sparks, heat ,fire, or blow a fuse. An "open" is a busted wire. Also a circuit may remain "open" if the switch never closes it to put the circuit to it's task.

The connections on the ballast resistor, as well as the resistor it's self often foul up. The gauge, on another circuit, in the tank also messes up., at least mine in my 90 does. If I remove the pump/ gauge assembly and test it it works fine. When I put it back in, the gauge sticks on 1/4 when I get below 1/2, then works below 1/4 tank. Really, I don't want to talk about it.

freegdr

My gauge reads when keys off always has?

bass_guy

lol....
there have been a few cars i have worked on where somebody tried a ghetto install ofa stereo and grounded a positive wire... fuse blows.... instead of putting another fuse in... something gets done like using the foil from a smoke pack to get it going... and ground it again... causing said wire to overheat.... at some point along the wire... it melts through not only its own casing... but a neighbouring caseing as well... thereby causing a "new path" for the circuit.. then more things start going wrong and becomes a "bug" that seems impossible to find....
this is the kind of thing that i am wondering might be going wrong....
from the description of how the xj died in the first place... and tracing where the pathways lead... it does not make sense that the power to the pump should end under the dash.... there is power from the ballast resistor... and there are no shorts from the kick panel to the pump.... ground is not an issue in this leg of the circuit either...
but there is a voltage bleed somewhere....
looking at the wiring diagram and putting tester to the connections is revealing nothing.... the gauge and the power to the pump are separate circuits... but the wires run parallel at a couple of points (nothing revealed in any wiring diagram about where the wires actually run in the vehicle)
i guess i should have been more clear in my initial question .....

is there another point along both circuits that the power could be crossed and cause voltage bleed? ..... to be more to the point...

DFlintstone

You probably know this, but the pump only comes on a few seconds when you turn the key to Ign, then again when it starts.

On your short. People have issues with the wiring at the rear all the time. The (later) HO rear 02 sensor, also wires pinched between things. I wonder if power to your pump back there could have shorted and burnt into the gauge wire...like you suggested. The pump has no fuse, it's on a fused link, one of the web off the big terminal on your starter relay.

Btw that resistor is not important, it was added when people complained about noise from the pump.

I wonder if you could energize the wire to the gauge, (maybe with 9 volts), then meter the fuel pump wire and juggle things under there and see if you can get it to read power from the pump wire. Maybe disconnect it from the tank....don't want you making the news!

bass_guy

so i managed to find a download the renix manual....
there in the pages of said book i have come to discover some interesting things about power routing and the relays...
for the ac, the fuel pump power routes through the ac relay, and the ac is far from working as it should...
so that’s 1 part of the problem...
the other part of the problem...
the aux fan is not coming on as it should.... thats what gave me a clue as to where to really start digging...
with these 2 issues being tied in with said relay...
i suspect that there is an open condition in more then 1 place...
ground to fuel pump being one of them... the power finding another route to ground via the fuel gauge, resulting in a false reading of the level...
so...
list of things to do....

1 ... clean and reground all the grounding points...
2 ... clean the electrical connections....
3 ... test relays, and replace what is needed...
4 ... disassemble harness where it goes into firewall, and check for damaged wires, repair as necessary.....

that should have pretty much all the issues resolved...
now that i have a much better idea of where all the sensors and relays are routed to... i can come up with a plan to rework some pathways so as to not have the systems take each other out....

the lead from the resistor to the ac relay and then to the fuel pump just seems like a really bad idea to me... but it makes sense if your trying to save a few $0.01 on the cost of manufacture....
after all if the load on the engine is increased, then other sensors would pick up the need for more fuel and deal with it ie: vacuum to fuel pressure regulator, o2 sensor...

i will update and let you know how it goes...

if i come up with anything uber cool... i will take notes of what i do and post...

cruiser54

Click on the pink link in my signature.

I recommend #1 and #3 for sure.

The ground point for the fuel pump and sender is behind the trim panel that's covered by the spare tire.

austinjoe13

Going off on a tangent here, if I remove the ballast causing more power to run through, will the fuel pressure be higher as well?

cruiser54

No. The fuel pressure regulator takes care of the pressure.

bass_guy

thanx cruiser54 ...
a rundown of almost everything on your pink list is what is in store for me...
and thanx for the ground location of the fuel pump...

my weekdays are everybody else s weekends... sucks sometimes...
means i am doing my weekend work while anybody that would give me a hand is at their 9 to 5 ....

DFlintstone

The fuel runs in a loop from the pump, past the injectors on the rail, then past the regulator, then back to the tank. The spring and vacuum at the regulator are what controls the pressure, (as said).

And Pete..thanks for the tip on the ground there.

cruiser54

Did you get the photos I sent earlier?