TrollHammer

Went wheeling a few weeks ago and laid my 93 XJ over on the passenger side. Got it flipped back on its wheels and drove another hour down the trail back to camp. Drove it around camp the next day, loaded it up and took it home. Washed the mud off and didn't see much wrong other than a little oil spilled onto the passenger side engine compartment. Didn't think much of it.

This weekend I was going to use the winch to pull a shed around, started it up, and got about 50-100 feet and it died shifting into reverse. Could not get it started.

After trying a few things like starter fluid and whatnot, I still have spark. I have no pressure at the fuel rail, and the pump does not run when you turn on the key. Figured that the pump went bad, and prepared to drop the tank.

Just for kicks, though, I went through and checked the relays and fuses. Here's what I found:
-Fuse box has dried mud and oil in it.
-All fuses look and measure OK.
-Comparing Fuel Pump relay with the others does not show anything unusual.
-Contact pins of the Fuel Pump relay are dark and scorched looking
-socket where the contact pins meet the fuse block is melted.

The contact pins that looked scorched were the Normally Closed set.

My questions are:

Is there a place that is common for the fuel pump wires to short (ohm meter does not show a short to ground)?

Can the pump itself short out without blowing a fuse or starting a fire?

Any other thoughts before I start poking around the wiring harness?

TrollHammer

Update: Relocated the relay to the next spot over (ABS slot), splicing some longer wire in and replacing the two connectors involved in the meltdown. I figured the relay contacts got dirty and heated up, and was about to just finish it off and start it up.

That was until I noticed one of the wires had a loop in it a few inches up the wiring harness. I uncoiled the loop and there is a small overhead-discolored insulation where the loop was. This tells me that the wire had current flowing through it over its rated capacity. Considering that it appears to be the wire leading to the balast resistor, and others seem to be able to bypass the resistor with no ill effects (though this increases the current through the circuit), it really has me scratching my head.

What should the measurements be across the wires leading to the pump, and what should they look like to power and ground?

Any other ideas? I'd really rather not have the thing catch fire ten miles out on a forest trail.

Radi

Wiring and connectors all eyeball OK? What happens if you remove the ballast resistor and jumper the wires together?
You could put a DC ammeter in place of the ballast resistor to measure current draw. I'm not sure what the fuel pump specs at, but 50-60% of the fuse rating would be a good ballpark.

TrollHammer

Ture enough about the current level, but taking the balast resistor out of circuit would increase current due to decreased resistance. If anything Id want to increase resistance and lower current, but it shouldnt need it, it should just work.

Have a different plan: after pulling the failed pump out of a comanchee Im also working on, I had a rude awakening. The wiring on this other pump had overheated, split and burned the insulation on the ground wire near the spade connector INSIDE the tank. The wire conductor was brittle and fracturing. This is all due to a weak connection between the spade connector and quick disconnect disapating heat, and I suspect also the actual reason for the pump to stop working in the comanche.

That said, kniwing that I did dent my tank on my cherokee and seeing hiw the pump sits in there, Im going to pull my pump and check for damage and shorts. I have another tank I might just swap in while I am at it.

Its scary that the wiring is so frail inside the tank, connectors exposed to the fuel and all...

TrollHammer

Jockeyed parts around, changed the tank out (along with the pump), and rechecked connections with an ohm meter with everything disconnected that I can find. This is what it looks like at the moment with the relay out, battery disconnected, fuel pump disconnected:

5.7 ohms to ground on one green/black wire at relay.
Open to ground on wire from relay to balast resistor (disconnected at both ends)
1.1 ohms across balast resistor
6 ohms to ground from green/grey wire on balast resistor
1.2 ohms to ground on one leg of connector to fuel pump (assumed ground)
120 ohms to ground on center pin (assumed sensor)
6 ohms to ground on other side of connector (all these are chassis side, did not check the disconnected pump)

I.cannot find my book at the moment, but it did not have all that much info. Im suspecting that there is a partial short to ground in the wiring harnes, or a short to a different circuit, but before I cut open all.my wiring harnesses I was hoping someone could tell me:

where does the green/black wire go from the relay block (the one that doesnt go to the balast resistor)

Where does the green/grey wire go from the balast resistor?

TrollHammer

(lost my last post, so hopefully this one goes)

Found my book, looks like the only other thing in the circuit is the O2 sensor as far as the book is concerned.

As far as I can tell, this may mean the O2 sensor would be drawing 2 amps. Seems a bit high to me, but I really don't know what it takes.

I need to draw it on paper, but it looks to me that if "something" is 6 ohms (2 amps at 12v), and the pump is 6 ohms (another 2 amps at 12v), then if the pump is connected the lowest resistance I should possibly get is 3 ohms (4 amps at 12v). According to http://www.powerstream.com/Wire_Size.htm, there should be no issue as far as chassis wiring is concerned, if I'm looking at it right.

This also does not tell me why my battery went dead. I'm still thinking I have a short in this line somewhere.

I guess if there are no other ideas I'll just have to hack into my wiring harness and visually inspect everything.

TrollHammer

Updating in case anyone else has the issue.

The problem was caused by weak electrical connections at the relay, caused by oil and mud in the fuse box.

I used rubbing alcohol to wash out the oil, I'm sure just about anything that dissolves oil or grease and doesn't harm plastic would work.

I moved the wires/connectors to an unused socket in the fuse box and replaced the overheated connectors, soldering and heat shrinking the splices.

The loop I found that overheated also got heat shrink. I think it was just due to being buried in the harness with extra wire and conducted heat coming back from the weak connection.

Have gone two wheeling trips and have no issues so far.