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Blowing the BIG fuses!!


NAXJA Member #120
NAXJA Member
NE Indiana
So I tore the interior out of the jeep the other day. After that I started it and it ran fine. Today I go out and try to start the jeep. It just turns over and over and over and over. Finally figure out that its a fuel problem. So i check the big fuse block under the hood. I find fuse number 2 is blown. Its a 30amp fuse that controls the fuel pump etc. I replace it with a 40amp cause that's all i have laying around. Go to start the motor...fires and dies, check fuse, fuse is blown.....wtf...

this thing just ran last week. where do i even start?

mac 'so sick of electrical gremlins' gyvr
Blown fuse = short circuit.

Trace the circuit, look for physical damage anywhere the wire runs through some kind of opening or fastener clip.

Start by unplugging the harness near the fuel pump. On the '88 there's a connector about 18" out from the tank, yours may be different but there should be a connector there somewhere. Unplug it, replace the fuse, and turn on the ignition. If the short is in the wire TO the pump, the fuse will blow even with the pump disconnected.
This happened on my 92 XJ a couple of weeks ago.

Turned out to be a short in the O2 harness. A mechanic on a previous repair had moved the wires, and it had melted/shorted-out agains the exhaust manifold.
i'll try both of your suggestions, as its in the garage with no interior i'm the mechanic that has to figure out what the heck the problem is....

mac 'short curcuited' gyvr
I'd first check areas that you were working near, probably easier to access and they may be related. If that doesn't solve it use a continuity checker to check your wires going to and from the fuse and see where they are going to ground. I don't know where the connections are in your wire harness but trace back through the different sections and that will narrow down your places to look for your short.
I hope this helps.
PS. There's an electrical contractor out here whose motto is, "We remove your shorts." :)
well i pulled the interior out, i had added power leather seats a while back and then took them out the other day for racing seats with the cage, i did pull the interior fuse box apart and put it back together to unwire the power seats (wires my dad had wired when the leather was put in), but after I did that, I did start the jeep to make sure everything was ok, and it ran just fine after that....

so i pulled the fuel pump wiring and the o2 sensor and it still started ran for just a sec, and died...and did blow the fuse....so next i'm going to unplug the alternator and see what happens...

this is a royal pain

mac '95s shouldn't have electrical issues' gyvr
macgyvr said:
so i pulled the fuel pump wiring and the o2 sensor and it still started ran for just a sec, and died...and did blow the fuse...
I would look in the area of the fuel pump wiring that was pulled.
Specifically, did flexing of the wires break the insulation and thus cause a short (either between the wires or wire-to-chassis)?
Short circuits cause blown fuses; *occasionally* there's a short in an alternator, but that *usually* manifests itself as a 'not charging' alternator, as opposed to blown fuses.

Could also be a poor connection at the back of the fuse block. If one of the re-done wires isn't solidly connected, it can cause a hella resistance which results in too many amps --> blown fuse.
on the fuel pump i just unplugged it, tried to start it and plugged it back in....

and to me if the fuel pump was the problem wouldn't it blow the fuse before the motor would even fire cause it turns on at one position before start...it should blow there if it was the problem....

i'm getting some scmatics tommorrow for the engine control system and i guess i'll just have to track it down and see what is screwed up.

mac 'short fuse' gyvr
Fuses 'blow' by heat of current melting the link.

Some fuses have a built-in heat sink, making them 'slow-blow' so a minor spike of amps (and resulting heating) won't kill the circuit.

Other fuses are fast-blow, so they trip *almost* instantly.

I don't know whether this fuse is supposed to be fast or slow, much less what the previous owner had in there.
Even a fast-blow may last a few seconds under load, if the load is too much stuff and not a short.

Re-check that fuel pump connector:
It might have had good contact before it was unplugged, but now it may have excessive corrosion between the contacts causing additional resistance.
Or, the connector may be broken internally and shorting out.
the fuel pump runs when you turn the key to the spot right before turning over, then it will fire and run a sec then blow.....i put 5 new fuses in this testing stuff...as well as double checking the fuel pump.....

mac 'still havent got to the alternator yet...trying to get cage built' gyvr
Called a buddy that is a Jeep mechanic, dont know why it took so long to remember to call him. He came over and check my jeep out. Found an unused plug under the instrument panel that had grounded out on a bolt.... pulled it off the bolt, jeep runs like a champ....

mac 'only cost me him drinking some of our beer' gyvr