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One more weirdness

glfredrick

NAXJA Forum User
Location
Louisville, KY
We have an 86 4 cyl. that has a very strange electrical issue that we cannot track down.

The fuel pump gets no power through the electrical harness.

Now, that should be obvious -- track down where it comes from, and make sure that it is getting power, ground, a good pump, etc. -- We did all of that...

Here's where it gets even weirder. In an attempt to bypass the electrical harness and send power directly to the tank pump, we ran a jumper wire direct to pump from the battery (just a test). The pump works fine -- BUT -- that kills all the electric to the ignition system. Engine cranks, but no spark.

So, we can have spark -- but no fuel -- or fuel -- but no spark.

This even happens when we totally isolate the fuel pump from the rest of the vehicle wiring harness (as in run direct and seperate wires to the pump).

Anyone have a clue?

Right now, we've got the ENTIRE electrical system unwound, uncovered, and laying loose -- and so far, nothing rubbed through anywhere.

The engine will start right up if we juice it manually, then lay off the pump and allow the spark to fire -- of course, it dies the minute it runs out of fuel.
 
With the older XJs, the wire used has not been noted for its longevity. The wire can corrode within the insulation creating an open that can't be seen. However, that doesn't seem to be your problem. Your problem is indicative of a short and it's probably within a connector. If not a connector, then in a wire bundle where the wires closely touch. A wire may have become overheated enough to melt through the insulation into another wire.

While I can empathize with you situation, I can't really tell you much more than what you already know. You'll just have to continue to search.

You may want to consider getting wiring from a donor vehicle or even check with Painless Wiring, www.painlesswiring.com , to see if they have a replacement kit.

Good luck with your hunt.
 
Thanks... We have even used a Snap On short tester to try to isolate this issue, but so far, to no avail. We'll keep on plugging along.

Any chance that it is internal in the computer?
 
Here's a wild one for you. Check the ground wires (grounds are permanent, power is temporary) on the block, the body and anyplace else it goes to from the fuel pump. It's feeding back ground from the ignition when you put power to the pump and creating a + + condition instead of a + - so the rest can't operate.
Also, the oil pressure sending unit on some models has a ground so the pump won't kick in till there is oil pressure. It also feeds the choke.
 
Thanks carnuk. That is the sort of stuff that has eluded us so far. I'll be checking over the weekend. Hopefully we'll get that thing running and out of the garage. I've got a project to build and the XJ is in my way!
 
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